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Materials and Fabrication

2005;():5-9. doi:10.1115/PVP2005-71029.

One result of the replacement steam generator program at South Texas Units 1 and 2 was an increased core flow at the units above the desired values. In order to reduce the core flow and reduce the sensitivity of the Alloy 600 reactor vessel closure head penetrations to primary water stress corrosion cracking (PWSCC), a reactor vessel upper head temperature reduction (UHTR) program was performed at the units. The UHTR program is a field modification which results in a significant reduction in the reactor vessel head temperature by reducing the bulk fluid temperature in the upper head region of the reactor. This is accomplished by providing additional bypass flow to the upper head region from the vessel downcomer which lowers the region’s temperature. The modification, with its increased bypass flow, also reduces the core flow to acceptable levels. Lowering temperatures in the head penetrations can be an effective way to increase the time for crack initiation, reduce the rate of crack propagation and to extend the useful life of the original reactor vessel head and/or the life of a replacement head. This paper discusses the UHTR field modifications performed at South Texas Units 1 and 2, the reduction in core flow realized, the results of the modification to reduce the plant’s susceptibility to Alloy 600 head penetration cracking and the significant extension to the reactor vessel head life with the implementation of a UHTR program.

Commentary by Dr. Valentin Fuster
2005;():11-16. doi:10.1115/PVP2005-71077.

The U.S. Nuclear Regulatory Commission (NRC) has undertaken a program to assess the integrity of control rod drive mechanism (CRDM) nozzles in existing plants that are not immediately replacing their RPV heads. This two-part paper summarizes some of the efforts undertaken on the behalf of the U.S.NRC for the development of detailed residual stress and circumferential crack-driving force solutions to be used in probabilistic determinations of the time from detectable leakage to failure. In this first paper, the finite element (FE) simulations were conducted to investigate the effects of weld geometry on the residual stresses in the J-weld for a centerhole CRDM nozzle. The variables of weld geometry included three weld heights (weld sizes) and three groove angles for each weld height while keeping the same weld size. The analysis results indicate that the overall weld residual stress decreases as the groove angle increases and higher residual stress magnitude is associated with certain weld height. The results also reveal that the axial residual stresses in the Alloy 600 tube are very sensitive to the weld height, and that the tube hoop stresses above the J-weld root increase with the increasing weld height.

Commentary by Dr. Valentin Fuster
2005;():17-22. doi:10.1115/PVP2005-71078.

The U.S.NRC has undertaken a program to assess the integrity of CRDM nozzles in existing plants that are not immediately replacing their RPV heads. This two-part paper summarizes some of the efforts undertaken on the behalf of the U.S.NRC for the development of detailed residual stress and circumferential crack-driving force solutions to be used in probabilistic determinations of the time from detectable leakage to failure. In this second paper, the weld residual stresses from the first paper were mapped onto detailed fracture mechanics finite element models that contained different length circumferential cracks in the CRDM centerhole nozzle. In each case, the cracks were unpinned after application of the operating pressure and temperatures and the K-solutions extracted. The results from these analyses suggest that the crack-driving force for a circumferential crack at the root of the J-weld slightly increases with increasing weld height but appears to be insensitive to bevel angle.

Commentary by Dr. Valentin Fuster
2005;():23-27. doi:10.1115/PVP2005-71406.

The susceptibility of Alloy 600 to Primary Water Stress Corrosion Cracking (PWSCC) has proven detrimental to several nuclear power plant components. Repair, modification or replacement of the components to mitigate the effects of PWSCC on Alloy 600 has been deemed necessary. In some cases, repair or replacement of plant components can be exorbitantly expensive; therefore, modification of the components is necessary to keep the plant operable. A form of modification is surface stress improvement, which alters the stress state of the material. Changing the stress state of the material eliminates one of the contributing factors required for the propagation of PWSCC. This paper discusses the application of surface stress improvement technologies to commercial nuclear power plant components and provides insight to where these technologies can be employed in the future.

Commentary by Dr. Valentin Fuster
2005;():29-39. doi:10.1115/PVP2005-71544.

Primary water stress corrosion cracking (PWSCC) continues to be a concern in nickel-based alloys (Alloy 600 and the associated weld metals, Alloys 82 and 182) in pressurized water reactors (PWRs). It has caused cracking and leakage in a number of components, including steam generator tubes, vessel head penetrations, and most recently, the dissimilar metal butt welds (DMWs) commonly used to connect vessel nozzles to PWR primary system piping. Weld overlays (WOLs) have been used extensively in the past twenty years to repair nuclear plant piping that has been found to be cracked or leaking due to stress corrosion cracking [1]. This paper summarizes the advantages of and technical justification for applying preemptive weld overlays (PWOLs) before cracking or leakage is observed, to mitigate PWSCC in Alloy 82/182 butt welds. PWOL design is governed by a number of considerations. The PWOL must supply sufficient thickness of resistant material (Alloy 52 weld metal) to provide new structural reinforcement of the original pipe weld sufficient to sustain design basis loads within ASME Code margins. Structural reinforcement calculations are presented demonstrating the achievement of this capability in accordance with ASME Section XI rules for evaluation of flaws in austenitic piping. The PWOL must supply sufficient thickness to effectively reverse the highly tensile residual stresses from the original DMW, including the potential detrimental effects of an in-process repair weld. Residual stress evaluations using elastic-plastic finite element models are presented that demonstrate the achievement of this objective for several typical nozzle geometries. Finally, analyses are presented to demonstrate that a dissimilar metal weld, with PWOL applied, meets the Nuclear Regulatory Commission (NRC) criteria for leak-before-break (LBB).

Commentary by Dr. Valentin Fuster
2005;():41-47. doi:10.1115/PVP2005-71697.

Stress corrosion cracking of Alloy 600 has lead to the modification and replacement of many nuclear power plant components. Among these components are the Bottom Mounted Nozzles (BMN) of the Reactor Pressure Vessel (RPV). Modifications of these components have been performed on an emergent basis. Since that time, Framatome ANP has developed state-of-the-art modification methods for the repair of BMNs using the Electrical Power Research Institute (EPRI) managed Materials Reliability Program (MRP) attributes for an ideal repair as a basis for evaluation of modification concepts. These attributes were used to evaluate the optimal modification concepts and develop processes and tooling to support future modification activity. This paper details the BMN configurations, modification evaluation criteria, several modification concepts, and the development of the tooling to support the optimal modification scenarios.

Topics: Reactor vessels
Commentary by Dr. Valentin Fuster
2005;():53-64. doi:10.1115/PVP2005-71027.

This paper evaluates non-destructive examination (NDE) detection capabilities for fatigue cracks in piping. Industry performance demonstration initiative (PDI) data for fatigue crack detection were used to develop a matrix of statistically based probability of detection (POD) curves that consider various NDE performance factors. Seven primary performance factors were identified — Material, Crack Geometry/Type, NDE Examination Access, NDE Procedure, Examiner Qualification, Pipe Diameter, and Pipe Wall Thickness. A database of 16,181 NDE performance observations, with 18 fields associated with each observation, was created and used to develop statistically based POD curves for 42 stainless steel and 14 carbon steel performance cases. Subsequent comparisons of the POD fits for each of the cases showed that excellent NDE performance for fatigue cracks can be expected for ferritic materials. Very little difference was observed between the POD curves for the 14 carbon steel performance cases considered in this study and NDE performance could therefore be represented by a single POD curve. For stainless steel, very good performance can also be expected for circumferential cracks located on the same side of the weld from which the NDE examination is made. POD depended primarily on component thickness. Three POD curves for stainless steel were prepared. Best estimate and the associated 95% confidence bounds for POD versus through-wall depth logistic regression digital data are provided. Probabilistic fracture mechanics (PFM) calculations were performed to compare best estimate leak probabilities obtained from both the new performance-based POD curves and previous PFM models. This work was performed under joint funding by EPRI and the U.S. Department of Energy (DOE), Office of Nuclear Energy Science and Technology’s Nuclear Energy Plant Optimization (NEPO) program.

Commentary by Dr. Valentin Fuster
2005;():65-67. doi:10.1115/PVP2005-71119.

As part of a wide ranging research programme aimed at developing a fracture mechanics methodology for blunt notches, earlier work for a general two-dimensional blunt notch Mode III model has shown that the stress at a distance × ≪ ρ (notch root radius of curvature) ahead of the notch root only depends on x, ρ and the peak stress σp , irrespective of the notch shape and the loading characteristics. This uniqueness has been confirmed for various notch profiles and loading scenarios. In this paper we show that the uniqueness of the local stress distribution is peculiar to a notch and does not apply to an inclusion.

Commentary by Dr. Valentin Fuster
2005;():69-72. doi:10.1115/PVP2005-71120.

With the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture as a basis, the author is involved in a wide-ranging research programme, the objective being to extend the sharp crack fracture mechanics methodology to blunt flaws, so as to take credit for the blunt flaw geometry. In earlier work, a blunt flaw fracture initiation relation has been derived, subject to the restriction that the process zone size s is small compared with the flaw depth (length) or any characteristic dimension of a configuration other than the root radius ρ. This relation gives the critical elastic flaw-tip peak stress σpcr , and has been derived using a two extremes procedure, whereby the separate solutions for small and large s/ρ are blended together so as to give a single relation that is valid for the complete spectrum of s/ρ values. The small s/ρ solution was obtained by inputting the stress field in the immediate vicinity of the flaw root into an analysis of the behaviour of a process zone at a planar surface. This paper provides formal justification for this small s/ρ procedure.

Commentary by Dr. Valentin Fuster
2005;():73-86. doi:10.1115/PVP2005-71141.

Appendix L of Section XI provides for serviceability assessments of piping components that are subject to fatigue stresses. This appendix introduced a damage tolerance examination strategy to assure that components perform reliably throughout operating periods between inspections. Operating periods are based on fatigue crack growth analyses of postulated pre-existing cracks. Evidence from service experience shows that fatigue cracking occurrences at operating nuclear power plants often result from mechanisms that cause cracks to initiate and then grow at multiple locations on the inside surface of a pipe, becoming longer and deeper, and eventually linking to form a single long crack. This paper documents important details of the technical bases for changes to Appendix L. Calculations identified aspect ratios for equivalent single cracks (ESC) between the extremes of a 6:1 ratio and a full circumferential crack that can be used in Appendix L flaw tolerance assessments to account for the initiation, growth, and linking of multiple fatigue cracks. Probabilistic fracture mechanics (PFM) calculations determined ESC aspect ratios that result in the same through-wall crack probability as multiple small cracks (0.02 inch depth) that initiate and coalesce. The computations considered two materials (stainless and low alloy steels), three pipe diameters, five cyclic membrane-to-gradient stress ratios and a wide range of primary loads. Subsequent deterministic calculations identified the ESC aspect ratio for the hypothetical reference flaw depth assumptions in Appendix L. This paper also describes computations that compare the Appendix L flaw tolerance allowable operating period for the ESC models with results obtained when a single default 6:1 aspect ratio reference flaw.

Topics: Fatigue cracks
Commentary by Dr. Valentin Fuster
2005;():87-94. doi:10.1115/PVP2005-71147.

This work presents an investigation of the ductile tearing properties for API 5L X70 and X80 pipeline steels using experimentally measured crack growth resistance curves (J-R curves). Testing of the pipeline steels employed compact tension (C(T)) fracture specimens to determine the J-R curves based upon the unloading compliance method using a single specimen technique in accordance with the ASTM E1820 standard procedure. Conventional tensile tests and Charpy V-Notch tests were also performed to determine the mechanical and impact properties for the tested materials. Severe splitting running parallel with the crack propagation path with varied lengths was observed in all tested fracture specimens, particularly for the API X80 material. The occurrence of splits makes the determination of JIc and resistance curves more difficult, as delamination of interfaces positioned normal to the crack front decreases the effective thickness of the test piece, inducing plane stress conditions deep inside the specimen. This experimental characterization provides additional toughness and mechanical data against which the general behavior of X70 and X80 class pipeline steel can be compared.

Commentary by Dr. Valentin Fuster
2005;():95-101. doi:10.1115/PVP2005-71152.

A model has been developed to assess the structural assessment of functionally graded materials in which yield strength and toughness vary spatially. The yield strength of the material at any point is deterministic, but varies spatially. The local cleavage toughness also varies spatially, but is statistically distributed following a two parameter Weibull model. The Weibull moduli quantify reliability and the second Weibull parameter is interpreted in terms of the mean toughness of the local material. The model intended to determine the crack propagation direction and failure probabilities of a stationary pre-crack. The effect of simple yield strength and toughness gradients are discussed, before the model is applied to experimental data on laser welded joints.

Commentary by Dr. Valentin Fuster
2005;():103-110. doi:10.1115/PVP2005-71201.

In this paper, governing equations and solutions for asymptotic singular and non-singular crack-tip sectors in perfectly plastic materials are first summarized under combined in-plane and out-of-plane shear loading conditions. The crack-tip fields under mixed mode II/III loading conditions are then investigated. An assembly of crack-tip sectors is adopted with stress discontinuities along the border of the two constant stress sectors. The solutions of the crack-tip fields under pure mode II, mixed mode II/III, and nearly pure mode III loading conditions are presented. The trends of the angular variations of the mixed mode II/III crack-tip stresses agree with those of the available computational analysis and the asymptotic analysis for low strain hardening materials. The pure mode II crack-tip stresses are similar to those of Hutchinson and the nearly pure mode III stresses are similar to those of the pure mode III crack-tip field of Rice.

Commentary by Dr. Valentin Fuster
2005;():111-121. doi:10.1115/PVP2005-71202.

Fracture properties of API X80 pipeline steel have been developed using a set of single edge notched bend (SENB) and single edge notched tension (SENT) specimens with shallow and deep cracks to generate different crack-tip constraint levels. The test data show that the J-R curves for X80 pipeline steel are strongly constraint dependent. To facilitate transfer of the experimental J-R curves to those for actual cracked components, like flawed pipeline, constraint corrected J-R curves are developed. The two-parameter J-A2 formulation is adopted to quantify constraint effect on the crack-tip fields and the J-R curves. The constraint parameter A2 is extracted by matching the J-A2 solution with finite element results for a specific crack configuration. A constraint corrected J-R curve is then formulated as a function of the constraint parameter A2 and crack extension Δa. A general method and procedure to transfer the experimental J-R curves from laboratory to actual cracked components are proposed. Using the test data of J-R curves for the SENB specimens, a mathematical expression representing a family of the J-R curves is constructed for X80. It is shown that the predicted J-R curves developed in this paper match well with experimental data for both SENB and SENT specimens. To demonstrate its application in assessing flaw instability, a pipeline with an axial surface crack is considered. For a crack depth of 50% of the wall thickness, the predicted J-R curve is found to be higher than that for the SENB specimen with the same crack length to width ratio. From this predicted J-R curve and crack driving force obtained by finite element analysis, the failure pressures of the pipeline at the crack initiation and instability are determined and discussed.

Commentary by Dr. Valentin Fuster
2005;():123-128. doi:10.1115/PVP2005-71255.

In a Mod.9Cr1Mo steel applied widely to boiler components, low hardness problem related with manufacturing and fabrication processes has become a critical issue recently. In this study, hardness, microstructure, tensile and creep rupture tests have been performed using specimens given various thermal cycles to investigate the detailed mechanism causing low hardness values of 150 to 170 Hv and the minimum hardness requirement from a standpoint of the tensile properties and the maximum allowable stresses. Low hardness values were mainly attributed to the formation of ferrite phase on cooling after heating at intercritical temperatures just above the Ac1, about 850°C. Ferrite transformation on cooling after intercritical heating occurred even at a relatively faster cooling rate of 3.5 °C/sec since the nose of ferrite transformation in the continuous cooling transformation (CCT) curve moved to the left due to the very low carbon content in austenite phase formed at intercritical region. Low hardness value of 160’s Hv occurred occasionally in practical applications was observed at a cooling rate of below 0.167 °C/sec after intercritical heating. At least 190 Hv of hardness values or more were needed to satisfy tensile properties and maximum allowable stresses specified in ASME Boiler & Pressure Vessel (B&PV) code.

Topics: Steel , Boilers
Commentary by Dr. Valentin Fuster
2005;():129-143. doi:10.1115/PVP2005-71273.

Flaws in Zr-2.5Nb alloy pressure tubes in CANDU nuclear reactors are susceptible to a crack initiation and growth mechanism known as Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth of the hydrided region and fracture of the hydrided region at the flaw-tip. An overload occurs when the hydrided region at a flaw is loaded to a stress higher than that at which this region is formed. Flaw disposition requires justification that the hydrided region overload from normal reactor operating and transient loading conditions will not fracture the hydrided region, and will not initiate DHC. Some preliminary test results on the effect of hydrided region overload on DHC initiation were presented in Reference [1]. In the present work, several series of more systematically designed monotonically increasing load experiments were performed on specimens prepared from an unirradiated pressure tube with hydrided region formed at flaws with a root radius of 0.1 or 0.3 mm under different hydride formation stresses and thermal histories. Crack initiation in the overload tests was detected by the acoustic emission technique. Test results indicate that the resistance to overload fracture is dependent on a variety of parameters including hydride formation stress, thermal history, flaw geometry and hydrogen concentration.

Commentary by Dr. Valentin Fuster
2005;():145-154. doi:10.1115/PVP2005-71278.

One of the thrust areas in the integrity analysis of cracked nuclear piping system is concern with the reduction in moment, at the crack section due to combined effects of local and global residual compliance. However an important consideration in the design of piping system, which is generally not considered, is the re-distribution of load that occurs due to finite compliance of the piping system. The load at the crack section reduces while it increases generally at support/anchor locations, which may be high stressed locations. In case of stiff-piping system this re-distribution of load may be quite significant. Hence for the complete integrity of the piping system these un-cracked locations should also be re-assessed. A generalized procedure is suggested to take care of the reduction in load at the cracked section and corresponding increase in reactions at the support/anchor locations in a 3-D cracked piping system. Thus the stability of cracked section as well as other highly stressed locations can be simultaneously assessed. Here it is assumed that the remaining piping system behaves in a linear elastic manner and the plasticity remains confined to the cracked section only. Detailed finite element analyses are performed on circuitous (3-D) cracked piping system to validate the developed approach. Results presented in this article clearly show that due to reduction in moment the crack driving force, for the same external load, reduces significantly.

Commentary by Dr. Valentin Fuster
2005;():155-163. doi:10.1115/PVP2005-71327.

An experiment was conducted to investigate the role of weld residual stress on stress corrosion cracking in welded carbon steel plates prototypic to those used for nuclear waste storage tanks. Carbon steel specimen plates were butt-joined with Gas Metal Arc Welding technique. Initial cracks (seed cracks) were machined across the weld and in the heat affected zone. These specimen plates were then submerged in a simulated high level radioactive waste chemistry environment. Stress corrosion cracking occurred in the as-welded plate but not in the stress-relieved duplicate. A detailed finite element analysis to simulate exactly the welding process was carried out, and the resulting temperature history was used to calculate the residual stress distribution in the plate for characterizing the observed stress corrosion cracking. It was shown that the cracking can be predicted for the through-thickness cracks perpendicular to the weld by comparing the experimental KISCC to the calculated stress intensity factors due to the welding residual stress. The predicted crack lengths agree reasonably well with the test data. The final crack lengths appear to be dependent on the details of welding and the sequence of machining the seed cracks, consistent with the prediction.

Commentary by Dr. Valentin Fuster
2005;():165-170. doi:10.1115/PVP2005-71346.

The application of the reference temperatures T0 and RTTo according to ASTM E 1921 and ASME Code Cases N-629 or N-631, respectively, shall be established in the current revision of German KTA rules. The Master Curve reference temperature T0 characterizes the fracture toughness of ferritic steels that experience onset of cleavage cracking at elastic, or elastic-plastic KJC -instabilities, or both. The plane-strain fracture toughness, KIC , defined by ASTM E 399, is assumed to represent a size insensitive initiation based lower bound value. The majority of existing fracture toughness data are based on KIC -values. More recent data are KJC , related to the issuing of ASTM E 1921 in 1997 and to the success of the Master Curve based T0 . Therefore, the possible difference between T0 determined from KJC and from KIC was investigated with available data bases for RPV-steels. The comparison of T0 (KJC ) and T0 (KIC ) showed a 1:1 correlation proving equivalence of KJC and KIC in the determination of T0 .

Commentary by Dr. Valentin Fuster
2005;():171-178. doi:10.1115/PVP2005-71347.

The brittle failure assessment for the reactor pressure vessel (RPV) of a 1300 MW pressurized water reactor (PWR) was revised according to the advanced state of the art. The RPV steel is 22 NiMoCr 37 (A 508 Cl.2). The expected neutron fluence at End of License (EOL) after 32 years of full operation is Φ < 2.3·1018 cm−2 . The assessment followed a multi-barrier concept to prove independently exclusion of crack initiation, crack arrest and exclusion of the load necessary to advance the arrested cracks through the RPV wall. For operational conditions, a combination of shut-down and anomalous operation was proven as the leading transient. This transient follows a load path decreasing with temperature and thus producing a warm pre-stressing for flaws in the RPV. Therefore, a warm pre-stress (WPS) effect is to be considered. For loss of coolant accident (LOCA) conditions, a leak size screening was performed to find the leading transients. Enveloping transients were determined and used for the brittle failure assessment. As covering crack postulates circumferential cracks in the flange joint to the vessel and axial cracks in the nozzle corner were investigated both under the assumption of a broken clad. For the LOCA loads, the leading leak size was determined by finite element analyses and fracture mechanics based assessment of different combinations of cold/hot leg leak position and cold/hot leg injection of emergency core cooling (ECC). Leading situations are in the flange joint for the cylinder and in the nozzle corner for the flange. In the nozzle corner, high thermal loading from cross corner cooling produces large plastic deformation and loss of constraint. In this situation, the fracture toughness KIC as determined from deeply cracked fracture mechanics specimens is not representative for the component. Thus, loss of constraint in the component was considered by the application of a constraint modified master curve to determine the constraint representative fracture toughness. For the investigated load cases the relevant part of the load paths is at upper shelf temperatures, with the exception of the anomalous operation starting at 50°C to lower temperatures. The results of the investigation demonstrate preclusion of initiation for normal and anomalous operation as well as for LOCA loads. For LOCA, two further barriers were proven with crack arrest for postulated crack extension well within allowable depth in the RPV wall and with the preclusion of the load necessary to advance the arrested crack through the wall.

Commentary by Dr. Valentin Fuster
2005;():179-182. doi:10.1115/PVP2005-71384.

This paper provides a review of findings of a comprehensive research effort by the authors and collaborators in the area of experiments, analysis and simulation of mixed-mode ductile fracture. Topics include mixed-mode Arcan stable tearing tests, the mixed-mode CTOD fracture criterion and its basis, normalization of ductile crack tip fields, ductile failure envelope, crack tunneling and slanting, effects of stress constraint, custom 2D and 3D mixed-mode crack growth simulation codes, and simulations of mixed-mode stable tearing crack growth tests.

Commentary by Dr. Valentin Fuster
2005;():183-186. doi:10.1115/PVP2005-71479.

Reactor pressure vessels under some special regimes (i.e. pressurized thermal shock) are loaded by a strongly biaxial tensile stresses whose ratio can reach even an opposite value in comparison with normal operating conditions. Pressure vessel integrity as well as its life assessment is performed on the basis of fracture mechanics where normally only uniaxial fracture toughness of materials are used as material inputs. Special biaxial tests on cruciform type specimens with thickness up to 90 mm were tested in Nuclear Research Institute (NRI) Rez - special testing equipment and testing methods including measurements have been developed and realized. Testing equipment with maximum loading up to 1.5 MN allows to reach different biaxial loading ratios between 0 and 2. During tests, carried out mostly at low temperatures, specimens deflections, strains, load and crack opening displacement are measured and then evaluated. In NRI Rez comparative experimental tests on cruciform and beam specimens were performed. The aim of these tests was to examine the effect of crack depth and biaxial loading on fracture toughness for reactor pressure vessel material 15Kh2MFA. For evaluating the tests, the FEM (program SYSTUS) was used. The performed tests confirm shallow crack effect, i.e. increase of fracture toughness for shallow cracks compared to that one of deep cracks. Further, the performed experiments show decrease of fracture toughness of shallow cracks loaded biaxially compared to uniaxial loading of shallow cracks. Quantitatively, the fracture toughness decrease was about 20%.

Commentary by Dr. Valentin Fuster
2005;():187-196. doi:10.1115/PVP2005-71484.

Fracture behavior of high strength linepipes with weld defects is of great interest for the integrity of pipeline system. Especially, in the seismic or permafrost area, where large ground displacement can be expected, linepipe materials need to have sufficient resistance against brittle and ductile fracture under large deformation. Wide plate tensile test with surface flaw in the girth weld metal of X100 linepipe demonstrated that tensile limit is dominated by ductile crack initiation and its propagation. Conditions for ductile crack initiation for the base materials and girth weld joints of Grade X80 and X100 linepipes were investigated in this study. It was shown that ductile cracking occurs in the notch tip region of the wide plate specimen when notch tip equivalent plastic strain reaches the same critical value as determined by the small-scale tests. Therefore, “the equivalent plastic strain” in the critical regions can be used as a transferable parameter to predict ductile crack initiation behavior. Assessment methodology for tensile limit of high strength linepipe girth weld with respect to preventing ductile cracking was proposed. The effect of strength matching of girth weld and base metal Y/T ratio on limit remote strain as well as allowable defect size was investigated analytically. Increasing strength matching and lowering Y/T ratio of base material can lead to higher limit strain to ductile cracking of girth weld. These effects of material properties were validated by weld wide plate tensile tests. Therefore, careful selection of material properties should be important to improve resistance against ductile cracking of linepipe girth welds under large deformation field.

Commentary by Dr. Valentin Fuster
2005;():197-204. doi:10.1115/PVP2005-71485.

The crack tip opening displacements and angles (CTOD/CTOA) are calculated with finite element method based on the test data of a set of constraint-dependent J-R curves for A285 carbon steel. The values of the CTOD/CTOA are initially high at initiation, but rapidly decrease to a nearly constant value. When the common practice is adopted by using only the constant part of CTOD/CTOA as the fracture criterion, the crack growth behavior is shown to be severely underestimated. However, with a bilinear form of CTOD/CTOA fracture criterion which approximates the initial non-constant portion, the experimental load vs. crack extension curves can be closely predicted. Furthermore, it is demonstrated that the CTOD/CTOA is crack tip constraint dependent. The values of CTOD/CTOA for specimens with various ratios of crack length to specimen width (a/W) are reflected by the J-R curves and their slopes.

Commentary by Dr. Valentin Fuster
2005;():205-208. doi:10.1115/PVP2005-71501.

Fatigue failures often take place in high temperature pressure vessels and equipment because of fluctuation of pressure and temperature. Fatigue crack growth properties of materials at high temperatures are very important for safety assessment of high temperature equipment. A series of fatigue crack growth tests were carried out, and fatigue crack growth rates were determined at 25∼500°C for typical steels 316L and 16MnR. The laws of fatigue crack growth of two materials at different temperatures and the effect of temperature on fatigue crack growth rates were studied. The results show that the crack growth rates increase with temperature for 316L steel. Both the exponent n and constant C for Paris law change with temperature. The fatigue cracks of 16MnR propagate at 150 °C and 300 °C more slowly than at room temperature and 425 °C. The fatigue crack growth rate at 425 °C is the highest for temperature range of 25–425 °C.

Commentary by Dr. Valentin Fuster
2005;():209-215. doi:10.1115/PVP2005-71521.

Tensile tests using welded joint specimens with a surface flaw were conducted in order to study the large deformation behavior and ductile fracture of welded structural component with a flaw. Two types of 490MPa class steel plates with different yield strength are prepared for this study. The surface flaw was introduced at the center of welded metal of welded joint specimen. Strain distribution during loading and the maximum strain at ductile fracture of specimen were measured in order to investigate the influence of stress-strain properties on ductile fracture behavior of the welded joint. Three dimensional elastoplastic finite element analyses were also carried out by using the welded joint specimen models in order to calculate the detailed stress and strain distributions around notch tip. In addition, a simplified analytical model to estimate fracture strain and critical flaw size of the welded joint was proposed using the stress-strain relationship combination between base material and welded metal by Swift’s equation. The results from the analytical model show fairly good agreement with experimental results.

Commentary by Dr. Valentin Fuster
2005;():217-224. doi:10.1115/PVP2005-71607.

Fatigue damage is generally described as the nucleation and growth of cracks to final failure. These two stages of fatigue damage are often modeled with completely different methods with no quantitative relationships between them. In addition, a number of fitting parameters are needed in order to consider different effects. The current work is aimed to develop a robust approach for the prediction of fatigue life from crack initiation to final fracture. The approach bridges the gap between the crack nucleation and growth regions. Based upon the conception that fatigue damage is directly related to the stresses and strains inside the material, it is assumed that both crack nucleation and crack growth are governed by the same fatigue damage mechanisms and a single fatigue damage criterion can model both stages. A basic rule is that any material point fails to form a fresh crack if the total accumulated fatigue damage reaches a limit. Crack growth is treated as a process of continuous crack nucleation without using the stress intensity factor or J-integral concept. The approach consists of two steps: stress analysis and fatigue damage prediction. Elastic-plastic stress analysis is conducted for the component to obtain the detailed stress-strain responses. By using a general fatigue criterion, fatigue crack nucleation and growth are predicted. Notched specimens made of 1070 steel were experimentally tested from crack initiation till fracture. The approach was applied to predict the fatigue life of 1070 steel and the predicted fatigue lives were in excellent agreement with the experimental observations.

Topics: Fatigue life
Commentary by Dr. Valentin Fuster
2005;():225-228. doi:10.1115/PVP2005-71726.

The creep deformation model of an electroformed nanocrystalline nickel-phosphorous repair material is examined in this paper, at the design temperature of 616K (650K), characteristic for the majority of Pressurized Water Reactor (PWR) nuclear power plants. The results from 15 constant load creep tests of this material, at stresses between 103 MPa and 483 MPa, were used to validate the model. The total creep rate determined with the model suggested in this paper, based on two predominant deformation mechanisms, is also compared with the literature.

Commentary by Dr. Valentin Fuster
2005;():229-238. doi:10.1115/PVP2005-71778.

A material configuration of central importance in composite materials or in protective coating technology is a thin film of one material deposited onto a substrate of a different material. Fabrication of such a structure inevitably gives rise to stress in the film due to lattice mismatch, differing coefficient of thermal expansion, chemical reactions, or other physical effects. Therefore, in general, the weakest link in this composite system often resides at the interface between the thin film and substrate. In order to make multi-layered electronic devices and structural composites with long-term reliability, the fracture behavior of the material interfaces must be known. This project is intended to address the problems associated with interface fracture toughness evaluation and offers an innovative testing procedure for the determination of interface fracture toughness applicable to thin coating materials in general.

Commentary by Dr. Valentin Fuster
2005;():241-247. doi:10.1115/PVP2005-71135.

The ASME code cases N-629 and N-631 permits the use of a Master Curve-based index temperature (RTTo ≡ T0 + 19.4°C) as an alternative to traditional RTNDT -based methods of positioning the ASME KIc , and KIR curves. This approach was adopted to enable use of Master Curve technology without requiring the wholesale changes to the structure of the ASME Code that would be needed to use all aspects of Master Curve technology. For the brittle failure analysis considering irradiation embrittlement additionally a procedure to predict the adjustment of fracture toughness for EOL from irradiation surveillance results must be available as by NRC R.G. 1.99 Rev. 2 e.g.: ART = Initial RTNDT + ΔRTNDT + Margin. The conservatism of this procedure when RTNDT is replaced by RTTo is investigated for western nuclear grade pressure vessel steels and their welds. Based on a systematic evaluation of nearly 100 different irradiated material data sets, a simple relation between RTToirr , RTToref and ΔT41JRG is proposed. The relation makes use of the R.G. 1.99 Rev. 2 and enables the minimizing of margins, necessary for conventional correlations based on temperature shifts. As an example, the method is used to assess the RTTo as a function of fluence for several German pressure vessel steels and corresponding welds. It is shown that the method is robust and well suited for codification.

Topics: Temperature
Commentary by Dr. Valentin Fuster
2005;():249-259. doi:10.1115/PVP2005-71146.

The integrity of the reactor pressure vessel (RPV) has to be demonstrated under the most severe type of loadings that can occur during the plant lifetime. The selection of the pressurized thermal shock (PTS) transients is questionable since no account is made of potential beneficial effect of the load history. If one considers a crack close to the inner wall of the vessel, the loading at elevated temperature will open the crack, then cooling will tend to close the crack and if a load is applied at low temperature, the fracture resistance of the cracked component will be much higher than predicted using toughness values at this temperature. This is known as the warm pre-stress (WPS) effect and has been well documented since decades ([1], [2]). A 3-year European Research & Development programme (SMILE) has been launched in January 2002 as part of the 5th Framework Programme of the European Atomic Energy Community (EURATOM) to comfort this phenomenon and develop methods of analysis for 3D configurations liable to experience significant local yielding [3]. This paper analyzes the WPS type experiments performed in SMILE on C(T) specimens and on the mock-up which consists in a hollow cylinder containing a total circumferential crack on the inner wall. The predictions, based on an extended version of the BEREMIN model appear to be good but slightly conservative.

Commentary by Dr. Valentin Fuster
2005;():261-266. doi:10.1115/PVP2005-71181.

In the standard test method for the determination of the reference temperature T0 in the transition range, ASTM E 1921-03 [1], the remark is given that different specimen types could lead to discrepancies in the calculated T0 values. Especially C(T) and SEN(B) specimens indicate by experimental evidence that a 10 °C to 15 °C difference in T0 has been observed. In the course of the European research project VOCALIST [2] a ferritic RPV steel has been investigated by conducting numerous fracture toughness experiments as well as intensive numerical studies. A local approach model based on the Weibull stress has been developed and calibrated for this material [3]. For the calculation of the constraint effect between SEN(B) and C(T) specimens with a crack to ligament ratio of approx. 0.5 the model has been applied to predict the constraint effects on fracture toughness and the resulting theoretical difference in the reference temperature T0 . For this purpose the according specimens have been calculated by several finite element models and a reference solution in the small scale yielding space allowed for the calculation of the “constraint free” reference transition temperature T0 . By means of theoretical constraint functions derived from the Weibull stress model, the difference for each specimen compared to the reference solution could be calculated. From the results a theoretical difference of ΔT0 = 10°C between SEN(B) (lower value) and C(T) specimens (higher value) caused by the different crack tip constraint has been obtained. This value confirms the experimental observations.

Commentary by Dr. Valentin Fuster
2005;():267-271. doi:10.1115/PVP2005-71196.

The recently initiated German project CARISMA (C rack Initiation and Ar rest of I rradiated S teel Ma terials) will create a data base on pre-irradiated original materials of the four generations of German nuclear pressurized water reactors, which allows the examination of the consequences if the Master Curve instead of the RTNDT concept is applied. Several original materials of the four generations of German nuclear power plants (typical for KWO, KKS - Biblis A, Biblis B, KKU - KKG, KWG, KKP2, KBR - KKE, KK12, GKN2) will be investigated. They have been irradiated in six large scale irradiation capsules in a German research reactor (the VAK plant) at corresponding plant conditions. The capsules contain regular tensile and Charpy impact specimens as well as Pellini and fracture toughness wedge opening load specimens up to a specimen thickness of 100 mm. The first fracture toughness tests have been performed on a weld metal NiCrMo1 UP(mod.)/LW320, LW340 (1. generation, lower bound of the weld materials)—with a fluence Φ = 2,12E19 cm−2 (E > 1 MeV). This weld has a Cu-content of 0.22 wgt.% and it was therefore supposed to show a large transition temperature shift. Some fracture toughness tests in the irradiated material condition were already available and during this project four 100 mm thick wedge opening load (WOL) specimens were tested. At one of the specimens brittle failure could not be achieved during the test, because the load capacity of the test machine was exceeded. Therefore the specimen was loaded by a load-unload-cool-fracture load path to demonstrate the warm pre-stress effect of this highly irradiated specimen. At the final fracture of the specimen at a lower temperature, the failure load was significantly higher than the original one (factor 3 higher), which clearly indicates that the benefit of warm pre-stressing will not be eroded with irradiation.

Topics: Stress
Commentary by Dr. Valentin Fuster
2005;():273-275. doi:10.1115/PVP2005-71197.

For the introduction of the new reference temperature RTTo of the ASME Code Cases N-629 and N-631 into the German KTA rules the equation RTTo = T0 + 19,4 °C (35 °F) was validated by the re-evaluation of the existing fracture toughness data base of German RPV steels, including unirradiated and irradiated base material and weld metal data. The test temperatures of the data base were standardized to the reference temperature T0 of the Master Curve of the data sets and the data base was compared with the ASME KIc -curve as adjusted by RTTo . The KIc -curve adjusted by RTTo enveloped both, the 1T-size adjusted data base and also the as measured data base, corresponding with the definition of RTTo . Thus the results also prove the validity of the KIc (RTTo )-curve for allowable flaw sizes and up to the crack length spectrum of the ASME KIC -data base without size adjustment of T0 . The results of both investigations confirmed the validity of RTTo for German RPV steels.

Commentary by Dr. Valentin Fuster
2005;():277. doi:10.1115/PVP2005-71245.
FREE TO VIEW

The Reactor Pressure Vessel (RPV) is an essential component liable to limit the lifetime duration of PWR plants. The assessment of defects in RPV subjected to PTS transients made at an European level generally do not take necessary into account the beneficial effect of load history (warm pre-stress WPS). A 3-year Research & Development program — SMILE — has been started in January 2002 as part of the Fifth Framework of the European Atomic Energy Community (EURATOM). The SMILE project (“Structural Margin Improvements in aged-embrittled RPV with Load history Effects”) is one of a “cluster” of Fifth Framework projects in the area of Plant Life Management. It aims to give sufficient elements to demonstrate, to model and to validate the beneficial WPS effect in a RPV integrity assessment. Finally, this project aims to harmonize the different approaches in the European Codes and Standards regarding the inclusion of the WPS effect in a RPV structural integrity assessment. Within the framework of the project, an important experimental work has been conducted including WPS type experiments on CT specimens and also a PTS type transient experiment on a large component. The experimental results on CT specimens confirm the beneficial effect of warm pre-stress, with an effective significant increase of the material resistance regarding the risk of brittle failure. The WPS type experiment on the cylinder has been successfully conducted, with a final brittle failure during the reloading. The present paper describes the aims and objectives of the SMILE project, the main experimental results, and the corresponding analyses based on engineering methods, finite element elastic and elastic-plastic computations, and local approach to fracture.

Commentary by Dr. Valentin Fuster
2005;():279-290. doi:10.1115/PVP2005-71246.

The Reactor Pressure Vessel (RPV) is an essential component liable to limit the lifetime duration of PWR plants. The assessment of defects in RPV subjected to PTS transients made at an European level generally not always account the beneficial effect of load history (warm pre-stress WPS) on vessel resistance regarding the risk of brittle failure. A 3-year Research & Development program — SMILE — has been started in January 2002 as part of the 5th Framework Program of the European Atomic Energy Community (EURATOM). The SMILE project — Structural Margin Improvements in aged embrittled RPV with Load history Effect — is one of a cluster of 5th framework projects in the area of Plant Life Management. It aims to give sufficient elements to demonstrate, to model and to validate the beneficial WPS effect in a RPV assessment. Within the framework of the project, an important experimental work has been conducted including WPS type experiments on CT specimens and one PTS type transient experiment on a large component. The WPS type experiment on the cylinder has been successfully conducted by MPA Stuttgart with a final brittle failure during the reloading. The present paper shortly describes the experiment and presents the corresponding analyses based on engineering methods, finite element elastic-plastic computations, and local approach to fracture. The results are in good agreement with the experimental observations. Very significant margins are underlined, with an effective important increase of the material resistance regarding the risk of brittle failure.

Topics: Stress , Cylinders
Commentary by Dr. Valentin Fuster
2005;():291-302. doi:10.1115/PVP2005-71296.

This document presents the energy approach of elastic-plastic fracture mechanics applied to the analysis of a “warm pre-stress” test on a cylinder made of a 17MoV84mod. steel, artificially degraded by special heat treatment to simulate an irradiated end of life RPV material and submitted to a mechanical loading close to a thermal shock transient. This test has been conducted at the MPA of Stuttgart in the context of the European project SMILE. The influence of the hardening representation (isotropic, kinematics, etc.) as well as the choice of the toughness values of the material are analysed. The numerical analysis was performed at EDF-R&D with the Code-Aster, the finite element code of EDF. First of all, we have identified the critical value Gpc of the Gp parameter at the temperature of failure (T = 35°C). Then we have applied the energy approach and compared the results obtained to experimental one’s. Concerning the global values, the agreement between the results of calculation and the experimental results is good if we consider the values of temperature or COD as well as the values of KJ . Concerning the local values, we have observed a rapid decreasing of the opening stress at the notch tip and a significant influence of the hardening on the Von Mises stress. The curves giving the fracture probability as function of KJ are quite relevant to estimate the predictive capacity of the approach used, as well as the importance of the choice of hardening. We have pointed out a 15% variation of KJ depending on this choice. Using tests on CT specimens at room temperature in order to identify the critical value Gpc of the Gp parameter, the value of 77.7 MPa.m1/2 is obtained for KJ if we consider the kinematics case, at 95% of fracture probability. This value is the highest and the nearest to the experimental value equal to 90 MPa.m1/2 . It is not a proof that the hardening is completely kinematics but it leads to the question of a more complete identification of the material to know what is the isotropic part and the kinematics part in the hardening. Besides, because the difference between the result obtained with the energy approach and the experimental result is equal to 14% we can conclude to a satisfactory predictive capacity of this approach. The “warm pre-stress” effect, associated with a significant increase of the cleavage fracture toughness at low temperature after a pre-loading at a higher temperature, is clearly predicted by using the energy approach. The comparison between the results obtained with the different approaches used (energy approach, Beremin model, etc.) will be made in another paper. This work is performed in the context of the SMILE European Project.

Topics: Cylinders
Commentary by Dr. Valentin Fuster
2005;():303-310. doi:10.1115/PVP2005-71298.

Validation of models for short crack behavior requires accurate measurement of crack opening displacement and crack tip strain fields. Development of reliable measurement procedures, using new techniques such as Image Correlation (IC), requires specimens containing cracks with a well defined geometry. In this paper, results of an experimental study concerning controlled initiation of short fatigue cracks at positive R-ratio in laboratory specimens made from 316L stainless steel are presented. Experimental techniques, including hardness testing and X-ray diffraction were employed in order to investigate the effect of surface preparation on the surface mechanical properties and residual stresses. Crack nucleation is difficult in smooth specimens of 316L austenitic stainless steel at positive R-ratio due to the high fatigue limit and low tensile strength. Specimens with a thin ligament were therefore developed to enable nucleation of a single short fatigue crack. An experimental study of the crack growth aspect ratio evolution was then carried out using a beach marking technique. The technique described in this paper enables single short fatigue cracks of well defined geometry to be nucleated under tensile cyclic loading. Stress corrosion cracks can be developed using the same specimen geometry. Miniature tensile specimens can then be extracted to perform in-situ measurements of the crack opening displacement and crack tip strain field by Image Correlation from Scanning Electron Microscopy observations.

Topics: Steel , Fatigue cracks
Commentary by Dr. Valentin Fuster
2005;():311-320. doi:10.1115/PVP2005-71364.

The Reactor Pressure Vessel (RPV) is an essential component, which is liable to limit the lifetime duration of PWR plants. The assessment of defects in RPV subjected to pressurized thermal shock (PTS) transients made at an European level generally does not necessarily consider the beneficial effect of the load history (Warm Pre-stress, WPS). The SMILE project — Structural Margin Improvements in aged embrittled RPV with Load history Effects — aims to give sufficient elements to demonstrate, to model and to validate the beneficial WPS effect. It also aims to harmonize the different approaches in the national codes and standards regarding the inclusion of the WPS effect in a RPV structural integrity assessment. The project includes significant experimental work on WPS type experiments with C(T) specimens and a PTS type transient experiment on a large component. This paper deals with the results of the PTS type transient experiment on a component-like specimen subjected to WPS- loading, the so called Validation Test, carried out within the framework of work package WP4. The test specimen consists of a cylindrical thick walled specimen with a thickness of 40mm and an outer diameter of 160mm, provided with an internal fully circumferential crack with a depth of about 15mm. The specified load path type is Load-Cool-Unload-Fracture (LCUF). No crack initiation occurred during cooling (thermal shock loading) although the loading path crossed the fracture toughness curve in the transition region. The benefit of the WPS-effect by final reloading up to fracture in the lower shelf region, was shown clearly. The corresponding fracture load during reloading in the lower shelf region was significantly higher than the crack initiation values of the original material in the lower shelf region. The post test fractographic evaluation showed that the fracture mode was predominantly cleavage fracture also with some secondary cracks emanating from major crack.

Topics: Stress
Commentary by Dr. Valentin Fuster
2005;():321-329. doi:10.1115/PVP2005-71558.

The EURATOM 6th Framework Integrated Project PERFECT (Prediction of Irradiation Damage Effects in Reactor Components) addresses irradiation damage in RPV materials and components by multi-scale modeling. This state-of-the-art approach offers many potential advantages over the conventional empirical methods used in current practice of nuclear plant lifetime management. Launched in January 2004, this 48-month project is focusing on two main components of nuclear power plants which are subject to irradiation damage: the ferritic steel reactor pressure vessel, and the austenitic steel internals. It is the purpose of the present paper to provide an overview of work being carried out in the RPV Mechanics Sub-project of PERFECT to predict the fracture behavior of PWR, BWR and WWER systems.

Commentary by Dr. Valentin Fuster
2005;():331-337. doi:10.1115/PVP2005-71606.

The effect of residual stresses on fracture behaviour of ferritic steels at low temperature has been investigated using a local approach based on the Beremin model [1]. The study aims to enhance the use of local approach in failure prediction when residual stress is present. A tensile residual stress field has been introduced in the laboratory specimens and their subsequent behaviour was investigated at low temperature. Local compression methods, including side-punching and in-plane loading, were employed to introduce residual stress fields. These methods are discussed and comprehensive range of experimental presented. The transferability of the Weibull parameters between the cracked specimens with different constraint, test temperatures and also from unstrained specimens to specimens with residual stresses are illustrated. The general scheme in failure prediction using the local approach is that the Weibull parameters in the Beremin type model calibrated to the as-received data should predict the failure following complex interaction of residual and applied stresses. The paper compares the predictions and the experimental results.

Topics: Steel , Stress
Commentary by Dr. Valentin Fuster
2005;():339. doi:10.1115/PVP2005-71616.
FREE TO VIEW

The in-service hardening and increase of the Ductile Brittle Transition Temperature (DBTT) of Reactor Pressure Vessel (RPV) steels are controlled at the atomic scale by the clustering of point defects with co-segregation of alloying elements like Cu, Mn, Ni, Si and P. The effort and strategy within the European Project PERFECT for developing advanced multi-scale numerical tools to predict this microstructure and the induced hardening as well as their experimental validation at the relevant scale will be presented. The first results obtained since January 2004, starting date of PERFECT, will be discussed and perspectives given.

Commentary by Dr. Valentin Fuster
2005;():341-346. doi:10.1115/PVP2005-71710.

Nuclear reactor pressure vessel steels are subjected to an irradiation-induced embrittlement in service and this may lead to a shift of the ductile-to-brittle transition temperature. The prediction of irradiation effect on toughness requires an accurate description of the elasto-visco-plastic behavior of irradiated steels. Recent progresses have been done to describe microstructural evolutions induced by irradiation. Ab-initio computations, molecular dynamics and discrete dislocations dynamics can predict the defects formation and the hardening induced by the dislocations – defects interactions. At this level, the irradiation effect is essentially reported as an increase of the critical resolved shear stress on the crystallographic slip systems. A numerical homogenization method is proposed to predict stress-strain curves of irradiated steels from the computed critical resolved shear stress evolution. Computations of realistic 3D aggregates and classical homogenization are performed with a Finite Element code [1]. Each grain is described as a single crystal with a crystal plasticity law, which naturally introduces the irradiation effect on the slip systems activity. The resulting average response over the whole aggregate corresponds to classical stress-strain curves. A Beremin type local approach is then applied to compute the fracture toughness of irradiated CT specimens. Assuming that the local approach parameters do not depend on the irradiation level, this methodology is able to take benefits of MD and DDD results to predict the irradiation effect on RPV steels toughness.

Commentary by Dr. Valentin Fuster
2005;():347-353. doi:10.1115/PVP2005-71768.

The repair or replacement of nuclear power components outwith code is a very cumbersome, costly and time consuming exercise which is responsible for long plant shut down time and corresponding loss of electricity production. The aim of the European Union sponsored project ENPOWER was to produce advanced repair techniques and procedures for application in such circumstances to improve the integrity and life of the component and eliminate the need for expensive and sometimes difficult to execute global Post Weld Heat Treatment (PWHT). This paper describes the development and validation of an Alternative Post Weld Treatment (APWT) for a repair weld in an austenitic header component consisting of a 217mm outside diameter by 202mm thick tube plate welded to a 64mm thick cylindrical header. The development of the method using finite element (FE) analysis and the predicted residual stresses before and after application of the APWT are given. Application of the method to a full size mock up of the header is described. Comparison is made between the FE predictions and Deep Hole Drilling through thickness residual stress measurements. This leads to the conclusion that the APWT method is a suitable alternative to a standard PWHT for the control of residual stress in repair welds.

Topics: Maintenance
Commentary by Dr. Valentin Fuster
2005;():355-360. doi:10.1115/PVP2005-71771.

ENPOWER is a EURATOM collaborative research project sponsored by the European Union. The aim is to produce advanced weld repair techniques and residual stress mitigation procedures for nuclear components in order to eliminate the need for expensive global Post-Weld Heat Treatment (PWHT). These procedures are based on weld repair optimisation combined with novel Alternative Post Weld Treatments (APWT). Repair procedures have been developed by carrying out parametric numerical studies for various repair and component configurations. APWTs are based on local heating at moderate temperatures (300–600°C) to produce local plasticity that redistributes the internal stresses and results in compression in the areas of interest. Two techniques have been identified as prime candidates: namely “hot compression” and “thermal shock” techniques. In combination with internal pressure the hot compression technique reveals itself to be very efficient and of most general use. The thermal shock method is effective for repairs in sections up to 25mm thick. The APWTs were established using numerical models applied to mock-up nuclear components, and validated by measurements. Numerical modelling methods have also been used to study the interactions between residual stresses, post weld treatments, operational loads, crack growth and fracture. Integrity assessment procedures using a modified J-integral definition are applied to confirm that optimised repair and APWT procedures can be developed to reduce welding residual stresses without any secondary detrimental effect, even with the presence of pre-existing cracks.

Commentary by Dr. Valentin Fuster
2005;():361-369. doi:10.1115/PVP2005-71772.

One of the tasks of the European Commission sponsored project ENPOWER was to manufacture repair welds on clad plate specimens simulating the inner wall of a Reactor Pressure Vessel (RPV) and to establish their structural integrity. The paper summarizes the main results from the repair welds carried out on clad plates with an anticipated sub-clad defect including the results from various residual stress measurements and from numerical welding simulations as well as from some fracture mechanical calculations. The results are discussed with respect to support the repair weld optimization in particular by minimizing the residual stresses. Moreover, the application ranges and capabilities of numerical simulations for this kind of weld processes are discussed.

Commentary by Dr. Valentin Fuster
2005;():371-383. doi:10.1115/PVP2005-71776.

The residual stress field around a single-pass weld filling a slit in a thin rectangular plate has been simulated using both 2D ABAQUS and 3D SYSWELD finite element models, with good agreement between the two codes. Through-wall cracks of varying lengths have been inserted into the plate along the weld centre-line, and the non-linear crack driving force due to residual stress evaluated using three formulations of the J-integral: the standard ABAQUS J, the G-theta approach coded into SYSWELD, and a modified J-integral, Jmod , that retains its path independence under non-proportional loading. Cracks were introduced into the FE meshes either simultaneously (all crack flank nodes released in the same step) or progressively (crack opened in small increments from mid-length to tip). The results were compared with crack driving force estimates made using linear elastic fracture mechanics (LEFM) and the R6 procedure. The crack driving forces predicted by all three J–formulations agree well for simultaneous opening, showing that the crack driving force rises to a peak for a crack length equal to the weld length, and falls for longer cracks. Linear elastic fracture mechanics gives a good estimate of the crack driving force for very short defects (confirming the absence of elastic follow up), but is conservative for longer defects, overestimating the peak driving force by 20%. The R6 estimates, which incorporate plasticity corrections, are more conservative than LEFM, overestimating the peak crack driving force by up to 60%. The crack driving force for a progressively opened crack is much lower than for simultaneous opening, indicating that there may be considerable excess pessimism in conventional assessments of defects of this type.

Commentary by Dr. Valentin Fuster
2005;():385-393. doi:10.1115/PVP2005-71786.

The European collaborative research project ENPOWER (Management of Nuclear Plant Operation by Optimizing Weld Repairs) has as one of its main objectives the development of guidelines for the application of repair welds to safety critical components in nuclear power plants. In this context letter box repair welds applied to thin ferritic steel plates to simulate repair of postulated shallow cracks have been manufactured for the purpose of experimental and numerical analysis of welding residual stresses. Two specimens have been procured, one of them prepared in accordance with a standard welding procedure, while in the second case a different procedure was followed in order to obtain extended martensite formation in the heat affected zone. Residual stresses have been determined in both specimens by neutron diffraction at the High Flux Reactor of the Joint Research Centre in Petten, The Netherlands. In parallel Institut de Soudure in France has performed a full 3-d analysis of the residual stress field for the standard welding case taking into account the materials and phase transformations. The experimental data obtained for both specimens clearly suggest that the non-conventional welding procedure rendered higher maximum stress values. In the case of the standard welding procedure numerical and experimental data show a reasonable qualitative agreement. The maximum stress value was in both cases found in the same region of the material — in the base metal just underneath the weld pool — and in both cases found to be of similar magnitude (∼800 MPa found in neutron diffraction and ∼700 MPa found in numerical analysis). In this paper the experimental and numerical approaches are outlined and the obtained results are presented. In addition an outlook is given to future work to be performed on this part of the ENPOWER project. A main issue pending is the application of an optimized advanced post weld heat treatment in one of the two cases and the subsequent numerical and experimental determination of its impact on the residual stress field. At the same time further evaluation of the materials transformations due to welding is pursued.

Commentary by Dr. Valentin Fuster
2005;():397-402. doi:10.1115/PVP2005-71040.

High temperature components in thermal power plants are subjected to creep-fatigue loading where creep cavities initiate and grow on grain boundaries. Development of a quantitative evaluation method of cavity growth is important for reliable maintenance of these components. In this study, a creep-fatigue test was carried out at 600°C on 2.25Cr-1Mo steel in a scanning electron microscope, and continuous observation of cavity growth behavior on the surface during the test was made. Based on the cavity growth observation, existing cavity growth models were modified and a simulation result by the modified model was discussed by comparing with observed cavity growth behavior. From the observation, spherical shape cavities initiate and grow up to their length of 2μm on the grain boundaries at initial stage of damage, and then these cavities change their shape to crack-like to grow until their length reaches around 10μm. Finally, crack-like cavities coalesce each other to form one micro crack along a grain boundary. It can be concluded that cavity growth rates of these cavities are controlled by diffusion and power law creep under constrained condition based on theoretical consideration of cavity growth mechanism. Through these discussions, a new cavity growth model was proposed by modifying conventional models. Both spherical and crack-like cavity growth rate equations were derived from the modified cavity growth model. It was indicated that measured cavity growth rate was well predicted by the growth rate equations derived from the modified model, and a cavity growth simulation result corresponds to the change in the maximum cavity size with cycles under the creep-fatigue loading.

Commentary by Dr. Valentin Fuster
2005;():403-410. doi:10.1115/PVP2005-71050.

An engineering study of the main steam line pressure tap cracking was performed. The failure mechanism was thermal fatigue and the circumferential cracks initiate at the pipe penetration on the inside surface of the pipe. Based on the results, a parametric fitness for service study was performed to develop a flaw tolerance graph to be used for the evaluation and disposition of detected defects. The parameters are pipe diameter, penetration diameter, and inspection interval. The objective is to determine the value of the current crack size when weld repair is needed. The fatigue crack growth calculations used the NASGRO computer program. The crack geometries were a single corner crack from a hole in a plate and a corner crack from a hole in a plate with two cracks. The analysis used the properties for Grade 22 plate material given in the program and applied a correction factor (69 cycles for Grade 22 at room temperature equals 1 fatigue cycle with hold time of 12 hours for P11) to account for the pipe material of P11, 1-1/4Cr-1/2Mo and the nominal operating temperature of 482°C. There were approximately 40 cold starts and 210 hot starts per year for the unit. The magnitude of the remote stress depended on the temperature range for the hot and cold starts. The applied stress consisted of tension and bending stresses and was estimated by the observed failure life of approximately 20 years for the 3/4 penetration in the pipe size 18 pipe.

Commentary by Dr. Valentin Fuster
2005;():411-418. doi:10.1115/PVP2005-71674.

Recent literature indicates that there is a concern regarding the short-term vs. long-term creep rupture base metal properties for Grade 91 material. Evaluations of recent creep rupture data suggest that the material properties degrade more severely than expected and extrapolated creep rupture properties may be very optimistic. One of the approaches to evaluate creep rupture data is with a parameterized master curve such as the Larson-Miller parameter. Evaluations of creep rupture data indicate that the effects of material degradation can be considered with appropriate stress, time and temperature relationships. Using the Larson-Miller parameter methodology, the selected heats of Grade 91 creep rupture data indicate a reasonable relationship that does not appear to degrade rapidly for the longer term data. If even longer term creep rupture data suggest severe aging degradation as compared to current extrapolations, a transition of the Larson-Miller parameter constant from 31 to 20 does not appear to be a good method to calculate the degraded life estimates. As longer term creep rupture data become available, resulting oxide thicknesses should be measured and reported. The adverse effect of oxidation at longer times, resulting in loss of material and effectively higher stress, should be evaluated.

Topics: Creep , Rupture
Commentary by Dr. Valentin Fuster
2005;():421-434. doi:10.1115/PVP2005-71505.

This paper presents a new continuum shape sensitivity method for calculating the mixed-mode stress-intensity factors of a stationary crack in two-dimensional, linear-elastic, orthotropic functionally graded materials with arbitrary geometry. The method involves the material derivative concept taken from continuum mechanics, the mutual potential energy release rate, and direct differentiation. Since the governing variational equation is differentiated prior to discretization, resulting sensitivity equations are independent of approximate numerical techniques, such as the finite element method, boundary element method, mesh-free method, or others. The discrete form of the mutual potential energy release rate is simple and easy to calculate, as it only requires multiplication of displacement vectors and stiffness sensitivity matrices. By judiciously selecting the velocity field, the method only requires displacement response in a subdomain close to the, crack tip, thus making the method computationally efficient. A finite-element based mixed-mode numerical example is presented to evaluate the accuracy of the fracture parameters calculated by the proposed method. Comparisons have been made between stress-intensity factors predicted by the proposed method and available reference solutions in the literature, generated either analytically or numerically using various other fracture integrals or analyses. Excellent agreement is obtained between the results of the proposed method and previously obtained solutions. Therefore, shape sensitivity analysis provides an attractive alternative to fracture analysis of cracks in homogeneous and non-homogeneous orthotropic materials.

Commentary by Dr. Valentin Fuster
2005;():435-444. doi:10.1115/PVP2005-71518.

This paper presents a new probabilistic method for reliability analysis of cracks in linear-elastic, isotropic, functionally graded media subject to random loads, material and gradation properties, and crack geometry. The method involves an interaction integral for calculating crack-driving forces and a novel function decomposition that facilitates lower-variate approximations of a general multivariate function. The fracture reliability analysis is based on response-surface models of univariate and bivariate approximations and subsequent Monte Carlo simulation. Two numerical examples illustrating both deterministic and stochastic aspects of fracture are presented. The numerical results indicate that the univariate and bivariate response-surface methods provide both accurate and efficient estimates of failure probability of cracks in functionally graded materials.

Commentary by Dr. Valentin Fuster
2005;():445-449. doi:10.1115/PVP2005-71564.

A theoretical and computational methodology for the analysis of the functionally graded material (FGM) is introduced, and its application is made to the problem of a dynamically propagating crack running transversely in the FGM, where the intensity of the estimated crack-tip severity is managed to keep in valance with the graded material toughness in the FGM during the propagation. To detect the crack-tip severity, an integral fracture parameter, T*, is used. The crack is propagated so that the value of T* is equated to the prescribed varying critical values of T* for the graded material. Emphasis is placed on the use of a fuzzy inference technique in order to control the crack speed, which is deduced from a few T* values immediately preceding the current crack position. As to describing the constitutive law for the FGM, micro-spherical particles of arbitrary size in mesoscale are considered to be randomly dispersed in the matrix medium. By assuming that the volume fraction of the inclusion is continuously varied from 0 to 100 percent in the material, the grading is modeled. For modeling the constitutive law for the FGM composite media of thermo-elastoplasticity, a closed form SCC-LRM constitutive model describing the nonlinear material mechanics of the particle-dispersed medium is used. The model is based on the self-consistent scheme and uses Eshelby’s equivalent inclusion method. Unprecedented analytical results of predicting the crack speed of a crack running transversely in the FGM plate are obtained. In some cases of material grading, apparent crack arresting is observed as the crack runs into the metal rich area of the FGM.

Commentary by Dr. Valentin Fuster
2005;():453-461. doi:10.1115/PVP2005-71419.

This paper describes part of an ongoing study to develop a simple tensile test which will maximize the effects of hydrostatic constraint. The test for such purposes is the notched bar test. Two notch geometries are in common use, the ASTM Standard notch, and the Bridgman blunt notch. Both of these tests have shortcomings, which are described in the paper. Alternative geometries, including notches, plane strain holes and slots have been evaluated, using the ratio of hydrostatic stress to Mises stress in a bar made of an elastic, perfectly plastic material. Examples are given of the stress evolution in selected geometries under creep according to a simple Bailey/Norton power law model, and comparison is made with the behavior when a more complex material constitutive law is used, which includes continuum creep damage. The model used in this case is a simplified version of the MPC Omega model, described in API 579 [1]. Since creep calculations involving damage are both computationally intensive and difficult to carry to completion due to numerical convergence problems, approximate methods of predicting specimen behavior under such complex material conditions is being explored. One promising method, based on isochronous stress/strain curves is described and the results compared with detailed predictions using a more accurate constitutive model.

Topics: Creep , Stress , Design
Commentary by Dr. Valentin Fuster
2005;():463-467. doi:10.1115/PVP2005-71780.

The development of a web-accessible materials handbook in support of the materials selection and structural design for the Generation IV nuclear reactors is being planned. Background of the reactor program is briefly introduced. Evolution of materials handbooks for nuclear reactors over years is reviewed in light of the trends brought forth by the rapid advancement in information technologies. The framework, major features, contents, and construction considerations of the web-accessible Gen IV Materials Handbook are discussed. Potential further developments and applications of the handbook are also elucidated.

Commentary by Dr. Valentin Fuster
2005;():469-476. doi:10.1115/PVP2005-71784.

Alloy 617 is being considered for the construction of components to operate in the Next Generation Nuclear Plant (NGNP). Service temperatures will range from 650 to 1000°C. To meet the needs of the conceptual designers of this plant, a materials handbook is being developed that will provide information on alloy 617, as well as other materials of interest. The database for alloy 617 to be incorporated into the handbook was produced in the 1970s and 1980s, while creep and damage models were developed from the database for use in the design of high-temperature gas-cooled reactors. In the work reported here, the US database and creep models are briefly reviewed. The work reported represents progress toward a useful model of the behavior of this material in the temperature range of 650 to 1000°C.

Topics: Creep , Alloys
Commentary by Dr. Valentin Fuster
2005;():477. doi:10.1115/PVP2005-71797.
FREE TO VIEW

Development for a web-accessible, interactive materials handbook has been planned for materials selection, design, and construction of the Generation IV Nuclear Reactors. A panel discussion will be held among potential users and the handbook development planners on functionality requirements for the handbook.

Commentary by Dr. Valentin Fuster
2005;():481. doi:10.1115/PVP2005-71155.
FREE TO VIEW

The paper will address commonly used materials in Hydrogen service and their limitations as the service conditions (like pressure) get into more challenging areas. For example, Aluminum has good history of use in hydrogen applications upto 3000 psig. It’s compatible with Hydrogen even at pressures higher than 15,000 psig. However, are we comfortable in continued use of Aluminum and similar low melting point materials at higher pressures for containing Hydrogen? The paper is expected to provide basis for a discussion on this safety issue. In addition, the paper will include a discussion around pressure vs stress level limits for common materials. A user friendly material selection chart will be presented for the benefit of the audience.

Topics: Hydrogen
Commentary by Dr. Valentin Fuster
2005;():483-491. doi:10.1115/PVP2005-71392.

Applications requiring the containment and transport of hydrogen gas at pressures greater than 70 MPa are anticipated in the evolving hydrogen economy infrastructure. Since hydrogen is known to alter the mechanical properties of materials, data are needed to guide the selection of materials for structural components. The objective of this study is to characterize hydrogen-assisted fracture in two austenitic steels, 21Cr-6Ni-9Mn (21-6-9) and 22Cr-13Ni-5Mn (22-13-5), as well as explore the role of yield strength and small concentrations of ferrite on hydrogen-assisted fracture. The testing methodology involves exposure of uniaxial tensile specimens to high-pressure hydrogen gas in order to precharge the specimens with hydrogen, then subsequently testing the specimens to measure strength and ductility. In all cases, the alloys remained ductile despite precharging to hydrogen concentrations >1 at%, this is substantiated by reduction in area of >50% and fracture surfaces dominated by microvoid coalescence. Low concentrations of ferrite and moderate changes in yield strength did not affect the hydrogen-assisted fracture of 21-6-9 and 22-13-5 respectively.

Commentary by Dr. Valentin Fuster
2005;():493-497. doi:10.1115/PVP2005-71628.

Hydrogen embrittlement (HE) of metals used in the system of fuel-cell vehicles, i.e., high-pressure hydrogen storage tanks and vessels, compressors, valves and pipes, is investigated in 70 MPa hydrogen at room temperature. The materials tested are austenitic stainless steels (i.e., SUS304; in the Japanese Industrial Standard (JIS), SUS316, SUS316L, and SUS316LN), a low-alloy steel (i.e., SCM440), carbon steels (i.e., SUY, S15C, S35C, S55C and S80C), a Ni-based superalloy (i.e., Inconel 718), and an aluminum alloy (i.e., A6061). Tensile tests were conducted at room temperature using a specially designed equipment developed by our laboratory, which was designed to measure the actual load on the specimen with an external load cell irrespective of the axial load caused by the high pressure and friction at sliding seals. SUS304 and SUS316 showed severe HE, while SUS316L and SUS316LN showed slight HE. Fracture occurred on strain-induced martensite of the austenitic stainless steels in hydrogen. SCM440 showed extreme HE depending on heat-treatment; in particular, quenched materials showed marked HE. The carbon steels showed extreme and severe HE depending on carbon content. Inconel 718 also showed severe HE, while A6061 showed negligible HE. These results and other HE testing results which AIST has done previously are summarized in the AIST HE data table. HE behavior of the material in high-pressure hydrogen is discussed in this paper.

Commentary by Dr. Valentin Fuster
2005;():499-503. doi:10.1115/PVP2005-71735.

To evaluate mechanical properties of the materials used for hydrogen systems such as fuel cell vehicles and hydrogen fuel stations, mechanical testing facilities in gaseous hydrogen at up to 45MPa pressure were newly designed and installed, and fatigue properties, which is one of the key properties for the onboard fuel tanks in the fuel cell vehicles, were actually evaluated for two kinds of liner materials of the on-board CFRP fuel tanks; AA6061-T6 aluminum alloy and 316L type of austenitic stainless steel. Axial S-N fatigue tests (R = −1) were conducted in air and also in gaseous hydrogen at 45MPa pressure at room temperature, and quite similar S-N curves were obtained in both circumstances within the maximum number of cycles to failure of 105 for AA6061-T6. 316L also exhibited excellent fatigue life and was not fractured with maximum applied stress of 90% of 0.2% proof stress at 105 cysles. Clear difference was not observed in fatigue crack growth rate in each material regardless of the circumstances investigated in this study including hydrogen gas at 45MPa pressure. Those results indicate that fatigue properties are not affected by gaseous hydrogen at around room temperature in both AA6061-T6 aluminum alloy and 316L type of stainless steel, and that both materials can fully be employed to the liner of the 35MPa on-board hydrogen fuel tanks from the viewpoints of fatigue properties.

Commentary by Dr. Valentin Fuster
2005;():507-513. doi:10.1115/PVP2005-71156.

The fabrication of near net shape powder metal (PM) components by hot isostatic pressing (HIP) has been an important manufacturing technology for steel and stainless steel alloys since about 1985. The manufacturing process involves inert gas atomization of powder, 3D CAD capsule design, sheet metal capsule fabrication and densification by HIP in very large pressure vessels. Since 1985, several thousand tonnes of parts have been produced. The major applications are found in the oil and gas industry especially in offshore applications, the industrial power generation industry, the pulp and paper industry and in pharmaceuticals and traditional engineering industries. Typically, the components replace castings, forgings and fabricated parts and are produced in grades such as martensitic steels, austenitic and duplex (ferritic/austenitic) stainless steels and nickel- based superalloys. The application of HIP PM near net shapes to manifolds for medium to high pressure use has a number of advantages compared to the traditional forging and welding approach. First, the need for machining of the components is reduced to a minimum and welding during final assembly is reduced substantially. Manifolds by HIP design reduce the necessary welding by 70–90%. Mechanical properties of the HIP PM part are isotropic and equal to the best forged properties in the flow direction as is demonstrated below. This derives from the fine uniform microstructure of the PM parts. The PM parts are significantly lighter in weight because of the need to stiffen the forged component at the location of the weldment for the intersecting passageway — the PM parts can be smoothly blended into the intersection without need for welding. Furthermore, the PM HIP components can be made with significantly reduced manufacturing lead-time, greater design flexibility and improved cost for the final component. The PM HIP near net shape route has received approval from both ASTM [1,2,3] and NACE [4] for specific steel, stainless steel and nickel base alloys. This paper reviews the manufacturing sequence for PM near net shapes and discusses the details of several successful applications. The application of the HIP PM process to subsea manifolds is highlighted.

Commentary by Dr. Valentin Fuster
2005;():515-525. doi:10.1115/PVP2005-71258.

More than twenty years have passed since the development and commercialization of water-cooling-type TMCP (Thermo-Mechanical Controlled Processing) steel. At present, the technology is used in various areas as a reliable process. Features of TMCP steel are excellent weldability and toughness, which make it extremely useful as a tank material. This paper aims to contribute to the safety and workability improvement in the tank construction field as in other areas by the effective use of TMCP steel. The paper first describes the characteristics of material properties and workability of TMCP steel and the metallurgy to realize these characteristics. It then introduces a several examples of recent development in the application of TMCP technology. The use of TMCP steel realizes better toughness in high-heat-input welding, improved low-temperature toughness and sour gas resistance, in addition to reduced weld pre-heating temperature thanks to its low carbon content. Recently, because of its excellent characteristics, it has become possible to apply TMCP steel to nuclear energy areas, medium- to high-temperature regions, and ultra-high strength types such as 780 MPa and 950 MPa. Examples of the latest commercial application include surface ultra fine grain steel whose excellent brittle crack arrestability is comparable to that of high-Ni steel for cryogenic use, and ultra-thick structural steel plate that allows super-high welding heat input over 100kJ/mm.

Commentary by Dr. Valentin Fuster
2005;():527-533. doi:10.1115/PVP2005-71412.

In an effort to maximize quality and production in both field and shop fabrication while minimizing cost, Fluor and PCL, in a joint venture, assessed new welding technology comparing power sources, arc transfer modes, and shielding gas mixtures using various semi-automatic welding processes. The results were combined with “No-Backing Gas” (NBG) root technique to provide ASME code acceptable welds with excellent corrosion and crack resistance for type 321 Stainless Steel [1]. Welds were deposited in approximately one third the time commonly demanded by conventional welding. The strategies for the selection of a welding power source, arc transfer modes, shielding gas mixtures, and the successful bridging from quality and integrity validation to shop and field implementation are discussed.

Commentary by Dr. Valentin Fuster
2005;():535-541. doi:10.1115/PVP2005-71711.

Hot Isostatic Pressing (HIP) has been used for many years to consolidate porosity in cast metal shapes to improve mechanical properties. When applied to fine metal powders, it becomes possible to produce Near Net Shape (NNS) items and more complex geometry components that are fully dense and offer an attractive set of properties at reduced cost. NNS items produced from powder deliver cost savings by reducing initial material usage and subsequent machining costs. Powder production and HIP processing are automated methods, which also provide protection against forging route obsolescence. Setup costs are lower and batch sizes smaller. HIPped powder microstructures are isotropic and equi-axed, with uniformly fine grain sizes not normally achieved in heavy section components, which makes ultrasonic NDE examination much easier. Inclusion contents are lower and of more benign geometry, which assists fracture assessment. Use of the technology has grown, particularly in the offshore oil industry where it is already established in high integrity applications, particularly in place of welded joints. Take-up in the more conservative nuclear industry has been slow. The quality of HIPped powder items can provide through life cost savings since there is greater assurance of structural integrity compared to welded or wrought components. In a broad program of testing, Rolls-Royce has established that HIPped powder 316L components, in items up to several tons in weight, have equivalent or slightly better strength, toughness and corrosion resistance than the forged counterpart. The Safety Case for a thin-walled pressure retaining component has been accepted and implemented.

Commentary by Dr. Valentin Fuster
2005;():545-550. doi:10.1115/PVP2005-71199.

Much work has been done to assess constraint effects on the crack-driving force for specimens and cracks in pipes. The material’s transition temperature where the fracture process changes from ductile tearing to cleavage fracture at crack initiation is affected by the constraint conditions, but is a material property that cannot be determined analytically. This paper presents a methodology to account for constraint effects to predict the lowest temperature for ductile fracture initiation and relates that temperature to Charpy impact data for typical ferritic pipe materials. It involves a series of transition temperature shifts to account for thickness, strain-rate, and constraint to give a master curve of transition temperatures from Charpy data to through-wall-cracked or surface-cracked pipes (with various a/t values) under quasi-static loading. These transition temperature shifts were based on hundreds of pipe tests and thousands of specimen tests over several decades of work by numerous investigators. It is equally applicable to ferritic nuclear pipe for Class 2, 3, or balance of plant piping, or for older linepipe materials. If found to be reasonable, then the procedure could be used in the ASME pipe flaw evaluation procedures as a screening criterion between LEFM and EPFM failure modes.

Commentary by Dr. Valentin Fuster
2005;():551-561. doi:10.1115/PVP2005-71200.

This paper assesses the effect of using primary water stress corrosion cracking (PWSCC) crack morphology parameters (roughness, number of turns, and actual flow path/pipe thickness) in determining the difference in the leakage crack length, and how the difference in the leaking crack lengths changes typical margins from past LBB submittals and published reports. Several past LBB submittal cases were selected; in addition, cases from generic LBB reports published by EPRI were also selected. The results of the analyses showed that the past submittals by nuclear steam system supply (NSSS) companies frequently used the surface roughness comparable to an air-fatigue crack with no turns and the actual flow path equal to the thickness of the pipe. This condition would give the shortest possible leakage flaw length. The roughness, number of turns, and actual flow path to thickness ratio for PWSCC cracks were determined from photomicrographs of service-removed cracks. When using the PWSCC crack morphology parameters that corresponded to the crack growing parallel to the long direction of the dendritic grains (V.C. Summer and Ringhals cases), then the leakage flaw length increased 69 percent over the air-fatigue crack length at the same leak rate. Using the same critical crack length as was used in the initial LBB submittals and the published documents, the margins on the crack length changed from 1.77 to 6.0 for the initial submittals (which we also reproduced) to 0.88 to 2.74 from our calculations for a PWSCC crack. If the crack grew in the buttered region of the bimetallic weld, then based on metallographic sections from service-removed flaws, there would be a more tortuous flow path. For this crack condition, in all but one case, the margins on the normal operating versus N+SSE crack lengths were below the safety factor of two required for LBB approval. The average margin decreased from 3.39 for the air-fatigue crack to 1.55 for the PWSCC crack growing transverse to the long direction of the dendritic grains. This was about an additional 20 percent decrease in the margin from the case of having the PWSCC grow parallel to the long direction of the dendritic grains. These results show that LBB is difficult to satisfy for PWSCC susceptible pipe using the current SRP 3.6.3 LBB approach. This LBB assessment did not consider the possible development of a long circumferential surface crack, which would be more detrimental to LBB behavior. Such cracking behavior would violate the LBB screening criterion.

Commentary by Dr. Valentin Fuster
2005;():563-577. doi:10.1115/PVP2005-71211.

Integrity assessment of piping components is very essential for safe and reliable operation of power plants. Over the last several decades, considerable work has been done throughout the world to develop a methodology for integrity assessment of pipes and elbows, appropriate for the material involved. However, there is scope of further development/improvement of issues, particularly for pipe bends, that are important for accurate integrity assessment of piping. Considering this aspect, a comprehensive Component Integrity Test Program was initiated in 1998 at Bhabha Atomic Research Centre (BARC), India. In this program, both theoretical and experimental investigations were undertaken to address various issues related to the integrity assessment of pipes and elbows. Under the experimental investigations, fracture mechanics tests have been conducted on pipes and elbows of 200–400 mm nominal bore (NB) diameter with various crack configurations and sizes under different loading conditions. Tests on small tensile and three point bend specimens, machined from the tested pipes, have also been done to evaluate the actual stress-strain and fracture resistance properties of pipe/elbow material. The load-deflection curve and crack initiation loads predicted by non-linear finite element analysis matched well with the experimental results. The theoretical collapse moments of throughwall circumferentially cracked elbows, predicted by the recently developed equations, are found to be closer to the test data compared to the other existing equations. The role of stress triaxialities ahead of crack tip is also shown in the transferability of J-Resistance curve from specimen to the component.

Topics: Pipes
Commentary by Dr. Valentin Fuster
2005;():579-590. doi:10.1115/PVP2005-71212.

Leak-before-break (LBB) assessment of primary heat transport piping of nuclear reactors involves detailed fracture assessment of pipes and elbows with postulated throughwall cracks. Fracture assessment requires the calculation of elastic-plastic J-integral and crack opening displacement (COD) for these piping components. Analytical estimation schemes to evaluate elastic-plastic J-integral and COD simplify the calculations. These types of estimation schemes are available for pipes with various crack configurations subjected to different types of loading. However, no such schemes are available for throughwall circumferentially cracked elbow (or pipe bend), an important component for LBB analysis. In this paper, simple J and COD estimation schemes are proposed for throughwall circumferentially cracked elbow subjected to closing bending moment. The ovalisation of elbow cross section has a significant bearing on its fracture behavior. Therefore, unlike conventional deformation theory plasticity analysis, incremental flow theory is adopted considering both material and geometric non-linearities in the development of the proposed estimation schemes. Although it violates Ilyushin’s theorem, it has been shown that the resulting estimation schemes is still reasonably accurate for engineering purposes. Finally, experimental/numerical validation has been provided by comparing the J-integral and COD between numerical/test data and predictions of the proposed estimation schemes.

Commentary by Dr. Valentin Fuster
2005;():591-599. doi:10.1115/PVP2005-71302.

This work was conducted in order to validate the flaw evaluation criteria for class 2, 3, and balance of plant piping in nuclear power plants. A methodology was developed as part of the Battelle Integrity of Nuclear Piping (BINP) program for predicting fracture initiation transition temperature (FITT) for a surface crack in a pipe from the 85% shear area transition temperature from Charpy specimen data. The predictive relationship is described in a separate paper in this conference [1]. In order to validate this methodology a series of Charpy, dynamic tear test (DTT), compact (tension) (CT), single-edge-notch [tension] (SEN(T)), and full-scale circumferential surface-cracked pipe experiments were conducted at different temperatures. As part of this paper, the results from these tests, along with their potential impact on the validation procedure to predict the quasi-static brittle-to-ductile FITT will be discussed.

Commentary by Dr. Valentin Fuster
2005;():601-616. doi:10.1115/PVP2005-71310.

The current Leak Before Break (LBB) assessment is based primarily on the monotonic fracture tearing instability. In it the maximum design accident load is compared with the fracture-tearing resistance load. The effect of cyclic loading has generally not been considered in the fracture assessment of nuclear power plant piping. It is a well-known fact that the reversible cyclic loading decreases the fracture resistance of the material, which leads to increased crack growth. Indian nuclear power reactors consider Operational-Basis-Earthquake (OBE) and Safe-Shutdown-Earthquake (SSE) event in the design of various structures, systems and components. Keeping this in view a series of cyclic tearing test have been conducted on straight pipes, made of ASTM SA333 Gr.6 carbon steel. This is the material of primary heat transport (PHT) piping material of Indian Pressurised Heavy Water Reactor (PHWR). In this series 13 tests have been carried out on circumferentially through wall cracked seamless and circumferential seam welded straight pipes under reversible cyclic bending loading. All the tests have been conducted under quasi-static i.e. slow loading rates and the dynamic effect is not considered. The cyclic test results have been compared with the corresponding monotonic pipe fracture test results. These test results and its comparison with corresponding monotonic tearing clearly illustrates the need of addressing the reduction in apparent fracture toughness of material under reversible cyclic loading and safe number of load cycles in the LBB assessment.

Topics: Pipes
Commentary by Dr. Valentin Fuster
2005;():617-621. doi:10.1115/PVP2005-71330.

This paper presents experimental results from pipe-system tests from the International Piping Integrity Research Group (IPIRG) and Battelle Integrity of Nuclear Piping (BINP) programs where the magnitudes of the thermal expansion (secondary) stresses were different in two pipe-system experiments with identical circumferential surface cracks in the same material. The pipe systems were loaded with a single-frequency dynamic forcing function to provide inertial and seismic anchor motion (SAM) moments. In the first tests, the hydraulic ram at the dynamic loading point was locked in place during the heating, so that the natural thermal expansion stresses developed. In the second test, the hydraulic ram at the dynamic loading point was statically offset after the heating to simulate a higher thermal expansion stress. The magnitude of the thermal expansion stress for both cases was within the ASME Section III limits. The results showed that the same total moment was reached in both tests, but the magnitude of the inertial moments at failure was reduced in the second test by the increase of the moment from the thermal expansion stresses. The reason for this effect is that the crack was relatively large, so the failure stress due to the crack was below yield of the uncracked pipe in the pipe system. In this case, the plasticity at the surface crack causes small displacements of the pipe compared to the overall elastic displacements of the pipe system; therefore, the yielding at the crack plane did not relieve the secondary stress, causing them to behave as a primary stress. This behavior is consistent with the B31.1 and ASME Section III Class 2 and 3 piping paragraphs on “Local Overstrain”. The implication from this work is that the safety factor on secondary stresses in the ASME Section XI Code pipe flaw evaluation procedures should be a function of the failure stress. Furthermore, secondary stresses should be included for all piping materials (including wrought stainless steels), and have the same safety factor as the primary stresses for stresses below the yield strength of the material.

Commentary by Dr. Valentin Fuster
2005;():623. doi:10.1115/PVP2005-71378.
FREE TO VIEW

As some CANDU plants in Canada are approaching the end of their design lives, various degradation mechanisms which were not anticipated during the design phase have been identified in the CANDU feeder piping and resulted in either actual structural failures or the early replacement of components. In particular, inter-granular stress corrosion cracking (IGSCC) and pipe wall thinning due to flow accelerated corrosion (FAC) are the most prominent degradation mechanisms in the CANDU feeder piping. The Canadian CANDU industry has developed and implemented programs to monitor and manage those unanticipated service-related degradations. Fitness for service guidelines are also developed to justify the structural integrity of the components until the next inspection. These programs include augmented periodic inspections that are targeted to specific components, thereby ensuring the early detection of cracks or excessive wall thinning. The inspection scope and frequency adopted in the degradation management programs exceed the requirements of ASME Section XI, “Inservice Inspection of Nuclear Reactor Coolant Systems” and CSA N285.4, “Periodic Inspection of CANDU Nuclear Power Plants Components”. However, those currently effective codes and standards do not specify requirements which are developed based on the consideration of the specific degradation mechanisms such as IGSCC and FAC wall thinning. Accordingly, it has been an issue for the nuclear regulator as well as in the industry to develop criteria for inspection and replacement/repair, which are based on the current level of understanding of degradation mechanisms and the inspection capability. Presented in this paper are the Canadian regulator’s perspective on the assurance of the safe operation of the CANDU feeder piping which endures degradations of IGSCC and FAC.

Topics: Pipes
Commentary by Dr. Valentin Fuster
2005;():625-629. doi:10.1115/PVP2005-71549.

This paper describes a new set of design formulae for a Full Structural Weld Overlay. The formulae presented herein are derived using similar assumptions as were used to develop the net section collapse formulae contained in Appendix C of the ASME B&PV Code, Section XI; however, they are developed specifically for a WOL geometry. By considering the WOL geometry during derivation of the plastic collapse load, a set of WOL design formulae are obtained which contain the WOL thickness as a solution variable. For WOL design this formulation is inherently easier and more efficient to use than adapting the part through wall flaw formulation contained in Appendix C. Sample WOL designs are presented in which both the new and existing formulae are used. The results obtained demonstrate that the new formulae produce a WOL thickness consistent with that calculated by properly using the existing method contained in Appendix C.

Commentary by Dr. Valentin Fuster
2005;():631-637. doi:10.1115/PVP2005-71581.

Feeder pipes in CANDU® reactors are an integral part of the circulating heat transport system, connecting the in-reactor fuel channels with the primary heat transport pipes. The feeder pipes are 1.5” to 3.5” in nominal size and made out of SA-106 Grade B material. A typical CANDU 6 station has 760 feeder pipes, half of which are located at the fuel channel inlet and the other half are at the fuel channel outlet. In one of the CANDU 6 stations, 2 feeder bends were replaced due to through-wall cracks and 6 others were replaced due to partial cracks detected by non-destructive examination techniques. In all cases, cracks were axial in orientation and were either at the inside surface of the bend flanks (approximately 60° from the intrados symmetry plane) or at the outside surface of the bend extrados. Examination of cracked elbows and measurement of residual stresses indicated that residual stress had a significant role in these failures, since the cracks were associated with locations of high residual stress. Hence, a significant effort was spent to measure the distribution and magnitude of residual stresses at feeder pipe bends. Residual stress in feeder bends is inevitably introduced during the bending process and significantly affected by the manufacturing and heat treatment techniques. Among the techniques used, hot-forming, intrados heating (warm-bending) and compression boosting are investigated. In this paper, the effect of the manufacturing process on the residual stresses of pipe bends is discussed. It was found that, among the bend forming techniques used, the intrados heating (warm-bending) technique results in the highest residual stresses.

Commentary by Dr. Valentin Fuster
2005;():639-644. doi:10.1115/PVP2005-71712.

J-Resistance properties of automatic narrow gap gas tungsten arc welds (NGGTAW) for ferritic primary loop piping were investigated. The piping diameter is 1066 mm and its thickness is 76 mm. To do this, tensile, micro-hardness and J-R tests were performed and the microstructures of the welds were examined. Tensile and J-R tests were conducted at 316°C and 177°C for temperature effect and at 1 mm/min and 1000 mm/min for strain rate effect. The results were compared with the weld cases which are fabricated by the conventional SMAW process. NGGTAW welds investigated in this paper have lower J-R curves than the SMAW welds do. Fracture surfaces of the NGGTAW welds have an unusual appearance with deep ditches and holes due to the inclusions and pores. It is believed that the high density of the inclusion and the pore which exists in the NGGTAW welds is responsible for this low toughness and the cracked surface. Therefore, for an improvement of the NGGTAW weld toughness, it is recommended that sulfur content should be limited to a considerably low level and the pore should be eliminated by modifying the welding procedure specification.

Commentary by Dr. Valentin Fuster
2005;():645-652. doi:10.1115/PVP2005-71769.

The mesh dependency of Rice and Tracey cavity growth factor (CGF) is overcome by integrating the CGF over a process zone surrounding the crack tip. This integral represents a modified damage potential. The critical value of the integral for crack initiation in weld material has been determined analyzing a welded CT specimen and comparing the computed crack initiation J with experimentally measured J-initiation value. This critical value is then employed to predict crack initiation load in 8” and 12” welded pipes having different measure of through-wall cracks at the center to predict the loads under four point bending loads. The computed values are compared with the experimentally measured values. A close agreement between the computed crack initiation loads with the experimentally measured values justifies the usefulness of the present modified damage potential.

Commentary by Dr. Valentin Fuster
2005;():653-659. doi:10.1115/PVP2005-71528.

Cast duplex stainless steels of CF8M and CF8 are used in major components because of their superior characteristics, such as corrosion resistance, weldability, and so on. However, these stainless steels are known to have tendency of thermal aging embrittlement after long term service. Therefore, the mechanical properties have been investigated using tensile test specimens and fracture toughness specimens aged at 300 to 450°C for up to 40,000 hours. From the results, the effects of thermal aging on the mechanical properties of these cast duplex stainless steels were identified. The true stress-true strain curve prediction method (TSS model) and fracture toughness prediction method (H3T model) after long term service were established. These prediction methods are used for the evaluation on the plant life management of nuclear power plants in Japan.

Commentary by Dr. Valentin Fuster
2005;():661-677. doi:10.1115/PVP2005-71633.

Inservice inspection requirements for pressure retaining welds in the regenerative, letdown, and residual heat removal heat exchangers are prescribed in Section XI Articles IWB and IWC of the ASME Boiler and Pressure Vessel Code. Accordingly, volumetric and/or surface examinations are performed on heat exchanger shell, head, nozzle-to-head, and nozzle-to-shell welds. Inspection difficulties associated with the implementation of these Code-required examinations have forced operating nuclear power plants to seek relief from the U.S. Nuclear Regulatory Commission. The nature of these relief requests are generally concerned with metallurgical factors, geometry, accessibility, and radiation burden. Over 60% of licensee requests to the NRC identify significant radiation exposure burden as the principal reason for relief from the ASME Code examinations on regenerative heat exchangers. For the residual heat removal heat exchangers, 90% of the relief requests are associated with geometry and accessibility concerns. Pacific Northwest National Laboratory was funded by the NRC Office of Nuclear Regulatory Research to review current practice with regard to volumetric and/or surface examinations of shell welds of letdown heat exchangers, regenerative heat exchangers, and residual (decay) heat removal heat exchangers. Design, operating, common preventative maintenance practices, and potential degradation mechanisms were reviewed. A detailed survey of domestic and international PWR-specific operating experience was performed to identify pressure boundary failures (or lack of failures) in each heat exchanger type and NSSS design. The service data survey was based on the PIPExp® database and covers PWR plants worldwide for the period 1970–2004. Finally a risk assessment of the current ASME Code inspection requirements for residual heat removal, letdown, and regenerative heat exchangers was performed. The results were then reviewed to discuss the examinations relative to plant safety and occupational radiation exposures.

Commentary by Dr. Valentin Fuster
2005;():681-690. doi:10.1115/PVP2005-71799.

We begin this expository essay by reviewing, with examples from the materials and fabrication testing literature, what a typical engineer already knows about statistics. We then consider a central question in engineering decision making, i.e., given a computer simulation of high-consequence systems, how do we verify and validate (V & V) and what are the margins of errors of all the important predicted results? To answer this question, we assert that we need three basic tools that already exist in statistical and metrological sciences: (A) Error Analysis. (B) Experimental Design. (C) Uncertainty Analysis. Those three tools, to be known as A B C of statistics, were developed through a powerful linkage between the statistical and metrological sciences. By extending the key concepts of this linkage from physical experiments to numerical simulations, we propose a new approach to answering the V & V question posed earlier. The key concepts are: (1) Uncertainty as defined in ISO Guide to the Expression of Uncertainty in Measurement (1993). (2) Design of experiments prior to data collection in a randomized or orthogonal scheme to evaluate interactions among model variables. (3) Standard reference benchmarks for calibration, and inter-laboratory studies for “weighted” consensus mean. To illustrate the need for and to discuss the plausibility of this metrology-based approach to V & V, two example problems are presented: (a) the verification of 12 simulations of the deformation of a cantilever beam, and (b) the calculation of a mean time to failure for a uniformly loaded 100-column single-floor steel grillage on fire.

Commentary by Dr. Valentin Fuster
2005;():693-708. doi:10.1115/PVP2005-71643.

Effects of rising and falling stress intensity factor (K) profiles on the SCC growth rates of stainless steel and nickel alloys has been studied in high temperature water. Sophisticated test control software was used that changes loading (P) based on crack length (a) to achieve a specific K trajectory by controlling dK/da, not simply dP/dt. The majority of SCC problems develop adjacent to welds, which have a complex residual stress profile vs. wall thickness. This, coupled with the dependence of K on crack length, causes K to change as the crack grows, not per se with time (t). The effect of “K-dot” on crack tip strain rate and the associated crack growth rate is discussed, along with the repercussions to understanding and dispositioning SCC response.

Commentary by Dr. Valentin Fuster
2005;():709-715. doi:10.1115/PVP2005-71781.

The structural materials selected for high-temperature heat-exchanger applications are expected to withstand very severe operating conditions including elevated temperatures and aggressive chemical species during hydrogen generation using nuclear power. Three different cycles namely sulfur-iodine, calcium-bromine and high temperature electrolysis have been identified for hydrogen generation. Three different structural materials namely Alloy C-22, Alloy C-276 and Waspaloy have been tested to evaluate their high-temperature tensile properties and stress corrosion cracking (SCC) resistance in an acidic solution. The data indicate that all three alloys are capable of maintaining appreciably high tensile strength upto a temperature of 600°C. The results of SCC testing indicate that all three materials are highly resistant to cracking in an acidic solution retaining much of their ductility and time to failure in the tested environment. Fractographic evaluation by scanning electron microscopy revealed dimple microstructure indicating significant ductility in all three alloys.

Commentary by Dr. Valentin Fuster
2005;():717-723. doi:10.1115/PVP2005-71782.

A torsion test has been devised that provides for plane strain constraint in small specimens during fracture toughness testing. This method has been extended for stress-corrosion cracking and a simple torsion load frame has been built to provide for step loading of the specimens. This paper describes using the torsion technique to measure KISCC for aluminum alloy 7075 having two thermo-mechanical treatments.

Commentary by Dr. Valentin Fuster
2005;():725-728. doi:10.1115/PVP2005-71783.

Stress corrosion cracking susceptibility of carbon steel and decarburized steel was studied in 8.5 M sodium hydroxide at 100 °C. Potentiodynamic experiments were performed to determine the potential values to be applied in slow strain rate (ssr) experiments. Optical and scanning electron microcopy were used to investigate the surfaces of corroded samples. Severe intergranular stress corrosion cracking was observed on the carbon steel samples in comparison to the decarburized steel samples.

Commentary by Dr. Valentin Fuster
2005;():731-738. doi:10.1115/PVP2005-71151.

This paper describes failure probability assessment of thinning pipe of a nuclear-power plant. The rupture of the feed water piping in the turbine building due to wall thinning happened in Kansai Electric Power Company (KEPCO) Mihama nuclear power plant Unit-3 on August 9, 2004. The variation between the prediction and measured wear rate was evaluated using the published data from KEPCO, and the probability density function was created. Using this probability density function, the failure probability of wall thinning pipe was evaluated using Monte Carlo simulation. The parameters of this evaluation are thinning patterns, wear rate, an applied stress and an inspection interval. The requirements to the inspection were clarified from the probability of failure.

Commentary by Dr. Valentin Fuster
2005;():739-744. doi:10.1115/PVP2005-71337.

Monotonic four-point bending tests were conducted using pipe specimens having orifice undergoing local wall thinning. The effects of local wall thinning on the fracture behaviors of pipe were investigated. Local wall thinning was machined on the inside of pipes in order to simulate erosion/corrosion metal loss. The configurations of the eroded area were l = 100 mm in eroded axial length, d/t = 0.5 and 0.8 in eroded ratio, and 2θ = 180° in eroded angle. The area undergoing local wall thinning was subjected to either tensile or compressive stress. Failure type could be classified into ovalization, local buckling, and crack initiation, depending on eroded ratio, and stress at the eroded area. Three-dimensional elasto-plastic analyses were also carried out using the finite element method, which is able to accurately simulate fracture behaviors. The crack initiation point could be successfully predicted by the criterion proposed by Miyazaki et al..

Topics: Pipes , Failure
Commentary by Dr. Valentin Fuster
2005;():745-752. doi:10.1115/PVP2005-71358.

It is important to assess the failure strengths for pipes with wall thinning to maintain the integrity of the piping systems and to make codification of allowable wall thinning. Full-scale fracture experiments on cyclic loading under constant internal pressure were performed for 4-inch diameter straight pipes and 8-inch diameter elbow pipes at ambient temperature. The experiments were low cycle fatigue under displacement controlled condition. It is shown that dominant failure mode under cyclic loading for straight pipes and elbows is crack initiation/growth accompanying swelling by ratchet or buckling with crack initiation. When the thinning depth is large, burst occurs after swelling. In addition, it is shown that pipes with wall thinning less than 50% of wall thickness have sufficient margins against seismic event of the safety shut down earthquake (SSE).

Topics: Pipes , Failure
Commentary by Dr. Valentin Fuster
2005;():753-759. doi:10.1115/PVP2005-71551.

This paper proposes a method based on the reference stress a approach to estimate residual strength of a pipe with local wall thinning. The method is based on the equivalent stress averaged over the minimum ligament in the locally wall thinned region. Inspired by the reference stress method for approximate creep stress analysis, approximate estimation equations for the equivalent stress in the minimum ligament are proposed, which are then calibrated using detailed elastic-plastic 3-D FE analysis. The resulting estimation equations are found to be insensitive not only to pipe and defect geometries but also to material. Comparison of failure loads, predicted according to the proposed method, with published test data for corroded pipes shows excellent agreement.

Topics: Stress , Pipes , Failure
Commentary by Dr. Valentin Fuster
2005;():763-770. doi:10.1115/PVP2005-71107.

Welding residual stresses have important consequences on the performance of engineering components. High residual stresses lead to loss of performance in corrosion, fatigue and fracture but as yet these consequences are poorly quantified. The major cause of this is that residual stress often remains the single largest unknown in industrial damage situations since they are difficult to measure or estimate theoretically. One of the key issues in the study of residual stress is that the detail of the stress distribution on a small scale (in the order of millimetres) can be important. In this paper, the neutron diffraction technique is used which while it is a very expensive technique, is capable of non-destructively measuring residual stresses at this scale up to a depth of 35mm. The investigation reported compares the residual stress characteristics due to various restraints for a single bead and in fully restrained samples with different numbers of beads. The findings have important consequences with respect to design of welding procedures and fitness for purpose assessments.

Commentary by Dr. Valentin Fuster
2005;():771-780. doi:10.1115/PVP2005-71315.

Residual stresses were analysed in a partial penetration weld attaching a tube inside a thick pressure vessel forging, both made of SA508 steel. 2D finite element (FE) analyses methods were used to simulate this multi-pass manual TIG weld. The weld preps are buttered and the forging subsequently heat-treated prior to making the closure weld. Buttering of the forging J-prep and subsequent PWHT creep stress relaxation were modelled. Generally the buttering was found to have minimal influence on the final stress state, although some difference in local peak stress and stress gradients were calculated. Representative test blocks were manufactured, with and without buttered weld preps. Each test block contained two tube penetrations and attachment welds, in order to examine interaction effects. Welding details were captured and peak temperatures recorded by thermocouples were reasonably consistent with the FE model predictions. Surface stresses were measured both in the as-welded condition and after machining, using the hole drilling strain gauge method. Good agreement with FE results was achieved in surface stress levels in the vessel forging, buttering and tube wall. However the 2D model overestimates weld hoop stresses. Large yield magnitude tensile stresses in the vicinity of the joint are balanced by lower compressive stresses in the surrounding PV forging. Interaction effects between the stress fields produced by adjacent tube welds are negligible.

Commentary by Dr. Valentin Fuster
2005;():781-790. doi:10.1115/PVP2005-71326.

Full two and three-dimensional single or multi-pass weld simulations are now feasible and practical given the development of improved analysis tools (e.g. ABAQUS), and significantly greater computer power. This paper describes a finite element analysis undertaken to predict the as-welded residual stress field following the welding of a tube attachment weld inside a thick pressure vessel (PV) forging. The coupled thermal-mechanical analysis was performed using the finite element (FE) code ABAQUS, A heat source modelling tool was employed to calculate welding fluxes, which were read into ABAQUS via a user subroutine. The ‘block’ dumped approach was utilised in the 2D thermal analysis such that complete weld rings are deposited instantaneously. Heat inputs were based on the actual weld parameters and bead sizes. The predicted fusion depths matched well with those found in sectioned weld test pieces. 2D FE sensitivity studies were performed examining the effect of variations in a number of parameters (bead sequence, hardening law, inter-pass temperature and annealing temperature). The hardening law was changed from isotropic to kinematic to investigate the effect of material behaviour. Large weld residual tensile stresses were calculated with significant compressive stresses in the adjacent vessel wall. Stress results were generally insensitive in the tube and forging, indicating that the vessel constraint dominates over local welding conditions. Weld hoop stresses were overestimated partly due to the ‘tourniquet’ effect of depositing rings of weld metal and the isotropic hardening law assumed.

Commentary by Dr. Valentin Fuster
2005;():791-797. doi:10.1115/PVP2005-71348.

This paper presents the results of a numerical study undertaken to assess the influence of residual stresses on the ductile tearing behaviour of a high strength, low toughness aluminium alloy. The Gurson-Tvergaard model [1, 2] was calibrated against conventional fracture toughness data using parameters relating to void nucleation, growth and coalescence. The calibrated model was used to predict the load versus ductile tearing behaviour of a series of full-scale and quarter-scale wide-plate tests. These centre-cracked tension tests included specimens that contained a self-balancing residual stress field that was tensile in the region of the through-wall crack. Analyses of the full-scale wide-plate tests indicated that the model provides a good prediction of the load versus ductile tearing behaviour up to approximately 3 mm of stable tearing. The influence of residual stress on the load versus crack growth behaviour was accurately simulated. Predictions of the load versus crack growth behaviour of full-scale wide-plate tests for crack extensions greater than 3 mm, and of the quarter-scale tests, were low in terms of predicted load at a given amount of tearing. This was considered to result from: (i) the ‘valid’ calibration range in terms of specimen thickness and crack extension, (ii) the development of shear lips and (iii) differences in the micro-mechanism of ductile void formation under plane strain and under plane stress conditions.

Topics: Stress
Commentary by Dr. Valentin Fuster
2005;():799-808. doi:10.1115/PVP2005-71355.

New test specimens to study initiation of reheat cracking in stainless steels have been developed using modern finite element (FE) weld modelling techniques. Residual stress simulations of two theoretical ring weld test specimens were conducted in order to optimise the specimen designs so that large zones of creep damage are predicted when subject to elevated temperatures. Each design was optimised to predict damage in the heat affected zone (HAZ) or weld metal region respectively. Fabrication of test components is proceeding on the basis of these studies. The general design is based on a thick disc of parent material with a multi-pass manual metal arc (MMA) ring weld on one surface. The overall objective was to design a specimen that produces macro cracks when soaked at temperatures of 600°C–650°C within several thousand hours. However, consideration was also given to the ease at which automated NDT procedures could be applied to monitor the development of creep damage, as well as the ability to measure residual stress levels by neutron diffraction and the contour method. The design analysis followed a pragmatic approach whereby an ‘appropriate’ geometry for the ring weld test specimen was analysed based upon a ‘baseline’ set of welding parameters, namely, heat input, lay-up sequence and inter-pass temperature. This ‘baseline’ design was then subject to a number of sensitivity studies to ascertain the performance of the specimen where key parameter’s were varied; these being lay-up sequence, number of capping passes, specimen thickness, radial weld position and weld fill. Also investigated was the introduction of an offset weld in order to control the location at which initiation occurred, either HAZ or weld metal. Residual stresses in this multi-pass MMA weld were predicted using a 2D axisymmetric model, simulating both the welding procedure, subsequent machining and furnace heat soak. The coupled thermal-mechanical analysis was performed using the FE code ABAQUS. A heat source modelling tool was employed to calculate welding fluxes, which were read into ABAQUS via a user subroutine. Fusion boundaries were assessed a correlation for total fused area.

Commentary by Dr. Valentin Fuster
2005;():809-816. doi:10.1115/PVP2005-71440.

Recently, stress corrosion cracking (SCC) of core internals and/or recirculation pipes of austenite stainless steel has been observed. SCC is considered to occur and progress near the welding zone because of the weld tensile residual stress. In the present work, thermo-elastic-plastic analysis of the residual stress was performed in order to clarify the effect of several parameters (diameter, thickness, number of multilayer welding) in the circumferential welding zone. Butt welding joint of SUS316L-pipes was examined. The residual stress was calculated by three dimensional-model and axisymmetric model and the results were compared and discussed in detail.

Commentary by Dr. Valentin Fuster
2005;():817-825. doi:10.1115/PVP2005-71461.

The fixed conditions of butt welds between straight pipe and valve or pump in the actual piping system are different from those of straight pipes. However, the effect of fixed condition on the residual stress and the stress intensity factor for evaluation of structural integrity of cracked piping is not clear. In this study, the finite element analyses were conducted by considering the differences in the distance from the center of weld to the fixed end L to clarify the effect of fixed condition on the residual stress and the stress intensity factor. For the 600A piping, the residual stress distribution was not affected by the distance L. Furthermore, the stress intensity factor of circumferential crack under the residual stress field could be estimated by using the existing simplified solution for piping.

Topics: Stress , Pipes
Commentary by Dr. Valentin Fuster
2005;():827-834. doi:10.1115/PVP2005-71462.

The stress intensity factor estimated by using the appropriate modeling of components is essential for evaluation of crack growth behavior in stress corrosion cracking. For the appropriate modeling of welded components with a crack, it is important to understand the effects of residual stress distribution and geometry of component on the stress intensity factor of surface crack. In this study, the stress intensity factors of surface crack under two assumed residual stress fields were calculated. As residual stress field, the bending type stress field (tension-compression) and the self-equilibrating stress field (tension-compression-tension) through the thickness were assumed. The geometries of components were plate and piping. The assumed surface cracks for evaluations were long crack in surface direction and semi-elliptical surface crack. Furthermore, the crack growth evaluations were conducted to understand the effects of residual stress distribution and geometry of component. Here, the crack growth evaluation means the simulation of increments of crack depth and length by using the crack growth property and stress intensity factors. From the comparison of stress intensity factors and crack growth evaluation for surface crack under residual stress field, the effects of residual stress distribution and geometry of component on the stress intensity factor of surface crack and appropriate modeling of cracked components were discussed.

Commentary by Dr. Valentin Fuster
2005;():835-841. doi:10.1115/PVP2005-71466.

Welding distortion of a T-joint is calculated numerically considering the effect of phase transformation. Welding distortion becomes smaller in the case of a T-joint with developed low-transformation temperature welding wire than with conventional wire, which has been produced for high accuracy fabrication. Results of weld distortion history by numerical simulation agree well with those by the measurement during the whole weld process. The effect of material properties such as martensitic transformation temperature on welding distortion is investigated by using numerical simulation.

Commentary by Dr. Valentin Fuster
2005;():843-851. doi:10.1115/PVP2005-71566.

In this paper, the effect of residual stress on the initiation of a crack at high temperature in a Type 347 austenitic steel weld is examined using the finite element method. Both two and three dimensional analyses have been carried out. Residual stresses have been introduced by prior mechanical deformation, using a previously developed notched compact tension specimen. It has been found that for the 347 weld material, peak stresses in the vicinity of the notch are approximately three times the yield strength at room temperature and the level of stress triaxiality (ratio between hydrostatic and equivalent stress) is approximately 1 (considerably higher than that for a uniaxial test). The finite element analysis includes the effects of stress redistribution and damage accumulation under creep conditions. For the case examined the analysis predicts that crack initiation will occur under conditions of stress relaxation if the uniaxial creep ductility of the material is less than 2.5%. Furthermore, the predicted life of the component under constant load (creep conditions) is significantly reduced due to the presence of the residual stress field.

Commentary by Dr. Valentin Fuster
2005;():853-860. doi:10.1115/PVP2005-71575.

This paper describes transverse residual stress and strain measurements aimed at quantifying end effects in single and multi-pass weld-runs. Two test specimens are examined: a 60 mm long weld bead deposited on the surface of a 180 mm × 120 mm × 17 mm thick stainless steel plate, and a 62° arc-length multi-pass repair weld in a 432 mm outer diameter, 19.6 mm thick stainless steel pipe girth weld. The residual stress measurements were made by employing the relatively new Contour method and by neutron diffraction using ENGIN-X, the engineering spectrometer at the ISIS facility of the Rutherford Appleton Laboratory (UK). The measured underlying transverse residual stress levels are observed to be essentially uniform directly beneath the weld bead in the plate specimen and in the heat affected zone beneath the capping passes moving from mid-length towards the stop-end of the pipe repair. However, results from both test components demonstrate the existence of short-range concentrations of transverse residual stress along the welding direction owing to individual weld capping bead start and stop effects. Such short length-scale stress variations must be allowed for when interpreting residual stress measurements from line-scans. The experimental work also demonstrates the importance of knowing the expected stress or strain distribution prior to choosing measurement lines for detailed study. The Contour measurement method and neutron strain scanning are powerful tools for mapping residual stress and strain fields.

Topics: Measurement , Stress
Commentary by Dr. Valentin Fuster
2005;():861-865. doi:10.1115/PVP2005-71605.

“ELIXIR – Extending Plant Life Through Improved Fabrication and Advanced Repair Methodology” was a European Union FP5 sponsored project. During the duration of the Elixir project, much work was directed at providing the necessary data for the validation of numerical modelling techniques applied to residual stress generation and hydrogen diffusion arising from the welding process. The project focussed around four industrial applications, namely petrochemical, boiler, offshore and submarine. This paper presents through-thickness residual stress measurements obtained by the University of Bristol on two of the large industrial components. The results were obtained using the deep hole drilling technique and compared to Finite Element predictions provided by other partners. The components considered are a large P275 steel set-in nozzle, typical of a boiler application and a large S690 steel set-on nozzle, typical of an offshore application. The boiler application consisted of a nozzle of diameter 600mm and thickness 50mm, on a pipe of diameter 1100mm and 100mm thickness. The offshore application was a nozzle of diameter 900mm and thickness 50mm, on a pipe of diameter 1050mm and 50mm thickness. Both the longitudinal and transverse stresses measured using deep hole drilling showed excellent agreement with Finite Element predictions through the thickness of the boiler sample. On the top surface, a zone of tensile residual stresses, over a distance of approximately 40mm, was revealed, which was equilibrated by a zone of compressive residual stresses over the final 50mm of thickness. Results for the offshore application demonstrated that at the front surface, both of the stress components were essentially zero, but both the longitudinal and transverse components rose rapidly to maxima of approximately 500MPa and 220MPa, respectively. Tensile residual stresses were supported over a distance of approximately 30mm. Over the final 20mm of thickness, compressive residual stresses existed, which again fell to approximately zero on the back face. There is excellent agreement between measurements and the Finite Element predictions for the transverse stress component, but less good agreement between measurements and predictions of the longitudinal stress component.

Commentary by Dr. Valentin Fuster
2005;():867-879. doi:10.1115/PVP2005-71631.

Welding distortion and residual stress has remained one of the major challenges in manufacturing. Extensive experimental efforts have been made throughout the history of manufacturing to understand and control the welding distortion and residual stresses. Extensive work has also been reported in the past couple decades in using analytical and computational methods to predict and quantify the effect of distortion and residual stress during the welding process. Examples of industrial welding applications using computational methods which highlight the challenges and benefits of using analysis and simulation tools to improve the quality and reduce the time and cost of the welding process development and problem resolution are discussed in [1–3]. One issue with regard to computational weld modeling needing addressed is with regard to material modeling. Since material melts and re-solidifies during welding, the use of a proper ‘weld’ constitutive model is critical. In particular, one issue regarding weld constitutive models is the phase transformations which can be important in some materials during weld solidification. This paper addresses this issue and shows some examples where the phase transformation effect is of second order importance and discusses when it must be properly considered.

Commentary by Dr. Valentin Fuster
2005;():881-887. doi:10.1115/PVP2005-71632.

High accuracy laser scanning and three-dimensional modeling are critical requirements of weld distortion research and ultimately to the management of weld distortion. The significance of these requirements is based on an on-going fabrication project designed to validate FEA predictions of weld distortions typical of large curved steel structures. As built 3D models of a series of 1,300 square foot test beams are measured using Coherent Laser Radar for the purpose of determining deformations caused by the thermal affects of welding, i.e.: bending, warping and buckling. This paper describes tools and techniques currently used to minimize the un-certainty of measurements and maintain accuracy during the processing of point clouds into surfaced models. Further techniques are defined for generating 3D parametric models from finite element analysis results, and utilizing these models for direct comparison to the as-built 3D modeled results.

Commentary by Dr. Valentin Fuster
2005;():889. doi:10.1115/PVP2005-71787.
FREE TO VIEW

Fracture assessment procedures such as BS 7910 and API 579 are formulated based on the Fracture Mechanics concept for assessing integrity of structures such as pipelines, pressure vessels, etc. In the current study those procedures are applied to through-wall and surface cracked pipe geometry under four-point bending. The predicted maximum tolerable applied loads are then compared with pipe full-scale fracture testing results published by Miura et al (2002). Other assessment schemes namely, GE/EPRI, Net-section plastic collapse, LBB.NRC and finally LBB.ENG2, as reported in the same publication are also included in the current paper for sake of comparison. The comparative study showed that BS 7910 and API 579 predict similar maximum tolerable load for through-wall pipes but different value for surface-cracked pipes. Difference in predictions for the latter geometry is owing to the use of different stress intensity factor/reference stress solution by BS 7910 than API 579. However, both procedures provided conservative results compared with the experimental data as well as other engineering routes mentioned in Miura et al (2002).

Commentary by Dr. Valentin Fuster
2005;():893-898. doi:10.1115/PVP2005-71103.

There are reported cases of serious accidents in ship hull structures of aged ships due to corrosion of steel after long period of use. This paper presents basic information on the effect of corrosion on static strength, that is, tensile strength, bending strength and shear strength, of fillet-welded joints of 500MPa-class steel used for ship hull construction. It is based on experimental results using specimens made of material of corroded fillet-welded joints of actual old ships and as-fabricated fillet-welded joint and simulated corroded fillet-welded joint. And also the effect of inclined angles on perpendicular direction shear strength of fillet-welded joint is investigated. Basically, static strength of the fillet-welded joint is affected by the (R) ratio of thickness of steel plates to total thickness of throat of the weld metal. When R was greater than 1.0, fracture occurred at weld metal under tensile stress condition or shear stress condition. However, under bending stress condition fracture at weld metal occurred when R was greater than 1.4. The effect of inclined angles on shear strength of fillet-welded joints is also presented.

Topics: Steel , Corrosion
Commentary by Dr. Valentin Fuster
2005;():899-907. doi:10.1115/PVP2005-71208.

Steady-state Eulerian analysis on thermal simulation of welds using moving coordinates is known as a very computationally efficient method. This paper presents a method of Eulerian analysis that uses commercial computational fluid dynamics (CFD) code. In order to show the practical availability of the Eulerian method, the method is applied to an analysis of a circumferential weld of a core shroud in a boiling water reactor (BWR). In this analysis, the double ellipsoidal power density distribution model proposed by Goldak et al. (1984) is applied for the weld heat source, the temperature dependency of thermal properties is considered, and the effect of latent heat is treated by enthalpy method. Comparison of the analyzed temperature histories at several locations on the surface of the weldment to the measured results shows that the numerical results reproduce the measured results well.

Commentary by Dr. Valentin Fuster
2005;():909-915. doi:10.1115/PVP2005-71321.

A mathematical model of the auto-repair welding system for the pressure vessel’s hemispherical head is proposed according to the principles of the robotics. The welding system can be simplified to a four-axis linkage system and two types of models can be obtained. Using these mathematical models, we analyze the position of the welding torch and derive the velocity formulas of the four axes in the system. All these formulas can be used in the development of the auto-repair welding system. The welding of the transition section which is mainly discussed in this paper is complex in the whole repair process, and a geometry model is proposed for it. The auto-repair welding experiments have been given to illustrate the validity of this model.

Commentary by Dr. Valentin Fuster
2005;():917-926. doi:10.1115/PVP2005-71482.

ASME Code Case N-666 provides alternative rules for repair of a cracked and leaking small bore socket weld by installation of a structural weld overlay [1]. The crack is not removed but is encapsulated and sealed under the weld overlay. Vibration fatigue testing reported by the Electric Power Research Institute (EPRI) demonstrates that socket welds repaired by the method specified in ASME Code Case N-666 have equivalent or better fatigue strength compared to standard socket welds. This paper investigates fatigue test data and fracture mechanics analyses for standard socket welds and compares this to the vibration fatigue strength exhibited by overlay repaired socket welds. A relationship based on fatigue testing of a standard socket weld with root defects was proposed by Japanese researchers to correlate the reduction in fatigue strength with increasing root defect size. This relationship is compared to an EPRI finite element model that was developed to evaluate the stress intensity factor at the root of a standard socket weld. A correction factor is proposed for estimation of the stress intensity factor at the crack tip of a socket weld repaired by weld overlay. The correction factor is derived from a three-dimensional solution for straight pipe with an inside surface circumferential crack and from the finite element model for standard sized socket welds. Finally, weld residual stress analyses reported by Japanese researchers for standard socket welds are compared to weld residual stress data from recent thermal-mechanical finite element analyses for overlay repaired socket welds. The threshold for fatigue crack propagation and the influence of weld residual stress is presented to explain the high vibration fatigue strength exhibited by socket welds repaired by the method of Code Case N-666.

Commentary by Dr. Valentin Fuster
2005;():927-937. doi:10.1115/PVP2005-71483.

Nickel-base Alloy 690 wrought material and Alloy 52 (ERNiCrFe-7) weld filler metal have in recent years become the material of choice in new fabrication and repair of commercial nuclear power plant components. Alloys 690 and 52 are preferred due to improved resistance to primary water stress corrosion cracking (PWSCC) as compared to other nickel-base alloys or filler metals, such as Alloy 600 and Alloy 82 (ERNiCr-3). Nickel-base alloys are commonly used in dissimilar metal joints between quenched and tempered low alloy steel and austenitic stainless steel components in nuclear power primary water systems. Nuclear power industry experience with Alloy 52 filler metal using manual or machine gas tungsten arc welding (GTAW) in multi-pass welds and highly restrained thick section welds has been troublesome. Ultrasonic and radiographic examination of Alloy 52 welds has, in some cases, revealed multiple subsurface micro-cracks in the weld metal heat affected zone (HAZ). Recent laboratory thermo-mechanical testing using modified varestraint test methods and a newly developed Gleeble-based strain-to-fracture test method indicate nickel-base alloys are susceptible to a ductility-dip cracking (DDC) phenomenon during the on cooling cycle of welding. Thermo-mechanical testing also demonstrates that initiation of DDC is dependent on exceeding a specific strain threshold or strain rate in susceptible alloys. Microanalysis and micro-characterization studies indicate that DDC is a solid-state thermo-mechanical phenomenon that occurs most commonly along migrated grain boundaries of single-phase austenitic stainless steel and nickel-base alloys. Though not fully understood, DDC is believed to initiate in the temperature range where material ductility drops concurrent with high shrinkage strains during the on cooling weld cycle. This paper reviews the most current thermo-mechanical laboratory test results and micro-characterization studies of nickel-base alloys for susceptibility to DDC. Alloy 52 weld filler metal is discussed in detail due to its importance to the nuclear power industry. Finally, welding parameters and specific filler metal chemistry to reduce potential for DDC are presented and information for evaluation of specific heats of Alloy 52 for susceptibility to DDC are discussed.

Commentary by Dr. Valentin Fuster
2005;():939. doi:10.1115/PVP2005-71514.
FREE TO VIEW

It has been shown in a series of recent publications that any arbitrary residual stress distribution can be decomposited into through-wall “self-equlibrating” and “bending”. In this paper, a mechanics based parametric description of through-wall residual stress profiles is provided for fracture and fatigue assessment of girth welds. This parametric equation contains two fundamental parameters: (1) relative heat input and (2) pipe thickness to radius ratio. For a given welding procedure and heat input, the through-thickness resdual stress distributions as well as magnitude are shown to exhibit a continuous transition from bending to self-equilibrating as r/t ratio increases. As a result, a single parametric equation can be established for describing through-wall residual stress distributions for a full range of pipe diameters and thicknesses as well as weld configurations. A series of detailed finite element simulations were also performed to validate the predictions by the parametric equiations proposed.

Commentary by Dr. Valentin Fuster
2005;():941-946. doi:10.1115/PVP2005-71572.

Duplex stainless steels (DSS) are finding increased application on offshore process plants in Brazil. Welding them is always a challenge. Pure argon and argon/low-nitrogen gas mixtures are available for welding them with GTAW process. The use of the same kind of gases on shielding and backing is usual. If no nitrogen is present in the gas mixtures there may be loss of nitrogen from the weld metal or HAZ. Three different welding procedures were tested for a 2% tungsten superduplex parent metal to verify the loss of nitrogen from weld metal and HAZ. First procedure used a 2%N-88%Ar-10%He as the shielding mixture and pure Ar as the backing gas while the second one used pure Ar as shielding and pure nitrogen as backing gas. A third procedure with no nitrogen for shielding or backing was also carried out for comparisons. Chemical analyses, corrosion test and determination of austenite/ferrite content were done to check metallurgical and chemical properties. Mechanical tests were performed to compare mechanical properties of the weld metal and HAZ. ASME Code, section IX, plus Norsok standard M-601 were referenced. Results indicate good mechanical behavior, corrosion resistance and austenite/ferrite phases balance in both procedures that used nitrogen as one of the gases. The procedure using only pure argon showed a decrease of austenite content in weld metal and heat affected zone. However, it fulfilled all the requirements. They didn’t present secondary phases or very significant variances on austenite presence even on weld metal and HAZ.

Commentary by Dr. Valentin Fuster
2005;():947-954. doi:10.1115/PVP2005-71626.

During manufacturing, complex shape welded pressure vessels are submitted to numerous intermediate heat treatments after each weld (de-hydrogenation treatment - DHT and/or intermediate stress relieving treatment - ISR) before final Post Weld Heat Treatment (PWHT). The present study aims at analysing and optimising the intermediate heat treatment conditions regarding the resulting mechanical properties (tensile strength and impact. strength) of CrMo and CrMoV creep resistant steels. Hydrogen behaviour in weld metal and HAZ, and residual stresses evolution have been assessed by numerical modelling and experimental measurements on welded specimens representative of big pressure vessels: butt welds and set in nozzle welds of 150 mm wall thickness. The optimised conditions are compared to usual construction codes and buyer’s requirements.

Commentary by Dr. Valentin Fuster
2005;():955-964. doi:10.1115/PVP2005-71654.

Issued from two successive RD studies in 1991–1995 for shipbuilding in France, a simplified method of numerical simulation of arc welding has been developed and validated on samples in full scale executed inside the shipyard. The metallurgical concept of the methodology is based on two main characteristic diagrams of iron-carbon steel: metallurgical phase transformation diagram and thermal dilatation diagram. In this paper, the simplified methodology is described with on its basic assumptions.

Commentary by Dr. Valentin Fuster

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