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ASME Conference Presenter Attendance Policy and Archival Proceedings

2011;():i. doi:10.1115/PVP2011-NS6.
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This online compilation of papers from the ASME 2011 Pressure Vessels and Piping Conference (PVP2011) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in the ASME Digital Library and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Materials and Fabrication

2011;():3-6. doi:10.1115/PVP2011-57194.

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 is 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 HIPed powder deliver cost savings by reducing initial material usage and subsequent machining costs. Powder production and HIP processing are automated methods, which provide protection against forging route obsolescence. Setup costs are lower and batch sizes are smaller, which makes HIPping particularly well suited to small numbers of high integrity components. HIPed powder microstructures are isotropic and equiaxed, with uniformly fine grain sizes not normally achieved in heavy section components, which facilitates ultrasonic NDE examination. Improved features to facilitate NDE are readily incorporated into the HIP assembly. Inclusion contents are lower and of more benign geometry, easing fracture assessment. In a broad program of testing, Rolls-Royce has established (1) that HIPed powder 316L/304L components, in items up to several tons in weight, have equivalent or slightly better strength, toughness and corrosion resistance than the wrought equivalents. Rolls-Royce are extending their activities to HIPing of Inconel alloys. The first phase has been to HIP test samples of Inconel 600 and Inconel 690 alloys. Initial testing has produced promising results in line with expectations of wrought material. There has also been the opportunity to vary the HIPing cycle to assess the effect of processing parameters on the final product. An ability to HIP Inconel components is thought to be of benefit in new plant construction, where material is often not readily available in required thick section. The adaptability and good control of the HIP technique also shows promise as a manufacturing route for future high temperature materials which will be required in Generation 4 civil builds.

Commentary by Dr. Valentin Fuster
2011;():7-11. doi:10.1115/PVP2011-57323.

In this study the combination effects of shot peening (SP) and crack-healing on the contact strength of ceramics were examined. Si3 N4 /SiC composite ceramics with high crack-healing ability were used and the specimens were subjected to shot peening with several peening pressures and shot diameters. Then, some specimens subjected to SP were heat-treated in air to heal surface cracks introduced by shot peening. Sphere indentation tests were carried out on the specimens and investigated the Weibull distribution of the contact strength. As a result, it is found that the combination of shot peening and crack-healing is effective to increase contact strength and decrease the scatter of the contact strength.

Commentary by Dr. Valentin Fuster
2011;():13-17. doi:10.1115/PVP2011-57563.

This paper provides details of an ongoing effort to characterise the microstructure of heavy section low alloy steel forgings used in high integrity applications and correlate these data to the mechanical properties of these materials. Current industry practice is to use chemical etching and visual assessment in acceptance standards to determine nominal averages for microstructure parameters. This work uses electron microscope techniques to provide information on a variety of grain and secondary-phase particle information allowing numerical comparison of key microstructure variables to mechanical properties. For instance, the transition toughness behaviour of low alloys steels is controlled by the combination of the largest grain and particle in the material, i.e. the most potent initiator for cleavage failure. Knowledge of only the average grain size is insufficient to correlate microstructure and transition toughness performance. The programme consists of three main stages: modelling key variables in the manufacturing process to predict microstructure from thermodynamic predictions, developing quantitative microstructure data on archive materials for which mechanical property data are known to allow empirical relationships to be constructed and, a final validation exercise of a complete model by production and assessment of trial forgings.

Commentary by Dr. Valentin Fuster
2011;():19-28. doi:10.1115/PVP2011-57892.

Oxide Dispersion Strengthened (ODS) alloys are a long established class of materials manufactured using powder metallurgy techniques. These alloys can offer exceptional high temperature strength and resistance to radiation damage, thus are envisioned to be used in a number of future nuclear and fossil energy power applications. However, due to the manufacturing steps involved, the overall cost to build components with these materials can be high. This paper presents work conducted to assess the feasibility of applying Selective Laser Melting (SLM) techniques to either coat or direct build on substrates with Fe-based Oxide Dispersion Strengthened (ODS) alloys. SLM is a rapid prototyping technique which can be used to manufacture near net-shape solid components from layered metallic powder beds. Two different geometries were of interest in this study — a simple button configuration with a nickel-base superalloy (IN939) substrate and a more complex hexagonal shaped wall with a mild steel substrate. Powders of PM2000 (a FeCrAl based ODS alloy) were deposited in both cases. Heat treatments were subsequently conducted on these structures to investigate effects of temperature on the bond characteristics and secondary recrystallisation. Electron microscopy examination revealed significant amounts of diffusion between the nickel and the ODS powders which enhances the bond strength. The studies have revealed the existence of a strong bond between the substrate and the interface even after prolonged exposure at elevated temperatures.

Topics: Lasers , Steel , Melting
Commentary by Dr. Valentin Fuster
2011;():31-36. doi:10.1115/PVP2011-57238.

This study was performed to clarify dependences of bonding strength on the interface wedge angle in the metal side of ceramic-to-metal joint. Each plate Si3 N4 and Ni used for this experiment is produced by wire electric discharge machining. The geometric interface shape at the edge of the interface is characterized by wedge angle on both side of the ceramic and metal defined as a configuration angle between the free surface of each material and the interface. As the wedge angle of Si3 N4 is a right angle, the wedge angle of Ni is set from 30° to 180°. Joint specimens were bonded at high temperature using braze metal of 0.05mm thickness under vacuum and cooled slowly. The tensile bonding strength of the ceramic-to-metal joint was evaluated to determine the optimum interface shape. The highest bonding strength appeared under identical interface conditions where the fracture pattern changed. This study provided a useful geometric interface shape to improve the tensile bonding strength of ceramic-to-metal joint.

Topics: Metals , Ceramics , Bonding , Wedges
Commentary by Dr. Valentin Fuster
2011;():37-46. doi:10.1115/PVP2011-57250.

Demand of natural gas continues to increase in the recent years due to the rise of environmental issue and the drastic increase of crude oil price. These events led to the increase of constructions of Liquefied Natural Gas (LNG) storage tanks worldwide. The inner tank material for above ground LNG storage tanks have mostly been made of a 9% nickel steel plate over the last 50 years as it has excellent mechanical properties under the cryogenic temperature of −160deg-C. During this period, the LNG storage tanks made of 9%Ni steel plate have been operated safely at the many LNG export and import terminals in the world. Meanwhile, technologies of steel making, refinement, design, analysis, welding and inspection have been improved significantly and enabled enlarging volumetric capacity of the tank 2–3 times. There was a tendency for nickel price to increase in recent years. In such a circumstance lowering Ni content has focused attention on the 9%Ni steel as nickel is an expensive and valuable rare metal and a 7%Ni steel plate was eventually researched and developed by optimizing the chemical compositions and applying Thermo-Mechanical Controlled Process (TMCP). As a result, it was demonstrated that 7%Ni-TMCP steel plate had excellent physical and mechanical properties equivalent to those of 9%Ni steel plate. In order to evaluate fitness of the 7%Ni-TMCP steel plate and its weld for LNG storage tanks a series of testing was conducted. Several different plate thicknesses, i.e. 6,10,25,40 and 50 mm, were chosen to run large scale fracture toughness tests including duplex ESSO tests, cruciform wide plate tests as well as small scale tests. It was concluded that the 7%Ni-TMCP steel plate warrants serious consideration for use in LNG storage tanks. This paper reports details of the research and development of the 7%Ni-TMCP steel plate.

Commentary by Dr. Valentin Fuster
2011;():47-50. doi:10.1115/PVP2011-57725.

Syntactic polymer foam has received intensive attention and extensive application due to its remarkable low cost, lightweight, mechanical properties as well as its thermal, acoustic properties for multifunctional purpose. Electrically conductive polymers have the advantages of light weight, resistance to corrosion, good processability, and tunable conductivity. In a recent separated study, we proposed a novel conductive polymer which was based on the metallic foam filled with syntactic polymer foam. In this study, instead of focusing its unique multi-physical properties, we focus on characterizing the mechanical properties of this new conductive syntactic foam. Before the exploration of this new hybrid foam, an understanding of the mechanical properties is quite necessary. To this end, hybrid foams were prepared by varying the volume fractions of microballoons in the syntactic foam and types of microballoon materials: glass and polymer microballoons. The metallic foam adopted in this work was based on aluminum with an average relative density of 7% (the porosity is about 93%). Both compressive and bending tests were conducted. The current test results may provide the valuable baseline and also facilitate the further understanding of this hybrid foams as a core material in the advanced sandwiched pipe/pressure vessel structures featured by lightweight, impact tolerant, self-monitoring, thermal and acoustic insulation, and electromagnetic shielding.

Commentary by Dr. Valentin Fuster
2011;():51-55. doi:10.1115/PVP2011-57730.

Sandwiched structures with syntactic foam core and fiber reinforced composite skins may have a great potential in pipe and pressure vessel structures due to the lightweight, competitive material cost, and it thermal and acoustic insulation effects. One concern of the syntactic foam as core materials is its brittleness (low toughness). In the current study, short basalt fibers are considered as a reinforcing phase to toughen the syntactic foam material. A compressive test and a notched three-point bending test were conducted to characterize the mechanical properties of the basalt fiber reinforced syntactic foam. In order to measure the fracture toughness, in the notched three-point bending test, two inclinometers were applied to record the rotation of the notched beam. Based on a moment-rotation based formulation, the J-integral was calculated. The compressive test results showed that the compressive strength slightly decreased with the inclusion of short basalt fibers. The three point bending test indicated that with a very low fiber volume fraction (0.25% and 0.5%), there was a dramatic increase in the syntactic foam’s tensile strength, ductility and toughness.

Commentary by Dr. Valentin Fuster
2011;():59-68. doi:10.1115/PVP2011-57423.

The world’s first filament-wound ASME Section X [1] Class II FRP(fiber reinforced plastic) vessels were built by Tankinetics Inc. in 2010. These vessels had semi-elliptical top and bottom, and were supported on skirts as shown in Fig.1. This paper is focused on the novelty of these vessels from design and fabrication standpoints. The design pressure is 50.76 psig. Ashland Derakane™ 470 resin is selected for the corrosion liner, and Derakane™ 510 N resin is used in the structural layer. The design is based on ASME Section X code [1] method A. For wind and seismic analysis, IBC 2006[3] and NBCC 2005[4] codes are followed. The domed top and bottom were made by hand lay-up method while the cylindrical shell section and skirt were made by filament winding technology. Filament winding is chosen for these pioneer vessels because it can produce stiffer, higher-strength laminates with much less fabrication time as compared to traditional hand lay-up process.

Commentary by Dr. Valentin Fuster
2011;():69-77. doi:10.1115/PVP2011-57540.

For automotive applications high pressure storage of compressed hydrogen (CH2) becomes more and more important especially regarding the future sustainable mobility based on renewable energy. The filament winding technology is highly industrialized today to meet the requirements for a high-quantity production of lightweight fiber reinforced pressure vessels used for hydrogen-powered cars using fuel cells or combustion engines. However, the main disadvantage of the conventional wet winding process is the low lay-down rate. A decrease of the cycle time to increase the production rate can be realized by the simultaneous feed of a large number of rovings circumferentially arranged around the mandrel. A team of engineers at the Institut fuer Verbundwerkstoffe (Institute for Composite Materials) now further developed the ring winding technology to manufacture pressure tanks with a diameter up to 500 mm (20 inch) with fully wrapped dome sections. This large ring winding head with 12 radial movable arms and multiple payout eyes allows an accordingly higher material output and reduces the cycle time significantly. The profitability analysis considering ring winding head configurations with different number of feed-eyes show the optimization potential regarding the reduction of the production costs of FRP pressure cylinders. A critical review regarding reachable process efficiency is also given in this paper. A modified “tube siphon impregnation unit” is the most important component of the ring winding head due to its compact and modular design. This clean impregnation of 48 carbon fiber rovings near the winder minimizes possible resin leakages. In comparison to a conventional resin bath the amount of hazardous waste, for example contaminated acetone, can be reduced. FRP laminates (fiber reinforced plastic) with a reduced number of crossing points and less ondulation of fiber bundles result higher load bearing capacity and increased mechanical properties. Regarding the degree of interweaving the multi feed-eye configuration of a ring winding head makes it more challenging to define an optimized winding pattern with the given winding angle. The split disk test method was used for comparative investigations regarding the influence of undulations on the material properties of filament wound laminates.

Commentary by Dr. Valentin Fuster
2011;():79-87. doi:10.1115/PVP2011-57543.

Filament winding is a well-established process for the production of high-end fully wrapped composite pressure vessels. This type of tanks can be designed for service pressures that exceed 700 bar and are ideal for storage of gas fuels like compressed hydrogen in automotive and lightweight applications. As the demand for composite pressure vessels increases, lower costs and better product quality become very important. Impregnation is one of the most important steps in the wet winding process. During this step the dry continuous fibers are combined with the liquid matrix in order to create a fully impregnated semi-finished product. The properties of the impregnated roving have a major effect on the laminate quality and the efficient processing of the liquid matrix has a big influence on the manufacturing costs. The present work is related to the development of a new impregnation method for the processing of carbon fiber rovings. The developed impregnation unit (siphon impregnation system) consists of a sinusoidal cavity without any moving parts. This combined with an automated resin mixing-dosing system this allows complete wet-out of the fibers, precise calibration of the resin fraction, and stable processing conditions. The paper focuses on the modeling of the impregnation process inside the siphon unit. Mathematical expressions for the fiber compaction, the gradual increase of the roving tension, the static pressure, the capillarity of the roving, and the fiber permeation are presented, discussed, and experimentally verified. These expressions were implemented in an algorithm which can model the impregnation process by taking input parameters into account like winding speed, resin dosing, viscosity, and roving tex. The model was solved and the processing parameters of winding tension, fiber volume fraction, and impregnation degree have been simulated. An experimental set-up based on a filament winding machine was used for the validation of the model. Trials with different processing parameters and long run tests have been performed. The results proved that the model can accurately simulate the impregnation process. The good impregnation degree of the wound samples confirmed the efficiency of the siphon impregnation unit.

Commentary by Dr. Valentin Fuster
2011;():89-95. doi:10.1115/PVP2011-57644.

Hydrogen storage is a key enabling technology for the extensive use of hydrogen as an energy carrier. However, none of the current technologies satisfies all of the hydrogen storage attributes sought by manufacturers, legislators and end-users. At present, compressed gaseous hydrogen storage (CGH2) is recognized as the most mature technology. This paper reviews recent developments and achievements regarding materials and technologies investigated by CEA to promote the development of a of type IV 70MPa hydrogen vessel. Particularly, results concerning innovative thermoplastic matrix composite vessel will be presented and discussed. On going developments on dedicated manufacturing process and material characterization will be shared in a first part of the presentation and a second part will be devoted to durability assessment and damage tolerance of such composite structures with respect to their potential applications.

Commentary by Dr. Valentin Fuster
2011;():97-106. doi:10.1115/PVP2011-57668.

Hydrogen storage remains a key issue for the high scale deployment of fuel cell applications. Gaseous hydrogen storage at high pressure with type IV vessels is the best technology. But it is necessary to reach a significant cost reduction of these storage systems. An optimization of the composite structure can be reached by numerical simulation. The goal of the OSIRHYS IV project is to develop and validate models and methods for composite high pressure design and optimization with behavior uncertainties knowledge. It was decided to limit this study to a particular topology, material and winding process. First burst simulations have been performed and results of linear static computations have been compared to experimental data. The numerical simulation models are compared with regards to vessel component masses, burst pressure, burst mode and local displacements. Results show that linear static analyses using axisymmetric and volume FE models could already predict with a reasonable accuracy the radial behavior of the tank in the case of a safe burst mode. Nevertheless, improvements of partner models are needed to reach better agreement with test data. These improvements need to be based on material and vessel geometry knowledge, behavior modelling and the FE model.

Commentary by Dr. Valentin Fuster
2011;():107-112. doi:10.1115/PVP2011-58008.

This paper discusses passive and active self-sealing techniques for pressure vessels. The history and state-of-the-art of self-sealing fluid containment vessels is followed by a discussion of challenges specific to implementing self-sealing on pressure vessels. These challenges include large pressure differentials, high speed flows through the leak, the need for relatively rapid response, and embedding the sealing techniques as a composite within a pressure vessel while satisfying practical constraints of weight and size. A benchtop pneumatic test bed provides a setting for evaluating self-sealing technologies. Testing focuses on experiments and models of passive techniques that use shear-thickening fluid coagulation for plugging. This is followed by results that demonstrate the use of active sealing methods with coordinated leak sensing and activated sealing. Acoustic emission (AE) monitoring detects the leak. Electrocoagulation and thermoplastic flow provide the means of controlled sealing. A separate study explores AE testing as a tool for damage assessment. Combining AE testing with neural-network pattern recognition algorithms enables leak detection, location, and size assessment.

Commentary by Dr. Valentin Fuster
2011;():113-116. doi:10.1115/PVP2011-58082.

The adhesively bonded structure has to be replaced after the crack initiation and propagation. In a previous study, a biomimic two-step self-healing scheme (close-then-heal) by mimicking human skin has been proposed for self-healing structural-length scale damage. The adhesively bonded joint are prepared and to invest its feasibility and repeatability by fabricating a composite adhesive bonded joint with thermoplastic particles dispersed in a most commonly used epoxy based adhesive material. The fractured specimens were healed per the close-then-heal mechanism and tested again to fracture. This fracture-healing test lasted for 3 cycles.

Commentary by Dr. Valentin Fuster
2011;():119-128. doi:10.1115/PVP2011-57033.

This paper is concerned with the development of a methodology for thermo-mechanical analysis of high temperature, steam-pressurised P91 pipes in electrical power generation plant under realistic (measured) temperature and pressure cycles. In particular, these data encompass key thermal events, such as ‘load-following’ temperature variations and sudden, significant fluctuations in steam temperatures associated with attemperation events and ‘trips’ (sudden plant shut-down), likely to induce thermo-mechanical fatigue damage. An anisothermal elastic-plastic-creep material model for cyclic behaviour of P91 is employed in the transient FE model to predict the stress-strain-temperature cycles and the associated strain-rates. The results permit characterisation of the behaviour of pressurised P91 pipes for identification of the thermo-mechanical loading histories relevant to such components, for realistic, customised testing; this type of capability is relevant to design and analysis with respect to the evolving nature of power plant operating cycles, e.g. associated with more flexible use of fossil fuel plant to complement renewable energy sources.

Commentary by Dr. Valentin Fuster
2011;():129-138. doi:10.1115/PVP2011-57166.

Many engineering components, such as power plant steam pipes, aero-engine turbine discs, etc, operate under severe loading/temperature conditions for the majority of their service life. As a result, cracks can initiate and subsequently propagate over time due to creep. Damage mechanics is a robust method for the prediction of behaviour of components subjected to high temperature creep conditions and in particular, the Liu and Murakami model has proven to be a useful tool for the prediction of creep crack growth under such conditions. Previous methods for obtaining the constant of multiaxiality required for the use of such models, i.e. α, have relied upon the steady load testing of specimens designed to give a specific multiaxial stress-state, such as notched bars, and the failure time obtained. A series of results from finite element (FE) analyses based on the same geometry and loading/temperature conditions as the experiment, each performed with a different α-value, are then interpolated in order to identify the α-value which results in the same failure time, tf , as that of the experimental test. However, the stress-state present within such a specimen geometry (and therefore the α-value obtained) does not reflect the multiaxial severity of the stress state ahead of a crack tip. Therefore, for the application of the Liu and Murakami model to crack tip (i.e., creep crack growth) conditions, it follows that the α-value should be obtained from a multiaxial stress-state of equal severity to that to which it is to be applied, i.e. a crack tip. Therefore compact tension (CT) specimen creep crack growth data has been used in order to obtain the α-value. The process for the α-value determination is similar to that discussed for the notched bar, except that the interpolation of the time to failure is replaced with an interpolation of the time to a given crack length, ta . The resulting FE predictions based on CT and thumbnail crack specimen geometries, for a 316 stainless steel, are shown to be accurate in comparison to experimental results.

Commentary by Dr. Valentin Fuster
2011;():139-144. doi:10.1115/PVP2011-57268.

Compressive plastic pre-strain induced at room temperature in type 316H stainless steel, significantly influences the tensile, creep deformation and crack growth behaviour of the material. It is known that the material is hardened after pre-strain to 8% plastic strain and thus exhibits little or no plasticity during loading of uniaxial or creep crack growth (CCG) tests. In addition pre-compression (PC) has been found to reduce the creep rupture time, creep ductility and accelerate creep crack growth rates compared to as-received (AR) (i.e. uncompressed) material. In order to understand pre-straining effects on mechanical behaviour of 316H, optical and scanning electron microscopy (SEM) studies have been performed on uncompressed and 8% pre-compressed material. Samples have been examined in three orientations (i.e. parallel and perpendicular to the pre-compression direction). Furthermore, the influence of cold pre-compression on local creep damage formation ahead of the crack tip on interrupted CCG tests on AR and PC material has been studied. The results are discussed in terms of intergranular and transgranular damage caused by the compression process and the importance of microstructural changes on the mechanical behaviour of the material in long term tests.

Commentary by Dr. Valentin Fuster
2011;():145-151. doi:10.1115/PVP2011-57269.

High temperature components generally undergo cyclic loading conditions. Prior tensile/compressive loading of a fracture specimen can induce compressive/tensile residual stress fields at the crack tip. These residual stresses will influence the subsequent fracture behaviour of the cracked body. This work forms part of a project to examine the influence of creep induced damage at a crack tip on subsequent fatigue crack growth and fracture toughness properties of austenitic type 316H stainless steel. Creep damage is introduced local to the crack tip of a fracture specimen by interrupting a creep crack growth test, performed at 550 °C. Prior to testing, the material was pre-compressed in order to strain harden the material. The compact tension, C(T), specimen geometry has been considered in this work. Since residual stresses are known to influence fatigue and fracture toughness properties of a cracked body, it is important that the residual stress levels at the crack tip are quantified. Neutron diffraction (ND) measurements have therefore been performed to quantify the extent of residual stress in these samples after initial loading, and compared to finite element model predictions. Two specimens have been considered with the crack plane orientated in parallel and perpendicular to the pre-compression direction. Compressive residual stresses of around 100 MPa have been measured directly ahead of the crack tip. Reasonable predictions of the principal residual stress distributions have been obtained by the simplified FE analysis. Though the tensile properties differ significantly in for specimens orientated parallel and perpendicular to the pre-compression direction, no significant differences in the residual stress field are predicted in the C(T) specimens orientated in both directions.

Commentary by Dr. Valentin Fuster
2011;():153-161. doi:10.1115/PVP2011-57330.

High temperature crack growth in weldments is of great practical concern in high temperature plant components. Cracking typically occurs in the heat affected zone (HAZ) and often propagates into adjacent parent material (PM). Recently, the importance of constraint effects on creep crack growth behaviour has been recognised and creep crack growth testing on a range of specimen geometries has been performed. Experimental crack growth testing has been performed at 550 °C on a range of fracture specimens using sections taken from a non-stress-relieved 316 steel weldment. These specimens include the compact tension, C(T), middle tension, M(T) and circumferentially cracked bar, CCB, geometries. Results are presented from two long-term creep crack growth (CCG) tests performed on M(T) weldment specimens and these are compared with available data on C(T) and CCB weldment specimens together with both long and short term tests on parent material for a range of specimen geometries. The creep crack initiation (CCI) and growth (CCG) behaviour from these tests has been analysed in terms of the C* parameter. As high levels of residual stress exist in non-stress-relieved weldments, the residual stresses remaining in the weldment specimens have therefore been quantified using the neutron diffraction technique. Long-term (low-load) tests are required on PM specimen to observe specimen constraint effects in 316 steel at 550 °C. When interpreted in terms of the C* parameter the CCG behavior of PM and Weldment materials follow the same trendline on low constraint geometries. However, significant difference is observed in the CCG behavior of PM and weldments on the high constraint C(T) geometry. Long term tests on C(T) specimen weldments are required to confirm the results found.

Commentary by Dr. Valentin Fuster
2011;():163-169. doi:10.1115/PVP2011-57402.

The present paper revisits a constrained use of Monkman-Grant coordinates, a relatively little employed or appreciated method for estimation of long-term creep life. This method is based on a logarithmic plot of remaining life versus the steady creep rate. A procedure, here called proportional similitude, is also discussed as a means to estimate the steady creep rate or time to rupture at an early stage of a test. Numerous studies as yet mostly unpublished increasingly demonstrate for many steel samples from prior creep service that a combination of these two methods permits extrapolations at least as soon, as accurate, and at overall cost similar to other popular procedures. One of the advantages of this procedure is that short term creep test results can be extrapolated to long-term creep life in a transparent manner without complex mathematical maneuvers or need for typical reference properties or initial behavior of the sample. Results now available for a variety of widely-employed materials suggest that these methods may have more-general validity for remaining creep life evaluations than industry has recognized. This paper presents remaining creep lives obtained though a combination of four procedures, i.e., Monkman-Grant, proportional similitude, Larson-Miller, and curve fit methods, for exposed hydrogen reformer tube samples. Results are compared with those of previous Omega analyses performed independently for the same sample.

Topics: Creep
Commentary by Dr. Valentin Fuster
2011;():171-176. doi:10.1115/PVP2011-57510.

This paper proposes a new analytical approach to predict creep void growth and remaining creep lifetime in a heat-affected zone (HAZ) of high Cr steel weldments. Concept of the new approach is based on a relationship between creep void growth rate and a parameter to represent multi-axial stress state obtained by finite element analysis. In this study, creep tests of ASME grade 91 (9Cr-1Mo-Nb-V) and grade 122 (11Cr-2W-0.4Mo-Cu-Nb-V) tubes with longitudinal weldments subjected to various internal pressures have been conducted to reveal creep void growth behavior in HAZ. Some specimens were intentionally interrupted before leakage at a damage level from 40 to 70% for clarification of void growth behavior at intermediate damage level. In addition, finite element creep analyses of the specimens at different creep strain rates in base metal, weld metal and HAZ have been carried out to investigate distribution of stress and stress triaxiality factor in HAZ. A comparison between stress distributions and void distributions revealed that stress triaxiality factor predominantly affects growth behavior of creep voids. From the result, the relationship between creep void growth rates and the parameter as a function of principal stress and triaxiality factor was established. Based on the relationship, a new prediction method was proposed. To verify proposed approach, the new method was applied to the elbow pipe with longitudinal weldment. As a result, predicted creep void growth in HAZ showed a good agreement with the results from diffusion simulation reported by literature. Therefore, the results demonstrated that the proposed approach is applicable to predict void distribution and remaining creep lifetime in the HAZ of high Cr steel weldments.

Topics: Creep , Heat , Steel
Commentary by Dr. Valentin Fuster
2011;():177-187. doi:10.1115/PVP2011-57532.

In a component design at elevated temperature, creep-fatigue is one of the most important failure modes, and assessment of creep-fatigue life in structural discontinuity is important issue to evaluate structural integrity of the components. Therefore a lot of creep-fatigue life evaluation methods were proposed until now. To compare and assess these evaluation methods, a series of creep-fatigue tests was carried out with notched specimens. All the specimens were made of Mod.9Cr-1Mo steel, which it is a candidate material for a primary and secondary heat transport system components of JSFR (Japan Sodium-cooled Fast Reactor). Mechanical creep-fatigue tests and thermal creep-fatigue tests were performed by using conventional uni-axial push-pull fatigue test machine and thermal gradient generating system with an induction heating coil. Stress concentration levels were adjusted by varying the diameters of notch roots in the both tests. In the test, creep-fatigue lives, crack initiation and propagation processes were observed by digital micro-scope and replica method. Besides those, a series of elastic Finite Element Analysis (FEA) were carried out to predict the number of cycles to failure by several creep-fatigue life evaluation methods. Then these predictions were compared with test results. Several types of evaluation methods which are stress redistribution locus (SRL) method, simple elastic follow-up method and the methods described in JSME FR (Fast Reactor) code were applied. The applicability and conservativeness of these methods were discussed. It was appeared that SRL method gave rational prediction of creep-fatigue life with conservativeness when the factor of κ = 1.6 was applied for all the conditions tested in this study. Comparison of SRL method and simple elastic follow-up method indicated that SRL method applied factor of κ = 1.6 gave the smallest creep-fatigue life in practicable stress level. JSME FR code gave an evaluation 70∼100 times conservative lives comparing with the test results.

Commentary by Dr. Valentin Fuster
2011;():189-196. doi:10.1115/PVP2011-57641.

A-parameter, void area ratio and other methods about creep void are used to estimate creep damage resulting from creep voids. However, these methods are based on not three-dimensional but two-dimensional geometry, though creep voids are three-dimensional cavities. By combining the 3D-EBSD method with SEM images, we have observed the three-dimensional shape of creep voids and their geometrical relationship with grain boundaries at first. The method is applied to 1Cr-1Mo-0.25V turbine rotor steel subjected to a creep rupture test (580°C, 180MPa). Also, interrupted creep specimens are prepared to observe the progress of void growth. Forty sections with 0.5 μm interval and 100μm × 100μm area are measured by mechanical polishing in order to reconstruct the three-dimensional shapes. In the results, four types of creep void are observed. One is sphere type whose radius is approximately 1μm. It is observed in the specimen whose creep life fraction is 25%. In the specimens with 50% and 75% creep damage, prolate and oblate spheroid whose radius is approximately 2.5μm are observed. Finally, connected voids are located within ruptured specimen. As the creep damage is progressed, not only void growth but also void nucleation is observed. Especially, on prior austenite grain boundary which is three-dimensionally perpendicular to the stress direction, creep voids are nucleated and grow in a concentrated manner. However, such nucleated small voids do not affect the void volume fraction.

Commentary by Dr. Valentin Fuster
2011;():197-204. doi:10.1115/PVP2011-57695.

Compact tension 316H austenitic steel specimens, extracted from an as-received ex-service pressure vessel header, have been pre-compressed to different load levels in order to introduce a residual stress field. Finite element (FE) analysis has been performed to predict the load level required to obtain a high magnitude tensile stress field over a significant distance ahead of the notch while preventing a large plastic zone in the specimen. The predicted residual stress profiles along the crack path are compared with those measured using neutron diffraction (ND). Comparisons have also been provided between the ND results of this work with recent work carried out on 316H and 347 stainless steels under different loading levels. The creep relaxation behaviour of the steel has been studied numerically. A proposed method to estimate the steady state creep crack tip parameter, C*, has been examined using the obtained displacement rates for the case of combined loading. Creep relaxation data for combined stresses are compared with the earlier studies.

Commentary by Dr. Valentin Fuster
2011;():205-213. doi:10.1115/PVP2011-57745.

A recently developed equilibrium and equivalency (E2 ) mechanisms based curve fitting method is extended to surface fitting for linear function z = f (x, y) = a + bx + cy with two independent variables x and y. The concept of equilibrium of ‘force’ and ‘moment’ is adopted to derive surface fitting formulae, which are exactly the same as that obtained with traditional least squares (LS) method for the linear function. However, E2 method has obvious physical meaning and therefore is more intuitive in quickly and correctly identifying data pattern and subsequent data analysis. Furthermore, the formula based on perpendicular offsets method in terms of data variation along surface normal direction is derived and the results are compared with the traditional methods. Finally, the application of these methods to data of fatigue and creep lives is presented.

Commentary by Dr. Valentin Fuster
2011;():215-225. doi:10.1115/PVP2011-57865.

It has been well established in materials such as austenitic steels and aluminium alloys that plastic strain leads to generation of internal stresses. A number of intergranular factors such as the anisotropic stiffness and yield behaviour of a single crystal, orientation of grain families to the loading direction, and the constraint each grain places on its neighbours are responsible for creating these stresses. The presence of these accumulated internal stresses in power plant components is important because they interact with the applied external stress and play a critical role in the initiation and development of material degradation that may lead to eventual failure. This study focuses on measuring the generation of internal (intergranular) strains and stresses in austenitic stainless steels subjected to creep deformation. Creep processes increase the overall inelastic strain in a material and this correspondingly alters the internal strain state. A combination of in-situ and static neutron diffraction measurements was conducted to assess the internal strain generation through a material’s creep life. These experiments have revealed similarities between internal strain generation during primary creep deformation and that during monotonic tensile deformation leading to the conclusion that common mechanisms may be responsible. Such studies are important in the current context when a number of power plants are being life extended. Components in high temperature service applications undergo a number of creep-fatigue cycles during their operation. It is vital to have accurate and robust life assessment procedures that can take account of long-term internal strain evolution effects to maintain economic but safe operation and avoid costly repairs or replacement.

Commentary by Dr. Valentin Fuster
2011;():227-234. doi:10.1115/PVP2011-57976.

Grade 92 steel is a class of the Creep Strength-Enhanced Ferritic (CSEF) steels developed for use in boilers and piping systems of ultra-supercritical steam fossil power plants. Although creep strength is a primary concern, consideration of the interaction of creep and fatigue damage is also important in evaluating the integrity of components as they will experience a range of cyclic loading. Although some studies have already been made on creep-fatigue behavior of this steel, test data under the conditions of creep damage dominance more relevant to plant evaluation, need to be supplemented. Girth welds often constitute critical locations dominating the integrity of piping systems and their creep-fatigue behavior is also of significant importance. Such a situation prompted the authors to initiate a study aiming at development of an extensive database on creep-fatigue behavior of base metal and welded joints of Grade 92 steel and establishment of an appropriate life estimation procedure. For the period of one and half year, a number of creep-fatigue data have been obtained on the base metal and cross-weld specimens at a wide range of loading conditions. Superiority of the energy-based approach to the conventional time fraction or ductility exhaustion approach for predicting creep-fatigue life was confirmed by their application to these creep-fatigue tests.

Topics: Creep , Fatigue , Steel
Commentary by Dr. Valentin Fuster
2011;():237-243. doi:10.1115/PVP2011-57107.

A finite element analysis has been performed to investigate the effects of warm prestressing of a pre-cracked PTS-D (Pressurized Thermal Shock Disk) specimen, for comparison with the experimental work conducted by the Belgium SCK-CEN organisation under the European NESC VII project. The specimen was loaded to a maximum loading at −50 °C, unloaded at the same temperature, cooled down to −150 °C, and then re-loaded to fracture at −150 °C. This is a loading cycle known as a LUCF cycle. The temperature-dependant tensile stress-strain data was used in the model and the finite element software ABAQUS was used in the analysis. The finite element results were used to derive the apparent fracture toughness by three different methods: (1) Chell’s displacement superposition method; (2) the local stress matching method; and (3) Wallin’s empirical formula. The apparent fracture toughness values were derived at the deepest point of the semi-elliptical crack for a 5% un-prestressed fracture toughness of 43.96 MPam1/2 at −150 °C. The detailed results were presented in the paper.

Topics: Disks , Cycles , Thermal shock
Commentary by Dr. Valentin Fuster
2011;():245-254. doi:10.1115/PVP2011-57112.

This paper describes numerical analyses performed to simulate warm pre-stress (WPS) experiments conducted with large-scale cruciform specimens within the Network for Evaluation of Structural Components (NESC-VII) project. NESC-VII is a European cooperative action in support of WPS application in reactor pressure vessel (RPV) integrity assessment. The project aims in evaluation of the influence of WPS when assessing the structural integrity of RPVs. Advanced fracture mechanics models will be developed and performed to validate experiments concerning the effect of different WPS scenarios on RPV components. The Oak Ridge National Laboratory (ORNL), USA contributes to the Work Package-2 (Analyses of WPS experiments) within the NESC-VII network. A series of WPS type experiments on large-scale cruciform specimens have been conducted at CEA Saclay, France, within the framework of NESC VII project. This paper first describes NESC-VII feasibility test analyses conducted at ORNL. Very good agreement was achieved between AREVA NP SAS and ORNL. Further analyses were conducted to evaluate the NESC-VII WPS tests conducted under Load-Cool-Transient-Fracture (LCTF) and Load-Cool-Fracture (LCF) conditions. This objective of this work is to provide a definitive quantification of WPS effects when assessing the structural integrity of reactor pressure vessels. This information will be utilized to further validate, refine, and improve the WPS models that are being used in probabilistic fracture mechanics computer codes now in use by the NRC staff in their effort to develop risk-informed updates to Title 10 of the U.S. Code of Federal Regulations (CFR), Part 50, Appendix G.

Commentary by Dr. Valentin Fuster
2011;():255-263. doi:10.1115/PVP2011-57189.

In the framework of NESC VII European project, a large experimental program has been dedicated to demonstrate the Warm Pre Stressing (WPS) effect in different testing configurations. One of the CEA (France) contributions to this project is the realization of five point bending tests on large cruciform specimens considering different WPS loading cycles. The five cruciform specimens, sponsored by EDF (France) and IRSN (France), are made of 18MND5 steel. Two of them have been tested on a same LCF (Load-Cool-Fracture) loading cycle, including an isothermal preloading at TWPS = −30°C followed by a cooling down to TFRAC = −150°C at a constant load before an isothermal reloading up to fracture at TFRAC . The results presented in this paper give a successful demonstration of the Warm Pre Stressing effect in biaxial loading conditions on a LCF cycle.

Commentary by Dr. Valentin Fuster
2011;():265-272. doi:10.1115/PVP2011-57190.

This study aims to evaluate the relevance of the active plasticity hypothesis in predicting the risk of cleavage fracture during a WPS loading cycle. We define a critical loading path, along which the active plasticity hypothesis might fail. On this critical loading path, Beremin local model is compared to a stress-based criterion model. The chosen critical loading path is a LTF (Load-Transient-Fracture) cycle, including an isothermal preloading up to F1WPS = 42 kN at TWPS = −25°C followed by a cooling down to TFRAC = −150°C with a progressive loading up to F2WPS = 45 kN before an isothermal reloading up to fracture at −150°C. The final loading level (F2WPS = 45 kN) is chosen so that the plasticity is not active during the cooling phase while the stress-based criterion is. Two sets of experiments, performed on normalized compact tensile specimens made of 16MND5 steel, have demonstrated that the active plasticity was a necessary condition for cleavage fracture.

Topics: Plasticity
Commentary by Dr. Valentin Fuster
2011;():273-281. doi:10.1115/PVP2011-57259.

Engineering components, particularly those containing weldments, may contain small crack-like defects that experience combinations of primary and secondary stresses during service. A new function, g(), has been introduced previously to quantify the influence of plasticity interaction under combined primary and secondary loading on a components crack driving force. This paper compares g() with experiments performed to consider g() over a range of plasticity values. This experimental programme was performed on scalloped notch three point bend specimens that had experienced a pre-compression to induce a residual stress field before being tested to failure over a range of temperatures (−150, −90 and −50 °C). Samples which did not undergo a pre-compression were also tested to provide an estimate of the materials fracture toughness at the temperature in question. Through analysing the experimental results it is clear that further material characterisation is required. This paper, therefore, only presents the initial results at this stage. However, as a pessimistic interpretation of the results has been made, and since both the existing R6 and the g() plasticity interaction parameters are acceptable, the experiments provide useful validation to both methods.

Topics: Stress
Commentary by Dr. Valentin Fuster
2011;():283-292. doi:10.1115/PVP2011-57373.

Oak Ridge National Laboratory (ORNL) is conducting a series of numerical analyses to simulate a large scale mock-up experiment planned within the European Network for S tructural Int egrity for L ifetime Manage ment – non-RPV Components (STYLE). STYLE is a European cooperative effort to assess the structural integrity of (non-reactor pressure vessel) reactor coolant pressure boundary components relevant to ageing and life-time management and to integrate the knowledge created in the project into mainstream nuclear industry assessment codes. ORNL contributes “work-in-kind” support to STYLE Work Package 2 (Numerical Analysis/Advanced Tools) and Work Package 3 (Engineering Assessment Methods/LBB Analyses). This paper summarizes the current status of ORNL analyses of the STYLE Mock-Up3 large-scale experiment to simulate and evaluate crack growth in a cladded ferritic pipe. The analyses are being performed in two parts. In the first part, advanced fracture mechanics models are being developed and performed to evaluate several experiment designs taking into account the capabilities of the test facility while satisfying the test objectives. Then these advanced fracture mechanics models will be utilized to simulate the crack growth in the large scale mock-up test. For the second part, the recently developed ORNL SIAM-PFM open-source, cross-platform, probabilistic computational tool will be used to generate an alternative assessment for comparison with the advanced fracture mechanics model results. The SIAM-PFM probabilistic analysis of the Mock-Up3 experiment will utilize fracture modules that are installed into a general probabilistic framework. The probabilistic results of the Mock-Up3 experiment obtained from SIAM-PFM will be compared to those results generated using the deterministic 3D nonlinear finite-element modeling approach. The objective of the probabilistic analysis is to provide uncertainty bounds that will assist in assessing the more detailed 3D finite-element solutions and to also assess the level of confidence that can be placed in the best-estimate finite-element solutions.

Commentary by Dr. Valentin Fuster
2011;():293-302. doi:10.1115/PVP2011-57546.

Local approach methods are becoming increasingly popular as practical tools for cleavage fracture toughness prediction. Their application involves two distinct elements: calculation of ‘individual’ probabilities of failure, dictated by the local mechanical fields; and summation of these failure probabilities to predict the probability of component failure. In this work, we demonstrate that development of the local approach methods to date has been essentially focused on improving the criterion for predicting local failure as a function of the local mechanical fields. Yet, the existing methods fail to predict with sufficient accuracy the effects of irradiation and defect geometry on fracture toughness when the calculations are based on a common set of model parameters. A possible reason for this, common to all methods, is found in the calculation of the cumulative failure probability, which is based on the weakest-link argument. We discuss the implications of the weakest-link assumption, identify those situations where it needs to be reconsidered, and propose future work that will increase our understanding for improving the calculation of global failure probability.

Commentary by Dr. Valentin Fuster
2011;():303-312. doi:10.1115/PVP2011-57551.

PERFORM 60 (Prediction of the effects of radiation for reactor pressure vessel and in-core materials using multi-scale modelling — 60 years foreseen plant lifetime) is a 48-month project of the 7th Framework of the European Atomic Energy Community (EURATOM) being carried out under the auspices of the Directorate General Research, Technology and Development (DG.RTD) of the European Commission. Launched in March 2009, and building on the achievements of PERFECT, a EURATOM 6th Framework project, PERFORM 60 has as its main objective the development of multi-scale modelling tools integrated onto a common software platform, aimed at predicting for PWRs (i) the effects of irradiation on RPV materials (low alloy bainitic steels), (ii) the combined effects of irradiation and corrosion on internals (austenitic stainless steels). Accordingly, PERFORM 60 is based on two main technical sub-projects: SP1 (RPV) and SP2 (Internals). An integration work package within both SP1 and SP2 serves to facilitate software development. A Users’ Group (SP3) supports the main technical sub-projects and allows representatives of constructors, utilities, regulators and research organizations from Europe and further afield to receive the information and training needed to make their own appraisal as to the validity of the developed tools. A significant effort is also being made to train young researchers in the field of physical modelling of materials degradation due to neutron irradiation. Against this background, the paper provides an overview of SP1, highlighting the various models and methods being developed, building on the achievements of PERFECT, to describe the evolution of flow properties of low-alloy steels with irradiation and address their subsequent effects on cleavage fracture behaviour.

Commentary by Dr. Valentin Fuster
2011;():313-322. doi:10.1115/PVP2011-57613.

This paper describes numerical and experimental investigations on transferability of material properties obtained by testing of small scale specimens to a real component. The presented study is related to the experimental and analytical work performed on Mock-up3, which is one of three unique large scale Mock-ups tested within the European project STYLE. Mock-up3 is foreseen to investigate transferability of material data, in particular fracture mechanics properties. An important part of this work is to study constraint effects on different small scale specimens and to compare their fracture behaviour with the fracture behaviour of a large scale (component like) structure. The Mock-Up3 is an original part of a surge line made of low alloy steel 20 MnMoNi 5 5 (which corresponds to SA 508 Grade 3, Cl. 1). The goal of the test is to introduce stable crack growth of an inner surface flaw until a break through the wall occurs. To design such a test reliable fracture mechanics material properties must be available. Usually, these material data are obtained by testing small specimens, which are subsequently used for the assessment of a large scale structure (component). This is being done under the assumption that these “small scale” material properties are fully transferable to “large scale” components. It is assumed that crack initiation in the ductile tearing regime is rather independent of the crack shape, a/W ratio, loading condition or size of the specimen (constraint effects). In order to check the aforementioned assumption and to improve understanding of the physical process leading to failure of cracked components comprehensive experimental and analytical work is being undertaken in STYLE. This paper summarizes Up-To-Date available results, which have been achieved during the first 15 months of the project.

Commentary by Dr. Valentin Fuster
2011;():323-337. doi:10.1115/PVP2011-57810.

In order to exclude the possibility of catastrophic failure of safety relevant pressure-retaining components in nuclear power plants during operation, the “integrity concept” is applied in Germany. It has been developed over the past 30 years on the basis of the safety criteria of the guidelines for damage precautions, as set by the German Advisory Committee on Reactor Safeguards (RSK-LL) and the basic safety concept. The integrity concept is based on the requirements of proven basic safety characteristics: design, construction, material, and manufacturing. Complementary elements (so-called redundancies) also have to be considered: the principle of inspections by multiple parties, the worst-case principle, the principle of plant monitoring and documentation, as well as the principle of verification. This includes consideration of possible operational damage mechanisms in terms of interaction between causes and consequences as well as consideration of any new knowledge, if necessary in the framework of additional safety analyses. The integrity concept is applied in German PWR plants, i.e. the main coolant lines and their connecting lines, as well as in German BWR plants with large main steam and feedwater lines within the pressure-retaining boundary (primary system) up to the outer containment. A fracture mechanics safety analysis with postulated defect sizes as well as the experimental basis of load behavior are essential parts of the integrity concept. The measures determined and verified in this way guarantee that no major deviations from design values occur. This is confirmed by periodic in-service inspections. The advantage of this concept is the application of reduced leakage assumptions for important safety-related pipe systems, i.e. either through application of a conservative postulate of 0.1 F according to RSK-LL or reduced leak sizes. The operational experience gained with this concept has been positive in all German PWR and BWR plants. This paper demonstrates that the German integrity concept has been proven to be successful over the past 30 years and reflects the state of the art. Its implementation in plants and its incorporation in the German Nuclear Codes (KTA rules) contribute to the safety of German nuclear power plants in terms of precautionary damage prevention.

Commentary by Dr. Valentin Fuster
2011;():339-346. doi:10.1115/PVP2011-58029.

The warm pre-stress (WPS) of a flawed structure occurs when it is pre-loaded at high temperature in the ductile domain then cooled and loaded up to fracture in the brittle to ductile transition temperature domain. This load history is a feature of RPV accidental transients of LOCA type. Numerous tests on non irradiated specimens and structures have shown the favourable effect of WPS on fracture behaviour. Theorical knowledge let expect that the WPS effect occurs by the same way on irradiated material, but experimental approach had to be completed in such conditions. The experimental program presented in the present article consists in fracture toughness tests under WPS loading conditions performed on two RPV steels irradiated up to a fluence of 6,5.1019 n/cm2 . The CT12.5 specimens used for these tests had been irradiated in the capsules of the pressure vessel surveillance program of two french reactors. Different types of WPS load history have been applied to cover typical accidental transients. All the results obtained confirmed for an irradiated steel the two assumptions generally made about the WPS effect: no fracture occurred during the cooling step of the loading even at high load level and the mean fracture toughness value is higher than that measured with conventional mono-temperature tests.

Commentary by Dr. Valentin Fuster
2011;():347-356. doi:10.1115/PVP2011-58080.

In this work the significance of residual stresses for ductile fracture was investigated. The treatment of residual stresses as expressed in fracture assessment procedures such as the R6 method is believed to be very conservative for ductile materials, when fracture occurs at high primary loads. Earlier numerical studies have reinforced this belief. Tests on notched 3PB specimens with and without residual stresses were conducted on two ferritic steels. The residual stresses were introduced by applying a compressive pre-load on notched specimens. The tests were designed to achieve crack initiation at load levels around the limit load. The crack growth in the tests was measured by a compliance method and by colour marking of the crack surface. The crack-tip driving force J was evaluated numerically for specimens with and without residual stresses. The experimental results show that the residual stresses clearly contribute to J at low primary loads. However, this contribution diminishes as the primary loads increase. The experimental results were also compared with results evaluated using the R6 procedure. These comparisons revealed an overly high conservativeness in R6 for cases with residual stresses compared to the conservativeness for cases without residual stresses.

Commentary by Dr. Valentin Fuster
2011;():359-366. doi:10.1115/PVP2011-57206.

Cracks found in a nuclear power plant reactor coolant system (RCS), such as primary water stress corrosion cracking (PWSCC) and intergranular stress corrosion cracking (IGSCC), usually have natural crack front shapes that can be very different from the idealized semi-elliptical or rectangular shapes considered in engineering handbooks and other analytical solutions based on limited shapes. Simplifications towards semi-elliptical shape or rectangular shape may potentially introduce unnecessary conservatism when the simplified shape has to contain the actual crack shape. On the other hand, it is very time-consuming to create a three-dimensional (3D) finite element (FE) model to simulate crack propagation in a natural shape using existing public-domain software like ABAQUS or ANSYS. In this study, a local deformation-based mesh-mapping (LDMM) method is proposed to model cracks with a natural front shape in any 3D structures. This methodology is first applied to model circumferential surface cracks with a natural crack front shape in the cross-sectional plane of a cylinder. The proposed new method can be applied to simulate both shallow and deep cracks. Also discussed in this paper is a direct method to reproduce welding residual stresses in the crack model using temperature fields combined with other sustained loads to predict crack propagations. With this novel LDMM method, natural crack fronts and non-planar crack faces can be easily modeled. The proposed new method can be used to generate a high-quality finite element model that can be used for both linear-elastic fracture mechanics (LEFM) and elastic-plastic fracture mechanics (EPFM) analyses. The study case illustrates that the proposed LDMM method is easy to implement and more efficient than the existing commercial software.

Commentary by Dr. Valentin Fuster
2011;():367-384. doi:10.1115/PVP2011-57465.

Weld overlays (WOL) of alloys believed to possess superior stress corrosion cracking (SCC) resistance are typically applied over welds made with SCC-susceptible alloys with the expectation that they will act as a barrier to SCC. The objective of this work is to investigate the behavior of a crack initiated in Alloy 182 as it approaches the Alloy 52M WOL interface. For this purpose, an Alloy 52M WOL was deposited on a double-J Alloy 182 weld. Compact tension specimens were fabricated with the notch in Alloy 182 and oriented towards the WOL, and tested in a simulated PWR environment. The first such test revealed that the SCC rates in Alloy 182 were found to decrease by an order of magnitude ahead of the interface, and that the crack advanced from Alloy 182 into Alloy 52M. The post test examination found that crack branching occurred at the interface between the two alloys. Growth in Alloy 52M along the interface appears severe, approx. 10−10 m/s. While for the most part (70%) the crack propagated along the interface, SCC cracking was also found to extend into Alloy 52M along the original direction. This cracking is substantial, yielding SCC rates of 10−11 m/s.

Commentary by Dr. Valentin Fuster
2011;():385-398. doi:10.1115/PVP2011-57649.

The objective of this work is to determine the cyclic and stress corrosion cracking (SCC) crack growth rates (CGRs) in a simulated PWR water environment for Alloy 690 heat affected zone (HAZ). In order to meet the objective, an Alloy 152 J-weld was produced on a piece of Alloy 690 tubing, and the test specimens were aligned with the HAZ. The environmental enhancement of cyclic CGRs for Alloy 690 HAZ was comparable to that measured for the same alloy in the as-received condition. The two Alloy 690 HAZ samples tested exhibited maximum SCC CGR rates of 10−11 m/s in the simulated PWR environment at 320°C, however, on average, these rates are similar or only slightly higher than those for the as-received alloy.

Commentary by Dr. Valentin Fuster
2011;():399-406. doi:10.1115/PVP2011-57693.

Slitting method residual stress measurements (Hill Engineering and UC Davis) and finite element weld simulation (US Nuclear Regulatory Commission) have been conducted in order to evaluate both the residual stress intensity factor and residual stress profiles for two compact tension coupon blanks. The two compact tension coupon blanks were provided by Argonne National Lab (ANL) and are similar to coupons used in ongoing stress corrosion cracking (SCC) studies in weld metal. The experimental data and finite element results are in reasonable agreement, showing similar trends in calculated residual stress profiles. Results from the work document the effect of specimen size and location on residual stress profiles, and could be used to determine the degree to which residual stresses affect crack growth measurements made in similar coupons.

Commentary by Dr. Valentin Fuster
2011;():407-420. doi:10.1115/PVP2011-57703.

Dissimilar metal welds of filler metal 182 (ENiCrFe-3) in the primary loop of pressurized water reactor (PWR) nuclear plants are susceptible to primary water stress corrosion cracking (PWSCC) after decades of service. Repair or mitigation has been routinely accomplished by installing a structural weld overlay (SWOL) on the filler metal 182 weld joint with the more PWSCC resistant filler metal 52M (ERNiCrFe-7A). The typical dissimilar metal joint consists of a low alloy steel vessel nozzle welded to an austenitic stainless steel safe end. The SWOL extends from the low alloy steel nozzle over the safe end and most often onto the adjoining wrought or cast stainless steel pipe. Field experience shows that filler metal 52M is susceptible to hot cracking when welding on certain heats of centrifugally cast stainless steel piping. This report evaluates 52M hot cracking when welding on CASS piping and provides the likely cause and mechanism for the cracking. The synergistic influence of silicon (Si) and sulfur (S) elements on the weld bead shape and dilution that leads to hot cracking is investigated. In addition, studies on the influence and use of the gas tungsten arc welding (GTAW) power ratio parameter for 52M overlays are presented.

Commentary by Dr. Valentin Fuster
2011;():421-426. doi:10.1115/PVP2011-57768.

This paper describes measurements of residual stress in coupons used for fracture mechanics testing. The primary objective of measurement is to quantify the distribution of residual stress acting to open (and/or close) the crack across the crack plane. The slitting method and the contour method are two destructive residual stress measurement methods particularly capable of addressing that objective, and these were applied to measure residual stress in a set of identically prepared compact tension (C(T)) coupons. Comparison of the results of the two measurement methods provides some useful observations. Results from fracture mechanics tests of residual stress bearing coupons and fracture analysis, based on linear superposition of applied and residual stresses, show consistent behavior of coupons having various levels of residual stress.

Commentary by Dr. Valentin Fuster
2011;():427-431. doi:10.1115/PVP2011-57782.

In nuclear power plants, the automated narrow gap welding (NGW) technique has been widely used in joining pipes in primary coolant system. Meanwhile, to apply the leak-before-break (LBB) design, mechanical properties of the structural materials of piping systems should be evaluated, especially at various welded joints. In this study, the fatigue crack growth rate (FCGR) and fracture resistance of stainless steel weld fusion zone and nearby heat affected zone (HAZ) were evaluated to support the LBB application. Tests were performed at plant operating temperature (315°C) and room temperature. FCGR test results showed higher crack growth rate in HAZ and the weld fusion zone compare to the base metal. Fracture resistance tests showed higher fracture toughness in HAZ compared to the weld fusion zone. By analyzing the microstructures in the weld fusion zone and HAZ, their effects on crack growth rate were discussed. Also, the crack growth behavior in circumferential direction was compared with that in radial direction which was previously reported.

Commentary by Dr. Valentin Fuster
2011;():433-441. doi:10.1115/PVP2011-57868.

The duration of post weld heat treatments (PWHT) applied to thick section multi-pass dissimilar metal welds (DMW), involving ferritic creep resistant steels of differing chromium content, are shown to have a considerable impact on the performance of the welded joint. Welding consumables of alloy types P22 and P24 have been used to form joints with P91 base alloy which were subsequently post weld heat treated for varying durations. High resolution transmission electron microscopy (TEM) has been exploited in the characterisation of precipitation in the weld material and the heat affected zone. It has been shown that uphill diffusion of carbon from the low to the higher alloy material during PWHT and creep test conditions occurs in all specimens. Selected area diffraction (SAD) and convergent beam electron diffraction (CBED) studies of carbon extraction replicas reveal extensive dissolution of M23 C6 and M7 C3 carbides in the decarburised zone of the weld alloy subsequent to post weld heat treatments. However, welds completed using Nb and V containing consumables retain a fine distribution of MX precipitation in the carbon depleted regions after PWHT. The retention of these microstructure stabilising carbonitrides facilitates the preservation of an ultra fine sub-grain microstructure, thus avoiding recrytallisation which is invariably observed in post weld heat treated P22:P91 DMWs. Cell size comparisons of the sub-grain microstructures have been investigated utilising channelling contrast back scattered scanning electron images of as welded and post weld heat treated material.

Commentary by Dr. Valentin Fuster
2011;():443-453. doi:10.1115/PVP2011-57957.

By getting the data from an ordered set of Gauss points on the flow line of a material point that passes near the weld pool, the evolution of the stress/strain tensor fields is visualized. The principal plastic strain tensor, principal deviatoric stress tensor, hydrostatic stress and temperature are visualized. This is done for three weld distortion mitigation strategies: i) pre-bending by applying a prescribed displacement, ii) applying a tensile load to the weld and iii) applying side heaters to the weld. Visualizing the evolution of the principal stress and strain vectors gives interesting insight into the mechanics of plastic deformation near a weld pool.

Commentary by Dr. Valentin Fuster
2011;():455-464. doi:10.1115/PVP2011-57960.

Using a frame-work for exploring a design space in Computational Weld Mechanics (CWM), a recent direct-search algorithm from Kolda, Lewis and Torczon is modified to use a least-square approximation to improve the method of following a path to the minimum in the algorithm. To compare the original and modified algorithms, a CWM optimization problem on a 152 × 1220 × 12.5 mm bar of Aluminum 5052-H32 is solved to minimize the weld distortion mitigated by a side heating technique. The CWM optimization problem is to find the best point in the space of side heater design parameters: power, heated area, longitudinal and transverse distance from the weld such that the final distortion is as low as possible (minimized). This CWM optimization problem is constrained to keep the stress level generated by the side heaters, in the elastic region to avoid adding an additional permanent plastic strain to the bar. The number of iterations, size of DOE matrix required and CPU time to find the minimum for the two algorithms are compared.

Commentary by Dr. Valentin Fuster
2011;():465-475. doi:10.1115/PVP2011-57973.

The characterization of the fracture toughness in weld and HAZ materials employed in nuclear power plant (NPP) piping is essential to assess the structural integrity of the piping systems under various loading conditions, when cracks/flaws are present in the weld/HAZ material regions. The current investigation was undertaken to determine the resistance to crack propagation using ASTM E1820 standard based compact tension, 1T-C(T), specimens with initial crack in the weld centerline (WCL) and adapted 1T-C(T) specimens with slanted cracks in the heat-affected-zone (HAZ) materials. The experiments were conducted at loading rates ranging from quasi-static to dynamic rates estimated for seismic loading at normal operating temperature. The fracture tests were also conducted at various loading rates to verify the occurrence of phenomena such as dynamic-strain-aging (DSA) that could potentially cause significant changes in the material toughness, J-R curves of the weld and HAZ materials. The investigation also compared the fracture toughness obtained from 1T-C(T) fracture tests with those obtained from impact loading on single-edge-notch-bend (SENB) specimens in a drop-weight-tear-test (DWTT) machine and others obtained from SEN(B) specimens tested at quasi-static loading rates. The range of crack growth resistance curves obtained from the various fracture tests and specimens were used to develop bounding JIc and J-R curves that are to be employed in computational finite element analysis (FEA) that are used determine crack propagation and stability in the weld/HAZ materials of NPP plant piping.

Commentary by Dr. Valentin Fuster
2011;():477-493. doi:10.1115/PVP2011-57990.

The paper focuses on hot cracking susceptibility analysis and a post-processor for a computational weld mechanics (CWM) framework to identify the transient 3D region susceptible to hot cracking for a welded structure. The Sigmajig hot cracking analysis analyzed by Zacharia in [1] is used. The specimen is 50 × 25 × 0.25 mm 316 stainless steel sheet welded with a TIG process with a constant transverse force applied on the side surfaces. The tensile traction, welding power, welding speed, and 4 double ellipsoid shape parameters are varied in a sensitivity analysis of hot cracking wrt the transverse tensile traction and several welding speeds for which power per unit length is kept constant. A control problem solved to adjust 4 double ellipsoid shape parameters for different welding speeds with constant power per unit length. The analysis includes 117 analyses for the control part and 28 analyses for the sensitivity part that are implemented in an automated mode of the CWM framework to save user time in implementation. The user prepares one single base project setup and 117+28 CWM analyses by the user specifying a DOE-matrix.

Commentary by Dr. Valentin Fuster
2011;():497-503. doi:10.1115/PVP2011-57022.

This paper analyzes the notch effect and presents a methodology, based on failure assessment diagrams and the notch analysis approaches based on the theory of critical distances, for the structural integrity assessment of notched components, which allows more accurate structural analyses to be made. The methodology is applied to a set of tests performed on PMMA single edge notched bending (senb) specimens, providing better results than those obtained when the analysis is performed considering that notches behave as cracks.

Commentary by Dr. Valentin Fuster
2011;():505-511. doi:10.1115/PVP2011-57099.

Fitness-for-service assessment of pressure vessels and piping often involves the evaluation of existing or potential crack-like flaws to guard against fracture or leaks that could be caused by the presence of such flaws. This paper presents an inelastic fracture mechanics model that has been developed to evaluate longitudinal surface cracks in pipelines, piping and pressure vessels subjected to internal pressure loading. The model uses the J-integral parameter to predict toughness-dependent failure and an effective flaw concept to predict flow-strength dependent failure. The concepts of the model are reviewed. Then, the model is used to evaluate the results of in-service failures and full-scale burst testing of steel pipe and pressure vessel samples. Application of the model to remaining life assessment based on inspection data and hydrostatic testing results is illustrated. Stress-corrosion cracking (SCC) and fatigue are considered as possible crack-growth mechanisms. Examples of typical remaining crack-growth life calculations are presented using both deterministic and probabilistic methods. The benefits of each method are discussed. Finally, planned future additions to the model are presented.

Commentary by Dr. Valentin Fuster
2011;():513-519. doi:10.1115/PVP2011-57333.

Clarification of creep damage mechanisms and establishment of remaining life prediction methods of weldment parts of P91 boiler pipings are important subjects to maintain reliable operation of boilers in thermal power plants. In order to develop a creep damage assessment method of weldment parts of P91 pipings, internal pressure creep tests were conducted on P91 steel longitudinally welded tubes and a previously proposed void growth simulation method is applied to predict void growth behavior. Failure occurred at the heat affected zone without significant deformation. It was found from observation of creep damage interrupted specimens that initiation of creep voids concentrated at the mid-thickness region rather than the surface. It was suggested that triaxial stress states caused acceleration of creep damage evolution in the heat affected zone resulting in internal failure of the tube specimens. Void growth behavior in the heat affected zone was well predicted by the previously proposed void growth simulation method. The void growth prediction method is applied to predict creep damage induced by void initiation and growth in a weldment part of an actual P91 pipe. From comparison of void number density between measurement for a weldment part of a retired elbow pipe and prediction by the simulation, good agreement is obtained indicating the void growth simulation method can be applied to creep damage assessment of weldment parts in actual boiler piping.

Commentary by Dr. Valentin Fuster
2011;():521-526. doi:10.1115/PVP2011-57438.

High-chromium steels have excellent high temperature strength properties and are therefore used for piping systems of ultra-super-critical (USC) power plants. Recently, type-IV damages occurred in heat affected zone (HAZ) and had often caused unexpected shutdown. The type-IV damages are due to dense concentration of creep voids on grain boundaries, and then quantitative prediction of void growth is an important subject to prevent a critical accident. To that end, it is necessary to obtain stress conditions under plant operation with high accuracy. The hoop or circumferential stress caused by steam or internal pressure can easily be estimated by a conventional FEM code. However estimation of system load caused by thermal expansion of pipe materials is difficult because of the complex structures of piping system. In this study, we developed a three-dimensional FEM code which can estimate both thermal expansion and creep deformation of complex piping system. Consequently, accurate stress analyses of weldments under plant operation become now enabled.

Commentary by Dr. Valentin Fuster
2011;():527-539. doi:10.1115/PVP2011-57697.

A high energy piping (HEP) asset integrity management program is important for the safety of power plant personnel and reliability of the generating units. HEP weldment failures have resulted in extensive damage of components and significant lost generation. The main steam (MS) piping system is one of the most critical HEP systems. Creep damage assessment in MS piping systems should include the evaluation of multiaxial stresses associated with the field conditions. Typical creep life assessment stress parameters and estimated failure times are evaluated and compared with those of three MS piping system girth weld creep failures. This paper presents empirical data indicating that lead-the-fleet girth welds of MS piping systems have creep failures which can be successfully predicted by a multiaxial stress parameter, such as maximum principal stress. The calibration study indicates that the parent metal maximum principal stress should be increased by more than 20% to predict reasonable circumferential weldment lives in 2-1/4Cr-1Mo material. The correlation of other stress parameters, such as hoop stress, longitudinal membrane stress, and the standard as-designed ASME B31.1 sustained load stress do not provide an adequate ranking of the most critical girth welds subject to creep. In some piping systems, it is possible that spool-to-spool and circumferential variations in pipe wall thicknesses may influence the weldment life consumption estimates. Therefore, field wall thickness measurements should be taken at the most critical stress locations and applied to the life consumption evaluations.

Topics: Creep , Welded joints , Pipes , Steam
Commentary by Dr. Valentin Fuster
2011;():541-546. doi:10.1115/PVP2011-58015.

For over three decades, the long seam-welded low alloy steel, Grades 11 and 22, high energy piping in fossil power plants has been considered at risk of premature damage and failure. The experience with piping damage and failures has been documented and extensively studied, but there remains a lack of perspective on how the overall experience with such piping, including that of the large “survivor” population, compares with what one may expect with the design rules used in their construction. Such a perspective can be useful in helping decide on suitable design rules for this class of piping. This paper focuses on an aggregate, global, semi-quantitative evaluation of the damage and failure experience in fossil plant low alloy steel long seam-welded piping in terms of a rate of failure measured against the performance of the overall population. A key aspect of the evaluation is the consideration of the survivor population, particularly important since the documented cases of failure and damage represent a very small fraction of the population of relevant components. The damage and failure rates have been derived from the Electric Power Research Institute database, using an exposure parameter represented by the product of operating time and length of piping. The rates are viewed against the backdrop of the statistical scatter band of base metal stress rupture data used in development of the ASME Code design allowable stresses and against the weld strength reduction factors recently adopted by the ASME Boiler Pressure Vessel Code, Section I and the Power Piping Code, B31.1.

Topics: Alloy steel , Pipes , Failure
Commentary by Dr. Valentin Fuster
2011;():549-557. doi:10.1115/PVP2011-57207.

The leak-before-break (LBB) applicability is stated in General Design Criterion 4 (GDC-4) of Title 10 of the Code of Federal Regulation Part 50 (10 CFR 50). GDC-4 requires that analyses reviewed and approved by the U.S. Nuclear Regulatory Commission (NRC) demonstrate that the probability of fluid system piping rupture is extremely low under conditions consistent with the design basis for the piping, in order that dynamic effects associated with postulated pipe ruptures in nuclear power units may be excluded from the design basis. Standard review plan 3.6.3 (SRP-3.6.3) further requires a simultaneous safety margin of two and ten on the flaw size and leak rate detectability, respectively, for deterministic analyses, believing that the very conservative and restrictive safety margins would lead to extremely low probability of fluid system piping rupture. The technology advancements of recent years make it possible to numerically quantify the probability of rupture with confidence. Planned for completion within the next six years, a long-term, large-scale assessment tool, xLPR, is currently being developed by the U.S. NRC, in cooperation with the nuclear industry, to assess the extremely low probability of rupture. The tool will include comprehensive evaluations both before and after through-wall cracks are developed in the degraded components. In this study, we are going to utilize a simplified methodology to investigate the probability of piping rupture for a postulated through-wall crack. The conditional probability, when multiplied by the probability of having a through-wall crack during the life time of plant service, produces an overall probability of piping rupture. The major quantifiable uncertainties, such as the uncertainties associated with the material tensile properties and fracture toughness, and flow-path crack morphology parameters will be modeled as correlated random variables in this paper. Efficient Dimension-Reduction methods will be applied to predict this conditional probability and the results will be compared with the Monte Carlo simulation method. As a sample application of the proposed method, the relationship between the magnitude of the conditional probabilities and the required leak rate detection capability will be established.

Commentary by Dr. Valentin Fuster
2011;():559-568. doi:10.1115/PVP2011-57591.

Dissimilar metal welds (DMWs) involving Alloy 600 and its equivalent weld metal, such as Alloy 82 are used in a number of outlet feeders in CANDU® reactors. The higher operating temperature (∼300°C) leads to concerns that primary water stress corrosion cracking (PWSCC) might occur. This paper demonstrates the low rupture probability of DMWs due to PWSCC using probabilistic fracture mechanics (PFM) method with several critical assumptions validated by experiments. The leak-before-break is also demonstrated by comparison of the leak probability with rupture probability. A sensitivity study is carried out to investigate the effect of various factors on failure probabilities. Credible leak detection and action play an important role in limiting the rupture probability, while in-service inspection (ISI) has little effect to reduce rupture probability.

Topics: Metals , Welded joints
Commentary by Dr. Valentin Fuster
2011;():569-575. doi:10.1115/PVP2011-57595.

Computation of large break probabilities in pipes when initiating cracks are the dominant degradation mode is difficult, because the problem is dominated by the probability of initiating multiple cracks around the pipe circumference and having them coalesce and grow to become long prior to penetrating the wall to become a leak. The purpose of this paper is to describe two techniques for evaluating very low large break probabilities in pipes with multiple initiating cracks: (i) combining initiation and growth probabilities by a convolution integral, and (ii) sorting through sets of sampled random variables and performing detailed (lifetime) calculations only for particularly “severe” sets. These techniques are demonstrated in an example problem involving primary water stress corrosion crack (PWSCC) initiation and subsequent growth in a piping weldment with high residual stresses by use of a probabilistic fracture mechanics code, PRAISE-CANDU, which is under development to address specific degradation issues in CANDU® reactors.

Topics: Failure , Probability
Commentary by Dr. Valentin Fuster
2011;():577-584. doi:10.1115/PVP2011-57740.

In US, definition of the Leak-Before-Break (LBB) approach and criteria for its use are provided in NUREG-1061. Volume 3 of NUREG-1061 defines LBB as “[[ellipsis]]the application of fracture mechanics technology to demonstrate that high energy fluid piping is very unlikely to experience double-ended ruptures or their equivalent as longitudinal or diagonal splits.” Current LBB evaluation uses a factor of safety of two (2) on critical flaw size and a factor of safety of ten (10) on detectable leakage to deterministically analyze, that for a given set of input those factors are achieved. Typical input for LBB evaluation consists of pipe geometry, material properties (both elastic and plastic), crack morphology, loads, and operating pressure and temperature. Since LBB has recently been applied for pipes with weld overlays (WOL), thickness, material properties, and crack morphology of WOL also becomes important. However, in real structure all the design parameters (input) for LBB evaluation are inherently random in nature. The current work includes randomness in the critical design input parameters for LBB evaluation. Based on the result of this study reliability (or its compliment, probability of failure) curves are obtained based on the randomness in the critical input parameters. A piping system is considered to fail the LBB evaluation if the actual leakage through the pipe is less than the required leak rate which is calculated as ten times the plant minimum leak detection capability. Separate reliability curves are obtained for various minimum plant leak detection capability piping (e.g.,[[ellipsis]], 1, 0.5,[[ellipsis]], 0.1 GPMs) and for various piping systems (large diameter pipes such as reactor coolant loop hot leg and cold leg; and small diameter pipes such as pressurizer surge line, etc.). The reliability curves give an insight into the likelihood for a deterministic design input based LBB evaluation to remain valid in view of the in-situ variations.

Topics: Reliability , Design
Commentary by Dr. Valentin Fuster
2011;():585-593. doi:10.1115/PVP2011-57937.

Risk based treatment of degradation and fracture in nuclear power plants has emerged as an important topic in recent years. One degradation mechanism of concern is stress corrosion cracking. Stress corrosion cracking is strongly driven by the weld residual stresses (WRS) which develop in nozzles and piping from the welding process. The weld residual stresses can have a large uncertainty associated with them. This uncertainty is caused by many sources including material property variations of base and welds metal, weld sequencing, weld repairs, weld process method, and heat inputs. Moreover, often mitigation procedures are used to correct a problem in an existing plant, which also leads to uncertainty in the WRS fields. The WRS fields are often input to probabilistic codes from weld modeling analyses. Thus another source of uncertainty is represented by the accuracy of the predictions compared with a limited set of measurements. Within the framework of a probabilistic degradation and fracture mechanics code these uncertainties must all be accounted for properly. Here we summarize several possibilities for properly accounting for the uncertainty inherent in the WRS fields. Several examples are shown which illustrate ranges where these treatments work well and ranges where improvement is needed. In addition, we propose a new method for consideration. This method consists of including the uncertainty sources within the WRS fields and tabulating them within tables which are then sampled during the probabilistic realization. Several variations of this process are also discussed. Several examples illustrating the procedures are presented.

Commentary by Dr. Valentin Fuster
2011;():597-603. doi:10.1115/PVP2011-57026.

Structural components in a variety of industries are routinely subjected to static and cyclic loadings over an extended period of time; all while exposed to aggressive reactants in the environment. Of critical importance is the interaction between a material discontinuity and its environment. Environmentally assisted cracking (EAC) has been observed in chemical processing, piping and power generation industries to result in the rupture and failure of components. Notably, stress corrosion cracking (SCC) and liquid metal embrittlement (LME) have been extensively researched over the last century; however, there exists uncertainty in the microstructural failure mechanisms. Discrepancies are rooted in theories accounting for some liquid-solid couples but not others subjected to identical conditions. Recently, fracture mechanics experiments have revealed a life-dependency on the level of an initial static stress intensity of a cracked member. Unstable crack growth is observed above a threshold stress intensity value, KILME , or critical stress, σLME , below which rupture does not occur as quickly or at all. Experimental findings suggest that rupture can still occur at a lower stress intensity, provided a favorable microstructural orientation and/or critical stress is achieved that promotes crack initiation. Crack initiation processes are, therefore, the critical limiting factor in assessing the life of a component subjected to corrosive environments. An experimental study implemented Al7075-T651 notched tensile specimens in the study on delayed fracture of specimens subjected to liquid mercury at room temperature. Through varying loads and loading patterns, the stress and strain at the notch root can be evaluated and correlated with time to crack initiation. Life predictions can then be made based on the level of stress experienced in a structural component, even before the existence of a crack is detected.

Commentary by Dr. Valentin Fuster
2011;():605-612. doi:10.1115/PVP2011-57091.

Primary water stress corrosion cracking (PWSCC) in the weld metal of alloy600 is an issue of concern in a pressurized water reactor (PWR). As a countermeasure against PWSCC, water jet peening (WJP), which can change tensile residual stress into compressive residual stress, has been applied to welded joints. Microstructure in the target area of WJP has an influence of not only WJP but welding and machining. Especially machining introduces severe plastic deformations to the materials. So microstructure in the target area might lack thermal stability due to severe plastic deformation. Additionally the region that compressive residual stress by WJP is nearly up to 1mm from the surface of the target material. As PWRs are operated at about 596K for long term, the compressive residual stress by WJP may be relieved due to creep. In order to keep operating PWRs safety, the stability of the compressive residual stress by WJP at elevated temperature has been clarified. In this work, the results were obtained written below. As a result of thermal aging test, a relaxation of compressive residual stress at specimen surface layer occurred due to recovery of the plastic deformations by machining. This stress relaxation behavior followed Johnson-Mehl equation. However residual stress relaxation due to creep was very few. Therefore it has suggested that the compressive residual stress introduced in Alloy600 by WJP is confirmed to remain stable during long term operation under elevated temperature.

Commentary by Dr. Valentin Fuster
2011;():613-623. doi:10.1115/PVP2011-57170.

With the aim to investigate the influence of strain hardening on the stainless steels susceptibility to stress corrosion cracking, tests were conducted in PWR environment on CT specimens, taken from a 316L stainless steel sheet cold rolled to 40% in thickness reduction. The initial cracks obtained by the fatigue pre-cracking have an atypical ‘V’ shape with smaller propagation in the center of the CT thickness compared to nominal propagation observed at both sides. The initial explanation was to consider a stress intensity factor derived from classical reference solution on the basis of a straight crack front, and considering the local value of the crack depth in the equation. This assumption raised several problems analsyes in this paper. This particular shape of the initial defect may be related to several factors, and partly to the 40% cold rolling. It is likely that the hardening is not uniform, with a higher rate at the specimen sides than in the central area. In addition, significant residual stresses due to the gradient of mechanical properties are observed. Due to the high rate of work hardening by rolling of the sheet metal, a gradient of the mechanical properties through the thickness was determined, and the residual stresses profile induced by this process was measured. The variations obtained are consistent with each other: the material is more hardened in the vicinity of specimen surface and residual stresses are compressive in nature in the central part of the specimen and of tensile type on the flanks. All these data were firstly considered in order to assess their role regarding the particular form of the initial crack front obtained after fatigue: the 3D finite element calculations taking into account the true shape of the crack front demonstrate the relationship between the characteristics of the experimental crack front obtained after fatigue pre-cracking and the residual stresses. Moreover, from the residual stresses measured on the plate where samples have been machined/prepared, the residual stresses field in the specimen after its machining is calculated and then taken into account in the mechanical analysis. The characteristics of this field in addition to the mechanical loading applied during SCC testing can explain the crack propagation behavior observed experimentally.

Commentary by Dr. Valentin Fuster
2011;():625-629. doi:10.1115/PVP2011-57198.

The stress corrosion cracking susceptibilities of type 16MnR low-alloy steel in simulated oil refining environment were investigated by slow strain rate test. Many research works have been done to study the single solution parameter effect on the stress corrosion cracking, such as H2 S concentration or Cl− content, but less had been done for the interactive effect of the multiparameter. In this work, the separate and interactive effects of H2 S concentration, Cl− content, temperature and pH value on stress corrosion cracking susceptibilities were studied. By stepwise regression analysis of test results, the effect notabilities of medium parameters on stress corrosion cracking susceptive index were revealed. It could be conclude that the effect of H2 S concentrations was most notable to susceptive indexes, there was no notable correlativity between Cl− content and susceptive indexes, temperature and H2 S played interactive role on the susceptive indexes.

Commentary by Dr. Valentin Fuster
2011;():631-637. doi:10.1115/PVP2011-57244.

Stress corrosion cracking (SCC) is a common failure in stainless steel and nickel based alloys in high-temperature oxygenated aqueous systems. Because the propagating mode and morphology is particular at the SCC tip, it is necessary to investigate and understand in detail the mechanical state close to the SCC tip for improving the prediction accuracy of SCC growth rate in stainless steel and nickel based alloys in the nuclear pressure vessels and piping. By using a sub-model technique in commercial finite element analysis code, the meso-scale stress and strain field in the SCC tip constituted by the oxide film and base metal is simulated and analyzed in this study. And reasonable and operational mechanical parameters in the prediction method of SCC growth rate based on the slip-oxidation model are also discussed. The results of the investigation provide a new insight into the quantitative prediction of SCC growth rate in nuclear structural materials in high temperature water environments.

Commentary by Dr. Valentin Fuster
2011;():639-653. doi:10.1115/PVP2011-57463.

Alloys 600 and 182 are used as structural materials in pressurized water reactors (PWRs) and have been found to undergo stress corrosion cracking (SCC). Alloys 690 and 152 are the replacement materials of choice for Alloys 600 and 182, respectively. The objective of this work is to determine the crack growth rates (CGRs) in a simulated PWR water environment for Alloy 152. In order to meet the objective, specimens made from a laboratory-prepared Alloy 152 double-J weld in the as-welded condition were tested. For the SCC CGR measurements, the specimens were pre-cracked under cyclic loading in a primary water environment, and the cyclic CGRs were monitored to determine the transition from the fatigue transgranular fracture mode to the intergranular SCC fracture mode. The environmental enhancement of cyclic CGRs for Alloy 152 was minimal; nevertheless, the transition from transgranular to intergranular cracking was successful. Weld samples tested from the single heat of Alloy 152 exhibited SCC CGR rates of 10−11 m/s in the simulated PWR environment at 320°C, which is only about an order of magnitude lower than typical for Alloy 182.

Commentary by Dr. Valentin Fuster
2011;():655-661. doi:10.1115/PVP2011-57728.

Reactor core internal components in light water reactors are subjected to neutron irradiation. It has been shown that the austenitic stainless steels used in reactor core internals are susceptible to stress corrosion cracking after extended neutron exposure. This form of material degradation is a complex phenomenon that involves concomitant conditions of irradiation, stress, and corrosion. Interacting with fatigue damage, irradiation-enhanced environmental effects could also contribute to cyclic crack growth. In this paper, the effects of neutron irradiation on cyclic cracking behavior were investigated for austenitic stainless steel welds. Post-irradiation cracking growth tests were performed on weld heat-affected zone specimens in a simulated boiling water reactor environment, and cyclic crack growth rates were obtained at two doses. Environmentally enhanced cracking was readily established in irradiated specimens. Crack growth rates of irradiated specimens were significantly higher than those of nonirradiated specimens. The impact of neutron irradiation on environmentally enhanced cyclic cracking behavior is discussed for different load ratios.

Commentary by Dr. Valentin Fuster
2011;():663-671. doi:10.1115/PVP2011-57824.

This paper is focused on the study of residual stress distribution at a dissimilar metal weld (DMW) of nuclear reactor nozzle. The paper extends some of the recent research on this subject by investigating the effect of weld sequence and nozzle length design on the residual stress distributions. It also investigates the effect of a partial excavation repair and a weld overlay on the residual stress distribution. As a result, some of the important residual stress features at DMW are revealed and these features are discussed and summarized in the paper.

Commentary by Dr. Valentin Fuster
2011;():673-679. doi:10.1115/PVP2011-57826.

This paper is focused on the discussion of weld residual stress relaxation in a uniform post weld heat treatment (PWHT). In particular, the paper is attempted to address a fundamental issue related to the PWHT stress relaxation behavior, i.e., what is the dominant stress relaxation mechanism in PWHT? Is it due to creep or material strength reduction at elevated temperature? The paper starts with a simplified 3-bar weld model to demonstrate how weld residual stress is developed and relaxed. It then follows with an example of thick section narrow groove weld to highlight the results and conclusions. The results clearly indicate that creep mechanism plays a dominant role in the stress relaxation of PWHT. Several other important observations related to the stress relaxation are also summarized.

Commentary by Dr. Valentin Fuster
2011;():681-687. doi:10.1115/PVP2011-58051.

Irradiation assisted stress corrosion cracking (IASCC) is a problem of growing importance in pressurized water reactors (PWR). An understanding of the mechanism(s) of IASCC is required in order to provide guidance for the development of mitigation strategies. One of the principal reasons why the IASCC mechanism(s) has been so difficult to understand is the inseparability of the different IASCC potential contributors evolutions due to neutron irradiation. The potential contributors to IASCC in PWR primary water are: (i) radiation induced segregation (RIS) at grain boundaries, (ii) radiation induced microstructure (formation and growth of dislocations loops, voids, bubbles, phases), (iii) localized deformation under loading, (iv) irradiation creep and transmutations. While the development of some of the contributors (RIS, microstructure) with increasing doses are at least qualitatively well understood, the role of these changes on IASCC remains unclear. Parallel to fundamental understanding developments relative to IASCC, well controlled laboratory tests on neutron irradiated stainless steels are needed to assess the main mechanisms and also to establish an engineering criterion relative to the initiation of fracture due to IASCC. First part of this study describes the methodology carried out at CEA in order to provide more experimental data from constant load tests dedicated to the study of initiation of SCC on neutron irradiated stainless steel. A description of the autoclave recirculation loop dedicated to SCC tests on neutron irradiated materials is then given. This autoclave recirculation loop has been started on July 2010 with the first SCC test on an irradiated stainless steel (grade 316) performed at CEA. The main steps of the interrupted SCC tests are then described. Second part of this paper reports the partial results of the first test performed on a highly neutron irradiated material.

Commentary by Dr. Valentin Fuster
2011;():691-698. doi:10.1115/PVP2011-57267.

Recent work conducted using the Advanced Finite Element Analysis (AFEA) method to simulate the ‘natural’ crack growth of a circumferential PWSCC demonstrated that a subcritical surface crack can transition to a through-wall crack with significant differences between the inner diameter and outer diameter crack lengths. In the current version of the xLPR (Extremely Low Probability of Rupture) code, once the surface crack penetrates the wall thickness, an idealized through-wall crack (which has an equivalent area as the final surface crack) is formed. This type of crack transition was selected since no general stress intensity factor (K) solutions were available for crack shapes that would form during the transitioning stages, i.e., non-idealized or slanted through-wall cracks. However, during the pilot study of the xLPR code, it has been identified that this crack transition method may provide non-conservative results in terms of leak-rate calculations. In this paper, in order to compare the ‘natural’ versus ‘idealized’ crack transition behavior, limited example cases were considered where both crack transitions were simulated using 3D finite element analyses. In addition, leak-rate calculations were performed to study how the two different crack transition methods can affect the leak-rates. The results of the present study demonstrate that the ‘idealized’ transition from surface to through-wall crack can significantly affect the leak-rate calculations.

Commentary by Dr. Valentin Fuster
2011;():699-704. doi:10.1115/PVP2011-57312.

A number of papers have been presented at previous ASME PVP conferences, which have evaluated the crack opening areas (COA) and stress intensity factors (K), using elastic finite element analysis techniques, for through-wall cracks in a region where an attachment is welded to a plate. This was a simplified geometry aimed at representing a more complicated geometry of a pipe-branch connection. A number of analyses were considered and conclusions made on the estimation of COA and K using simple handbook solutions. More recently the analyses included the application of nonlinear geometry and the addition of crack face contact when applying bending loads. This paper is a continuation of these previous studies, assessing through-wall cracks in a more realistic pipe-branch connection geometry. The calculated COA and K values for the more complex geometry are compared to values from pipe models with no branch connections, in a similar manner to that applied in the previous work on the simplified plate geometry. Judgments are made on the conservatism, or otherwise, of the estimated COA and K for the more complex geometry solutions compared to the simple geometry solutions.

Commentary by Dr. Valentin Fuster
2011;():705-714. doi:10.1115/PVP2011-57507.

For sodium pipes of Japan Sodium cooled Fast Reactor (JSFR), the continuous leak monitoring will be adopted as an alternative to a volumetric test of the weld joints under conditions that satisfy Leak-Before-Break (LBB). The sodium pipes are made of ASME Gr.91 (modified 9Cr-1Mo) steel. Thickness of the pipes is small, because the internal pressure is very low. Modified 9Cr-1Mo steel has a relatively large yield stress and small work hardening coefficient comparing to the austenitic stainless steels which are currently used in the conventional plants. In order to discuss about the LBB of the sodium pipes made of modified 9Cr-1Mo steel, the coolant leak rate from a through wall crack must be estimated properly. Since the leak rate is strongly related to the crack opening displacement (COD), an appropriate COD assessment method must be established to perform LBB assessment. However, COD assessment method applicable for JSFR sodium pipes — thin wall and small work hardening material — has not been proposed yet. Therefore, the authors have proposed a COD assessment method applicable to thin walled large diameter pipe made of modified 9Cr-1Mo steel. In this method, COD is calculated by classifying into three components of elastic, local plastic and fully plastic. This paper describes the improved COD assessment method and verifies the validity of the method based on the results of a series of four-point bending tests at elevated temperature using thin wall modified 9Cr-1Mo steel pipe containing a circumferential through wall crack. As a result, COD values calculated by the proposed method were in a good agreement with the experimental results for the uniform pipe without weld. In the case that the crack was machined at weld metal or heat affected zone (HAZ), proposed method predicted relatively larger COD than the experimental results. The causes of such discrepancies were discussed comparing with the results of finite element analyses. Based on these examinations, the rational leak rate evaluation method in LBB assessment was proposed.

Commentary by Dr. Valentin Fuster
2011;():715-722. doi:10.1115/PVP2011-57513.

For sodium pipes of Japan Sodium cooled Fast Reactor (JSFR), the continuous leak monitoring will be adopted as an alternative to a volumetric test of the weld joints under conditions that satisfy Leak-Before-Break (LBB). The vessels of JSFR are connected by thin wall pipes with a large diameter made of modified 9Cr-1Mo steel and the internal pressure of the pipes is very low. Modified 9Cr-1Mo steel has relatively large yield stress and small work hardening coefficient compared to the austenitic stainless steels which are currently used in the conventional plants. Therefore, these material characteristics of modified 9Cr-1Mo steel must be taken into account in LBB assessment, as well as geometrical and structural features of JSFR pipes. In order to demonstrate LBB aspects of the JSFR pipes, the authors have proposed a LBB assessment flowchart and developed assessment methods of unstable fracture and crack opening displacement (COD) for the thin wall pipes with large diameter made of modified 9Cr-1Mo steel. This paper studies the master curve to estimate the crack length when a postulated initial crack unexpectedly grows and penetrates the pipe thickness. In order to obtain the fatigue crack and creep crack growth characteristics of modified 9Cr-1Mo steel pipes, fatigue crack and creep crack growth tests were conducted using compact tension (CT) specimens and crack growth rates for both fatigue and creep at elevated temperature were obtained. Based on the obtained material characteristics and the results of a series of crack growth calculations, a relationship between the penetrated crack length and the ratio of membrane to total stress, so called as master curve, was proposed. In this study, master curves were proposed for pipes made of modified 9Cr-1Mo steel as a function of pipe geometry, i.e. the ratio of radius to thickness.

Commentary by Dr. Valentin Fuster
2011;():725-736. doi:10.1115/PVP2011-57029.

Fluoride Salt-Cooled High-Temperature Reactors (FHRs) are a promising new class of thermal-spectrum nuclear reactors. The reactor structural materials must possess high-temperature strength and chemical compatibility with the liquid fluoride salt as well as with a power cycle fluid such as supercritical water while remaining resistant to residual air within the containment. Alloy N was developed for use with liquid fluoride salts and it possesses adequate strength and chemical compatibility up to about 700°C. A distinctive property of FHRs is that their maximum allowable coolant temperature is restricted by their structural alloy maximum service temperature. As the reactor thermal efficiency directly increases with the maximum coolant temperature, higher temperature resistant alloys are strongly desired. This paper reviews the current status of Alloy N and its relevance to FHRs including its design principles, development history, high temperature strength, environmental resistance, metallurgical stability, component manufacturability, ASME codification status, and reactor service requirements. The review will identify issues and provide guidance for improving the alloy properties or implementing engineering solutions.

Commentary by Dr. Valentin Fuster
2011;():737-743. doi:10.1115/PVP2011-57152.

The web-enabled materials properties database MatDB of the European Commission Joint Research Centre (EC-JRC) is a database application for the storage, retrieval and evaluation of experimentally measured materials data coming from European R&D projects. Data exchange and interoperability are important database issues to reduce costs of expensive material tests. Many organizations world-wide are participating in the development of GEN IV reactors. To reduce costs the GEN IV International Forum has agreed to interoperate and exchange data for the screening and qualification of candidate materials. To simplify the complexity of data mapping between differently structured databases, adoption of a standardized XML schema is the favored option. The paper focuses on MatDB XML related tools and items: • Upgrade, extension and implementation of the MatDB XML schema within a planned US/EC cooperation; • European standardization activities for data exchange, interoperability and the development of standard formats for engineering materials data; • MatDB data cite participation.

Commentary by Dr. Valentin Fuster
2011;():745-753. doi:10.1115/PVP2011-57156.

Mod 9Cr-1Mo steel (T91) is a candidate material for steam generator of SFR (Sodium Fast Reactors). In order to validate this choice, it is necessary, firstly to verify that it is able to withstand the planned environmental and operating conditions, and secondly to check if it is covered by the existing design codes, concerning its procurement, fabrication, welding, examination methods and mechanical design rules. A large R&D program on mod 9Cr-1Mo steel has been undertaken at CEA in order to characterize the behavior of this material and of its welded junctions. In this frame, a new measurement system for tensile testing was developed in the laboratory of structural integrity and standards (LISN) of the CEA (French atomic commission), in order to characterize the local behavior of the material during a whole tensile testing. Indeed, with the conventional measurement system (typically an extensometer), the local behavior of the material can only be determined during the stable step of the testing. So, usually the behavior of the material during the necking step of the step is unknown. This new measurement is based on the use of some laser micrometers which allow measuring the minimum diameter of the specimen and the curvature radius during the necking phase with a great precision. Thanks to the Bridgman formula, we can evaluate the local behavior of the material until the failure of the specimen. This new system was used to characterize the tensile propriety of a bimetallic welded junction of Mod 9Cr-1Mo steel and austenitic stainless steel 316L(N) realized with GTAW process and inconel filler metal. These works lead to propose a tensile curve for each materials of the welded junction.

Commentary by Dr. Valentin Fuster
2011;():755-758. doi:10.1115/PVP2011-57348.

Reactor Pressure Vessel (RPV) materials are well proven suitable materials for nuclear applications. Surveillance data from both pressurized Water Reactors (PWR) and Water-cooled Water-moderated Energy Reactor (WWER) up to 0.08–0.45 dpa and research data up to approx 1–2 dpa are available for irradiation temperature of 300°C. The response to radiation is very well understood as well as the effect of deleterious impurities. The data have been analysed in details, showing the strong role of impurities like copper and phosphorus besides basic matrix damage (MD). Reliable embrittlement models have been developed for the range of pressure vessel application. In fact, nowadays, very ‘clean’ materials with minimum radiation sensitivity are utilized. For ‘clean’ RPV steels, basic matrix damage is the main damage mechanism, described as a power function of the dose with exponents normally from 1/3 to 1/2. New advanced ferritic-martensitic (FM) materials have been designed for fusion and GEN IV applications, as for example Eurofer steel. Irradiation embrittlement data at high doses for this material are already available, also at 300°C. Both RPV and FM materials may be suitable also for GEN IV applications; in particular for the pressure vessel. In this study, the embrittlement curves of RPV and Eurofer are compared showing continuity and a common embrittlement trend till intermediate doses. For very high doses, over 1–2 dpa where only Eurofer data are available, damage rate saturation tendency is observed. The observed continuity and partial overlapping can be explained by a common damage underlying mechanism of the different steels, e.g. ferrite damage, in spite of different steel structures and content (e.g. from 1% Cr for PWR, 2.25% Cr for WWER and 9% Cr for Eurofer steel. Form the modeling point of view the matrix term need to be adjusted for doses far higher than those experienced for RPV applications; in fact the predictions are far too conservative and a new descriptive term need to be developed. In conclusion, clean RPV materials together with new developed steels can be conveniently utilised at 300°C also outside their envisaged dose range.

Commentary by Dr. Valentin Fuster
2011;():759-766. doi:10.1115/PVP2011-57393.

High-energy synchrotron radiation has proven to be a powerful technique for investigating fundamental deformation processes for various materials, particularly metals and alloys. In this study, high-energy synchrotron X-ray diffraction (XRD) was used to evaluate Alloy 617 and Alloy 230, both of which are top candidate structural materials for the Very-High-Temperature Reactor (VHTR). Uniaxial tensile experiments using in-situ high-energy X-ray exposure showed the substantial advantages of this synchrotron technique. First, the small volume fractions of carbides, e.g. ∼6% of M6 C in Alloy 230, which are difficult to observe using lab-based X-ray machines or neutron scattering facilities, were successfully examined using high-energy X-ray diffraction. Second, the loading processes of the austenitic matrix and carbides were separately studied by analyzing their respective lattice strain evolutions. In the present study, the focus was placed on Alloy 230. Although the Bragg reflections from the γ matrix behave differently, the lattice strain measured from these reflections responds linearly to external applied stress. In contrast, the lattice strain evolution for carbides is more complicated. During the transition from the elastic to the plastic regime, carbide particles experience a dramatic loading process, and their internal stress rapidly reaches the maximum value that can be withstood. The internal stress for the particles then decreases slowly with increasing applied stress. This indicates a continued particle fracture process during plastic deformations of the γ matrix. The study showed that high-energy synchrotron X-ray radiation, as a non-destructive technique for in-situ measurement, can be applied to ongoing material research for nuclear applications.

Commentary by Dr. Valentin Fuster
2011;():767-774. doi:10.1115/PVP2011-57655.

Ferritic-martensitic steels are the lead structural materials for next-generation nuclear energy systems. Due to increased operating temperatures required in advanced high-temperature reactor concepts, the high temperature performance of structural alloys and reliable high temperature structural design methodology have become increasingly urgent issues. Ferritic-martensitic steels experience significant cyclic softening at high temperatures, and this cyclic softening behavior affects consecutive stress relaxation response during hold time under creep-fatigue loading. It is found that the stress relaxation response during hold of the mod.9Cr-1Mo steel can be accurately described by a stress relaxation model. The creep damage associated with the stress relaxation during hold time can then be accurately calculated using the stress relaxation data and creep rupture data. It is shown that the unit creep damage per cycle in mod.9Cr-1Mo steel decreases considerably with increasing number of cycles due to cyclic softening, and the creep damage is sensitive to the initial stress of stress relaxation. Proper evaluation of the creep-fatigue damage in mod.9Cr-1Mo steel must consider the cyclic softening effect and its associated variations in creep damage from stress relaxation during the hold time.

Topics: Creep , Fatigue , Steel
Commentary by Dr. Valentin Fuster
2011;():775-783. doi:10.1115/PVP2011-57795.

Materials property data, and broader materials information, are essential to the wide range of people and functions in engineering enterprises involved in the design and construction of power generation facilities. This paper focuses on how this complex and specialist information must be managed, and how it can be deployed most effectively to those who need it. There are many different types of data to be considered, but they all begin as test results of one form or another. Generating this test data represents a major cost for many organizations. Applying it effectively can give them a major competitive advantage — for example, avoiding problems in design, gaining more understanding of the performance of materials, and ultimately reducing maintenance costs or enabling asset life extension. Yet few organizations have in place any systematic system to manage materials data. Not only does this mean that they are not making best use of their investment in this asset, it means that they can actually waste large amounts of time and money, in searching for the right data, or in duplicating tests that have already been done. This paper concentrates on the practical challenges involved, and on the technical details of how these challenges can be solved, drawing on the experience of developing the GRANTA MI materials information management system in collaboration with a consortium of leading aerospace, energy, and defense organizations. Three use-case scenarios are explored in which the management and application of materials data are important. These are: support for engineering design, statistical process control, and materials selection. For each use-case, the technical requirements for any corporate materials data management system are identified. Significant overlaps are found between the requirements for the different areas, indicating that a well-designed system can both meet a broad range of specific needs, and help to integrate different aspects of an organization’s operations. It is then appropriate to discuss the actual software tools required to meet these needs. These include tools to capture and consolidate materials data, to analyze and apply the data, to maintain a corporate materials information resource, and to deploy materials data enterprise-wide to the different functions (e.g., Design, and Quality Assurance) and roles (e.g., design engineers, stress analysts, and process improvement managers) that require it. The paper closes by providing guidelines on best practice regarding implementation of materials data management technology.

Commentary by Dr. Valentin Fuster
2011;():785-791. doi:10.1115/PVP2011-57891.

This paper focuses on reliability assessment of creep rupture life under the service conditions at high temperatures and stresses for Gr. 91 steel which is considered as one of the prime structural materials for next-generation nuclear reactors. An interference model based on Z parameter, which is considered for the fluctuations of both service temperature and stress besides the scattering of rupture data, was analyzed and used to assess the reliability on creep rupture life of Gr. 91 steel. The scattering distribution of the creep rupture data of the Gr. 91 steel was investigated by using the Z parameter. It appeared that the Z parameter of creep rupture data for Gr.91 steel exhibited normal distribution. Using the normal distribution, a Monte-Carlo simulation (MCS) was carried out to generate a number of random variables for Zs and Zcr , and the reliability of the creep rupture life under the fluctuations of service temperature and stress conditions was estimated by using the interference model. It showed that the value of reliability decreased with increasing service time. Higher temperature caused the trend of faster deterioration. The value of reliability decreased rapidly at higher temperature fluctuation amplitude, and the reliability decreased as the scattering of the creep rupture data became serious.

Topics: Creep , Steel , Reliability , Rupture
Commentary by Dr. Valentin Fuster
2011;():793-802. doi:10.1115/PVP2011-58089.

Most nuclear power plants being designed and constructed in the world today utilize advanced light water reactors with improved economics and safety; they are referred to by the US Department of Energy as Generation III+ Nuclear Plants. Overall, the Generation III+ power plants are expected to be safer and more affordable than those presently in operation. The offsite construction of structural and mechanical modules is a key element of the Generation III+ plant design; this feature significantly reduces the amount of onsite laborers and compresses the construction schedule. Each mechanical module must be structurally qualified to support its attached components for transportation, lifting, and operation scenarios. Qualification of the modules is very complicated because of the applicable codes and criteria as well as the diversity of components that may be attached to them. The purpose of this paper is to provide analysis instruction and to recommend special modeling techniques for structurally analyzing mechanical modules; the recommendations provided in this paper should not be taken as absolute rules but rather as guidelines to be altered, as needed, in order to more accurately simulate specific plant requirements. A mechanical module may be classified as a large gang hanger which supports many system components; a module may have dozens of pipe supports attached to it as well as tanks, piping, valves, pumps, conduit, ductwork, and cable trays. Mechanical modules are a fundamental aspect of the Generation III+ plant, and therefore must be properly analyzed and qualified. Due to the practically infinite possible arrangements of structural members and components, special modeling techniques are often required for considering all the possible loadings that may exist for the transportation, lifting, and operation scenarios. A structural analysis computer program such as GTStrudl or StaadPro must be used to build an analytical model of the module; the module frame and its attached components must be simulated in the model. Loadings such as dead weight, live load, thermal expansion, earthquake, wind, pressure, and flow transients are commonly applied to modules and their components. The analysis and qualification of the mechanical module frame must address structural member stresses, weld stresses, connection local stresses, and module support design. Results of the module qualification must be documented and verified according to plant procedures and criteria.

Commentary by Dr. Valentin Fuster
2011;():805-818. doi:10.1115/PVP2011-57092.

The efficiency of petrochemical reactors is intimately related to process parameters, i.e. service temperatures and pressures. Low alloyed ferritic materials, such as 2 1/4 Cr1Mo(V) and 3Cr1Mo(V) steel grades, are widely used for many years to build heavy wall reactors. This is mainly due to their good mechanical properties at high temperatures under high hydrogen partial pressures and good resistance to High Temperature Hydrogen Attack (HTHA). Depending on the grades, the ASME Code gives limitations in terms of maximum temperature that can limit the use of these low alloy grades. Moreover, above a given temperature, maximum allowable stresses are driven by the creep behaviour, leading to a strong lowering of the assumed resistance and hence to extra-thickness and weight. Many developments were done concurrently to increase the efficiency of petrochemical processes. In particular, this can lead to increase service temperatures and therefore actual pressure vessel wall temperatures. Indeed, more and more temperatures around 500°C are likely to be used, leading to reduced choice in terms of permitted steel grades. The low alloy vanadium-enhanced grades are not allowed (except using specific code case) whereas the usual grades have reduced creep allowable stresses. With a view to allowing strong improvements in admissible process parameters, a vanadium-modified 9Cr1Mo creep strength enhanced material with advanced hydrogen resistance and improved toughness was developed. Very thick plates (up to 200mm thick) were produced and tested. This contribution reports both mechanical and metallurgical assessments performed on these heavy plates. Evaluations of hydrogen resistance (HTHA) as well as creep resistance under high hydrogen pressure are also reported. The V-modified 9Cr1Mo grade exhibits an excellent behaviour in hydrogen rich environment, showing therefore some advantages in terms of service conditions. The manufacturing of heavy plates has made significant progress in the recent years, allowing thick products to be manufactured with good homogeneity and mechanical behaviour. Taking into account the maximum use temperature as well as the allowable stresses as described in the ASME BPV Code section VIII division 2, the V-modified 9Cr1Mo grade will clearly be of great interest to companies wishing to enhance the efficiency of their refining/petrochemical processes. The question of welding must also be addressed more specifically to finish validating the 9Cr1MoV option.

Commentary by Dr. Valentin Fuster
2011;():819-833. doi:10.1115/PVP2011-57209.

For more than fifty years the oil refining industry has been using American Society of Mechanical Engineers (ASME) B31.3 “Process Piping” for the design of piping systems i