ASME Conference Presenter Attendance Policy and Archival Proceedings

2015;():V06AT00A001. doi:10.1115/PVP2015-NS6A.

This online compilation of papers from the ASME 2015 Pressure Vessels and Piping Conference (PVP2015) 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to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Advanced Manufacturing and Material Technology

2015;():V06AT06A001. doi:10.1115/PVP2015-45064.

Coil-wound heat exchangers (CWHE) for low temperature applications such as the liquefaction of natural gas (LNG) are often made of aluminium alloys. The fabrication of these aluminium coil-wound heat exchangers holds several challenges, one of which is joining the tubes to the tube sheet. For this specific task, conventional joining technologies such as laser beam welding (LBW) or tungsten inert gas (TIG) welding cannot be easily performed in fully-mechanised mode or are not cost-effective. A joint project between the Helmholtz-Zentrum Geesthacht (HZG) and LINDE Engineering aims at the development of a new solid state joining process, Hybrid Friction Diffusion Bonding (HFDB), to fabricate tube-to-tube-sheet connections for aluminium coil-wound heat exchangers. In the present study, the HFDB process has been developed to industrial maturity and the quality of the joints has been demonstrated by gas leak tightness tests and tensile pull-out tests. The joints meet the requirements for industrial application. Furthermore, the thermal field development in the weld area and the applied process forces have been monitored and correlated to process parameters. The microstructure of the joint has been investigated, and dynamic recrystallization is assumed to be the primary grain refinement mechanism in the thermomechanically affected zone.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A002. doi:10.1115/PVP2015-45675.

After the Fukushima events in 2011, DOE-NE in collaboration with nuclear industry shifted R&D emphasis to accident performance of LWR fuels under extended loss of active cooling and steam exposure. DOE-NE has created a roadmap for the “Development of Light Water Reactor Fuels with Enhanced Accident Tolerance.” The mission of the Accident Tolerant Fuel (ATF) Roadmap is to develop the next generation of LWR fuels with improved performance, reliability, and safety characteristics during normal operations and accident conditions and with reduced waste generation. The ultimate goal of the ATF roadmap is to support the insertion of lead fuel rods (LFRs) or lead fuel assemblies (LFAs) of an Accident Tolerant Fuel into a commercial LWR within 10 years (i.e., by the end of FY-2022). As a step toward this goal, an irradiation test series has been developed to assess the performance of proposed ATF concepts under normal LWR operating conditions. Data generated by this test program will be used to establish the feasibility of certain aspects of proposed ATF concepts, as well as provide information to support screening among concepts; as such, it is an integral part of Phase I: Feasibility Assessment and Down-Selection outlined in the ATF Roadmap. This irradiation test series is planned to be performed as a series of drop-in capsule tests to be irradiated in the Advanced Test Reactor (ATR) operated by the Idaho National Laboratory (INL), and it has been designated as the ATF-1 test series.

Current fission reactors use zirconium-based fuel cladding because of its extremely low macroscopic thermal neutron absorption cross-section, good high temperature strength, and decent corrosion resistance. However, advanced, innovative materials may provide these same benefits while increasing reactor safety margin, core power density, and fuel utilization. These advanced fuel cladding systems will allow revolutionary cladding performance and enhanced fuel mechanical designs, however, challenges exist in design, analysis and fabrication of innovative, never before tested, fuel cladding systems for in-reactor testing. This paper highlights the challenges associated with design, fabrication and welding, and inspection of innovative materials and actions taken to address those challenges in preparation for the Phase I ATR irradiation testing. The lessons learned from Phase I of this experiment can be used to guide researchers for design and analysis of future in-reactor testing of advanced fuel cladding systems.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A003. doi:10.1115/PVP2015-46001.

Laser Engineered Net Shape processing was evaluated as a means to repair scratched gas sample bottles and mis-machined test bases. Defects to be repaired were intentionally introduced. These defects simulated scratches and holes that had been either over bored or misaligned. These parts were repaired using LENS and then tested in the baseline, as damaged, and hydrogen charged conditions. The effect of LENS repair was not detrimental to the properties of the components. Further development work and implementation of the process is recommended.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Advanced Sensor Technologies for Monitoring Structural Integrity of PVP Systems

2015;():V06AT06A004. doi:10.1115/PVP2015-45595.

The thermal effects at elevated temperatures mostly exist for pressure vessel and pipe (PVP) applications. The technologies for diagnosis and prognosis of PVP systems need to take the thermal effect into account and compensate it on sensing and monitoring of PVP structures. One of the extensively employed sensor technologies has been permanently installed piezoelectric wafer active sensor (PWAS) for in-situ continuous structural health monitoring (SHM). Using the transduction of ultrasonic elastic waves into voltage and vice versa, PWAS has been emerged as one of the major SHM sensing technologies. However, the dynamic characteristics of PWAS need to be explored prior its installation for in-situ SHM. Electro-mechanical impedance spectroscopy (EMIS) method has been utilized as a dynamic descriptor of PWAS and as a high frequency local modal sensing technique by applying standing waves to indicate the response of the PWAS resonator by determining the resonance and anti-resonance frequencies. Another SHM technology utilizing PWAS is guided wave propagation (GWP) as a far-field transient sensing technique by transducing the traveling guided ultrasonic waves (GUW) into substrate structure. The paper first presents EMIS method that qualifies and quantifies circular PWAS resonators under traction-free boundary condition and in an ambience with increasing temperature. The piezoelectric material degradation was investigated by introducing the temperature effects on the material parameters that are obtained from experimental observations as well as from related work in literature. GWP technique is also presented by inclusion of the thermal effects on the substrate material. The MATLAB GUI under the name of Wave Form Revealer (WFR) was adapted for prediction of the thermal effects on coupled guided waves and dynamic structural change in the substrate material at elevated temperature. The WFR software allows for the analysis of multimodal guided waves in the structure with affected material parameters in an ambience with elevated temperature.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A005. doi:10.1115/PVP2015-45623.

The increasing number, size, and complexity of nuclear facilities deployed worldwide are increasing the need to maintain readiness and develop innovative sensing materials to monitor important to safety structures (ITS). Assessing and supporting next generation nuclear materials management and safeguards for future U.S. fuel cycles with minimum human intervention is of paramount importance. Technologies for the diagnosis and prognosis of a nuclear system, such as dry cast storage system (DCSS), can improve verification of the health of the structure that can eventually reduce the likelihood of inadvertently failure of a component. In the past decades, an extensive sensor technology development has been used for structural health monitoring (SHM). Fiber optical sensors have emerged as one of the major SHM technologies developed particularly for temperature and strain measurements. However, the fiber optical sensors and sensing system has not been developed with adequate solutions and guideline for DCSS applications. This paper presents an experimental study of temperature effect on fiber Bragg grating (FBG) sensors. The reflective spectrum of FBG sensors on the structure was measured with a tunable laser source. The shift of FBG reflective spectrum reflected the thermal expansion on the structure. The shift of the spectrum due to the temperature effect was correlated to the temperature changes. In addition, the FBG sensing methodology including high frequency guided ultrasonic waves (GUW) under different temperatures were also performed to check the performance of high frequency, small strain sensing. The potential of FBG sensors for DCSS applications was explored. The paper ends with conclusions and suggestions for further work.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A006. doi:10.1115/PVP2015-45798.

Piezoelectric Wafer Active Sensors (PWAS) are a viable option for monitoring the structural integrity of pressure vessels and piping systems. They are inexpensive, small and unobtrusive sensors which can be permanently attached to structures for long term monitoring without interfering with operations, such as operating in areas with limited head space. PWAS are used to inspect the structure through several methods which include; pitch catch or pulse echo wave propagation, and electromechanical impedance spectroscopy. Since the PWAS could be exposed to a range of environmental and/or operating conditions while attached to the structure, the change in the properties and electromechanical characteristics of the sensor must be known at a given condition. Accordingly, there is a need for a testing system which can measure the PWAS properties while exposing the sensor to a wide range of temperatures.

The focus in this paper is on elevated temperatures, but the same methodology could be used for low temperature or environmental testing. The requirements which were imposed on the design include: providing an electrical connection from each electrode to the exterior of an industrial oven; allow the sensor to expand and contract both in plane and out of plane; withstand an extended duration at elevated temperatures; the equipment must not influence the measured quantities. The challenges include how to place the sensor in an oven and make electrical contact while allowing free motion, how to implement wiring and electrical connections at elevated temperatures, how to allow the thermal expansion of components and account for thermal mismatching, and how to maintain electrical isolation of the two electrodes. This paper discusses how these requirements were met and challenges overcome, as well as experimental validation of the system.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A007. doi:10.1115/PVP2015-45841.

This paper discusses the temperature effects of using piezoelectric wafer active sensors (PWAS) technologies for structural health monitoring (SHM) in pressure vessels and piping (PVP) applications, e.g. dry cast storage system (DCSS). The research into monitoring of DCSS health has experienced a dramatic increase following the issuance of the Blue Ribbon Commission (BRC) on America’s Nuclear Future Final Report in 2012. The interim storage of spent nuclear fuel from reactor sites has gained additional importance and urgency for resolving waste-management-related technical issues. PWAS have emerged as one of the major SHM technologies developed particularly for generating and receiving acousto-ultrasonic waves for the purpose of continuous monitoring and diagnosis. Durability and survivability of PWAS under temperature effects was first tested in experiments. The analytical model of PWAS based sensor and sensing system under temperature effects was then developed. This paper compared the analytical model and experimental results of PWAS under temperature changes. Since the environmental variability of a sensing system includes changes in both the sensors and the sensing methodology including acoustic emission (AE), guided ultrasonic waves (GUW), and electro-mechanical impedance spectroscopy (EMIS), we also performed several temperature exposure with different PWAS sensing configurations under a controlled oven. The potential of PWAS for DCSS applications has been explored. The paper ends with conclusions and suggestions for further work.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A008. doi:10.1115/PVP2015-45845.

This paper presents an efficient damage detection technique for power-plant-tubes by using guided waves and magnetostrictive transducer arrays. Particularly, our detection technique focuses on the small diameter and thick wall power-plant-tubes, such as superheater tubes, reheater tubes and water wall tubes. Firstly, the damage effects on guided waves in small diameter and thick wall tubes were studied by using three-dimensional finite element method. The wave reflections and mode conversions induced by damage were investigated. Secondly, based on T (0, 1)-F (n, 2) modes, magnetostrictive transducers were designed for guided wave generation and sensing in small diameter and thick wall tubes. The designed magnetostrictive transducers can effectively generate and measure guided waves, especially the non-dispersive torsional T (0, 1) wave mode. Finally, a magnetostrictive transducer array was developed for damage detection in small diameter and thick wall tubes. Through a virtual focusing array imaging algorithm, intensity images were constructed, which can show both the location and size of damage.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A009. doi:10.1115/PVP2015-45846.

The U.S. Nuclear Regulatory Commission (NRC) issued Generic Letter 2008-01 due to the safety concern associated with gas accumulation events in emergency core cooling pipes of nuclear power plants. Since the gas accumulation may critically damage pipes, pumps, and valves, and affect the safety operation of nuclear power plants, the gas accumulation needs to be detected as well as quantified.

This paper presents a quantitative gas accumulation detection method for water loaded pipes by using guided waves. To establish the detection method, we investigated the differences between guided waves in a free pipe and those in a water loaded pipe. The guided waves in both cases were measured by using a scanning laser Doppler vibrometer, and analyzed by using frequency wavenumber analysis. Analysis results show that guided wave characteristics such as wavenumbers and wave speeds are different between the free and water loaded pipes. Based on those findings, we developed a gas accumulation detection method that can also provide quantitative information of the gas accumulation. Through a proof-of-concept test, the quantitative gas accumulation detection method was verified.

Topics: Waves , Pipes , Water
Commentary by Dr. Valentin Fuster

Materials and Fabrication: Application of Fracture Mechanics in Failure Assessment

2015;():V06AT06A010. doi:10.1115/PVP2015-45149.

Assessment which includes structure analysis is carried out to evaluate the fitness for service for the defects found during in-service inspection based upon the methodology of Chinese assessment standard, which exceed the acceptance criteria in construction code. In search of potential of tolerance of defect, the limit analysis is also performed. Finally, safety suggestions are provided for the future operation.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A011. doi:10.1115/PVP2015-45178.

When assessing the effect of residual stresses on the driving force for fracture of a structure containing a defect, it is necessary to determine the residual stress field either via measurement or process simulation (e.g. weld modelling). However, residual stresses can be difficult and time-consuming to measure and model, especially in welded components which are subject to complex patterns of deformation during manufacture in addition to material inhomogeneity. To avoid this, many assessment procedures provide compendia of conservative estimates of the residual stress distribution for a limited set of welded joints. Unfortunately the use of these conservative estimates can lead to unrealistic results.

In this article we propose alternative methods to account for the effect of residual stress on linear elastic defect assessments. We determine the maximum possible stress intensity factor that can be generated in a cracked component by any unknown self-equilibrating residual stress field. The limiting value of stress intensity factor can then be used to assess the acceptability of defects without the need for any residual stress data.

Topics: Stress
Commentary by Dr. Valentin Fuster
2015;():V06AT06A012. doi:10.1115/PVP2015-45204.

This paper is based on a ductile failure simulation under dynamic loading conditions using finite element (FE) analyses. Recently a simple finite element method in a quasi-static test has been proposed to implement fracture simulation based on the well-known stress modified fracture strain model. The stress-modified fracture strain model is determined to be incremental damage in terms of stress triaxiality and fracture strain for dimple fracture from tensile test result with FE analyses technique. Since dynamic loading effect is especially important to assess pipe with crack-like defect, this work propose the integrated model which combines quasi-static with dynamic loading effect. In order to validate stress-modified fracture strain model in dynamic loading conditions, this paper compares results of FE analysis using proposed method with strain dependent smooth bar tests and notch tensile tests using Johnson-Cook equation. In conclusion, the stress-modified fracture strain model criterion can be calibrated by FE analyses with strain rate dependent fracture toughness test results.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A013. doi:10.1115/PVP2015-45233.

The evaluation of the crack tip deformation is essential to the estimation of crack growth under either static or cyclic loading. A 3-D elastic-plastic finite element analysis was developed to simulate the crack tip deformation along mixed mode inclined edge cracks in a steel plate subjected to either monotonic or cyclic loading at selected R-ratios. Bilinear kinematic hardening model was used to describe the material behavior. The development of the monotonic (Δm) and cyclic (Δc) crack tip plastically deformed zones and opening displacements were traced to find the effect of the crack inclination angle, which significantly affected the size and shape of the crack tip plastic zone. The finite element results compared well with the analytical results based on modified Dugdale’s model. It was observed that Mode II has a significant effect on the plastic zone in the case of equal inclined crack length (EICL), i.e., Mode II increases as the crack angle decreases. Also, it is interesting to note that for the EICL, the magnitude of Δc is delayed to appear with decreasing the inclination angle. Whereas, the variation of monotonic and cyclic plastic zone size in the equal crack horizontal projection (ECHP) case is not affected by the crack inclination angle. Furthermore, it was observed that the static crack tip opening displacement (CTOD) and the cyclic CTOD are independent of the crack inclination angle.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A014. doi:10.1115/PVP2015-45274.

A range of methods are currently available within R6 to calculate the inelastic secondary stress intensity factor under secondary loads in isolation, Display FormulaKJS. Each of these methods has different levels of associated conservatism depending on assumptions made, the complexity of the approach and the ability to account for different levels of elastic follow-up. Approaches that include an elastic follow-up factor, Z, for treating the interaction of combined primary and secondary stresses have recently been investigated by Ainsworth and James. However, the maturity of this recent work under combined primary and secondary loading means that one of the most significant aspect of the conservatisms in calculating the combined elastic-plastic stress intensity factor, KJ, is now in the calculation of Display FormulaKJS. This work considers existing approaches in R6 to calculate Display FormulaKJS and proposes a further approach allowing the value of Z to be altered.

For comparison this work considers finite element analyses of a circumferentially cracked cylinder with four thermal distributions and two shallow cracks. These conditions were controlled to manipulate the level of Z. The magnitude of the temperature difference in these profiles has been increased over the analysis time to provide a relationship between the elastic and inelastic secondary stress intensity factors, Display FormulaKIS and Display FormulaKJS, with increasing secondary load to demonstrate any enhancement and subsequent redistribution of the secondary stress. These finite element estimates have been compared to existing methods in R6 to calculate Display FormulaKJS/KIS which reinforce the available advice in R6 for each case. The proposed approach also compares favourably with the finite element results through modification of Z. The proposed approach is also seen to be compatible with the other approaches within R6 as it has been shown to reproduce the Option 2 failure assessment curve for cases where the elastic follow-up is significant (i.e. Z ≫ 5) and conforms to the displacement controlled estimate of Display FormulaKJS in Section III.14.5 of R6.

Topics: Stress
Commentary by Dr. Valentin Fuster
2015;():V06AT06A015. doi:10.1115/PVP2015-45300.

For nuclear welded components the complex nature of the residual stresses involved means it is often advantageous to produce mock-ups in order that the structural integrity and performance may be assessed. The weight and size of these components can make the production of mock-ups prohibitively expensive, and so the use of scaled models is considered here. Numerical analysis and finite element simulations have been carried out to investigate the scaling laws encountered affecting the applied loads, stress fields and crack driving forces that are of interest in the full sized component.

To illustrate the effects of scaling we consider the introduction of a residual stress through prior plastic deformation in rectangular beams of different sizes. A simple scaling law provides the loads required to introduce the same magnitude and distribution of residual stresses in different sized specimens. This is pertinent to uncracked beams. In contrast, if a crack is introduced this scaling law is no longer applicable and the stress intensity factor associated with residual and applied stresses differ for different sized specimens. Alternatively, to create the same crack driving force in different sized specimens different initial residual stress fields are required. The implications of these findings are discussed in the context of future work.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A016. doi:10.1115/PVP2015-45437.

To estimate maximum load-carrying capacity of pipes with multiple circumferential cracks, the net-section collapse load approach has been proposed. Although the proposed method has been validated against pipe test data, experimental data are quite limited due to large sets of variables to be considered. In this paper, a numerical method is proposed to generate virtual pipe test data with wide ranges of crack geometry and interspacing. To get confidence of the proposed numerical method, it is firstly applied to simulate existing 4-inch diameter schedule 80 pipes with two circumferential cracks. Predicted maximum loads agree well with experimental data. Then the proposed method is applied to generate maximum loads for wider ranges of crack geometry and loading conditions. It is found that the net-section collapse load approach works well for all cases considered.

Topics: Stress , Pipes , Collapse
Commentary by Dr. Valentin Fuster
2015;():V06AT06A017. doi:10.1115/PVP2015-45450.

Dynamic loading effects on ferritic steel toughness have been evaluated in the brittle-to-ductile transition, considering loading rates representative of object drops. To verify that the brittle-to-ductile transition curve, initially defined from static tests, tends to shift to higher temperatures due to dynamic effects even in the case of object drops, experiments on 16MND5 steel have been performed.

A three-point bending set-up and a thermal chamber have been designed in order to perform dynamic fracture tests on large Single Edge-notched Bending SE(B) specimen, at very low temperature using a drop-shock machine. In a first step, considering that the reference temperature of the material (according to the master curve concept) is −122 °C, dynamic tests at −120 °C have been performed. These tests have confirmed that the fracture mode is still brittle at this temperature, when an impact speed of 4.85 m/s is used.

Elastic-plastic or viscoplastic dynamic simulations of the tests, compared to classical static analysis, have demonstrated that the effects of inertia and viscosity on fracture toughness are negligible considering the very low values obtained on these tests at −120 °C. These results also confirm the decrease of fracture toughness due to dynamic loading compared to experimental data from static tests. A further step will be to complete this demonstration with dynamic tests at higher temperatures in the brittle-to-ductile transition.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A018. doi:10.1115/PVP2015-45466.

To address the issue of the lack of guidance in fitness-for-service codes [1], [2], for biaxial and triaxial effects on fracture, this paper expands on results presented at PVP 2014 [3] for a centre cracked plate by including the effect of plate length and then presents results for single edge and double edge cracked plates. The results demonstrate that load biaxiality can have a significant effect on the limit load and the crack driving force; reducing the limit load and increasing the crack driving force relative to that for uniaxial loading for negative biaxiality and high positive biaxiality but increasing the limit load and reducing the crack driving force for some intermediate levels close to equibiaxial loading.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A019. doi:10.1115/PVP2015-45480.

Crack path deviation in Single Edge Notch Tension (SENT) specimens, and its influence on the determination of J, has been investigated as part of the development of a new British Standard for SENT testing, BS 8571 [1]. Crack path deviation by angles up to 50° have been observed during stable tearing in parent material SENT specimens. This paper investigates the effect of crack path deviation on the measured fracture toughness, and offers a correction formula when crack path deviation invalidates the default standard J equations. Mixed mode effects in crack path deviation are also investigated.

A parametric study using finite element analysis has been carried out to compare the value of J calculated using standard equations (which assume a straight crack propagation path) with the value of J calculated using the contour integral method for different levels of crack path deviation. Crack path deviation from the initial crack plane resulted in a non-conservative estimate of fracture toughness using the standard equations. This means that any SENT test exhibiting crack path deviation may need to be discarded, wasting valuable test specimens. Instead, a correction factor has been developed to adjust the calculated value of J if path deviation is observed.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A020. doi:10.1115/PVP2015-45618.

Elasto-plastic fracture mechanics is commonly used for defect assessments in Nuclear Industry. Most of the approaches are based on the crack driving force J which is compared to a fracture resistance parameter. This fracture resistance parameter cannot be measured directly, but requires either J engineering formulae or Finite Element computations. The η(Eta) factor approach [1] is the most popular method allowing deriving fracture resistance from test records. One of the issue is the dependency of this factor to strain-hardening, namely for the shallow cracks.

This paper analyzes the effect of strain hardening on Eta factor solutions for two types of loading. Detailed Finite Element results are provided for Single Edge Cracked specimens under bending or tension. The results are compared to other formulae from the literature and new formulae are proposed.

Topics: Work hardening
Commentary by Dr. Valentin Fuster
2015;():V06AT06A021. doi:10.1115/PVP2015-45635.

This paper introduces a method to characterize the effect of notch bluntness on hydrogen embrittlement for high strength structural steel, FeE 690T, C(T) specimens. Hydrogen concentration depending on notch radius is assessed via finite element (FE) hydrogen diffusion analysis already developed and validated by the authors. Reduction in fracture toughness, KIC or JIC, due to hydrogen embrittlement is evaluated by means of a coupled hydrogen diffusion-ductile damage analysis. The ductile damage simulation used in this study is based on the model known as ‘stress-modified fracture strain model’. Tensile properties and fracture strains are modified according to the level of hydrogen concentration in the simulation and its effect on the fracture behavior of the specimen is simulated for different notch radii.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A022. doi:10.1115/PVP2015-45712.

“Hot” residual stresses exist in metal welds due to welding thermal stresses, and “cold” residual stresses occur in mechanical damaged metallic pipes due to large plastic deformation. For a crack in ether a hot or cold residual stress field, residual stresses might have a strong effect on the crack-tip field and the fracture parameter, J-integral. To consider the effect of residual stresses and to ensure the path-independence of J, different correction methods have been developed over the years. Recently, the finite element analysis (FEA) commercial software ABAQUS adopted one of correction methods for determining the residual stress corrected J-integral.

This paper intends to evaluate this new function of ABAQUS and to see if the residual stress corrected J-integral is path-independent. A brief review is first given to the J-integral definition, the conditions of J-integral path-independence or dependence, and the modifications of J-integral to consider the residual stress effect. A modified single edge-notched bend (SENB) specimen is then adopted, and a FEA numerical procedure is developed and used in the numerical tests to evaluate the path-independence of the residual stress corrected J-integral using ABAQUS. Detailed elastic-plastic FEA calculations are carried out for the modified SENB specimen in three-point bending. The residual stress field, crack-tip field, and J-integral with and without consideration of the residual stress effect are determined and discussed.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A023. doi:10.1115/PVP2015-45763.

The J-integral solutions for cracked pipes are important in crack growth calculation and failure evaluation based on the elastic-plastic fracture mechanics. One of the most important crack types in structural integrity assessment for nuclear piping systems is circumferential semi-elliptical surface crack on the inside of the pipes. Although several J-integral solutions have been provided, no solutions were developed at both the deepest and the surface points of circumferential semi-elliptical surface cracks in pipes. In this study, with backgrounds described above, the J-integral solutions of circumferential semi-elliptical surface cracks on the inside of the pipe were developed by numerical finite element analyses. Three dimensional elastic-plastic analyses were performed considering different material properties, pipe sizes, crack dimensions and, especially, combined loading condition of internal pressure and bending moment which is a typical loading condition for nuclear piping systems. The J values at both the deepest and the surface points were extracted from finite element analysis results. Moreover, in order to benefit users in practical applications, a pair of convenient J-integral estimation equations were developed based on the calculated J values at the deepest and the surface points. Finally, the accuracy and applicability of the convenient equations were confirmed by comparing with the provided stress intensity factor solutions in elastic region and with finite element analysis results in elastic-plastic region.

Topics: Pressure , Pipes
Commentary by Dr. Valentin Fuster
2015;():V06AT06A024. doi:10.1115/PVP2015-45820.

In this paper, stress and strain distributions near a crack tip in a round compact tension specimen of elastic-plastic materials are obtained by finite element analyses. The strain distributions are used to explore the use of the crack tip strain distributions for crack growth rate models due to stress corrosion cracking in unirradiated and irradiated steels with different yield stresses and hardening behaviors. Both power-law hardening and perfectly plastic materials are considered. The computational results indicate that the critical radial distance to the tip based on the crack tip opening displacement is outside of the Hutchinson-Rice-Rosengren (HRR) dominant zone for power-law hardening materials in a round compact tension specimen under the stress intensity factor typically considered for stress corrosion cracking. For both the power-law hardening and perfectly plastic materials, the computational results show that the strain distributions are different from those of the analytical solutions for the range of the radial distance larger than the critical radial distance based on the crack opening displacement within the plastic zones. The computational results suggest that for the stress intensity factor typically considered for stress corrosion crack growth rate models, computational results are needed to estimate the strain rate for developing crack growth rate models to correlate to the experimental data.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A025. doi:10.1115/PVP2015-45859.

This report addresses a mixed mode driven cracking and its fracture failure assessment for applications in aging nuclear power facilities. Following our earlier discussion on the use of mode-I based criteria in the current practice of fracture assessments, a finite element analysis of a full-scale laboratory test, a Benchmark four-point bending test of a straight pipe with an obliquely inserted crack in a dissimilar metal weld of ferritic steel (A508) and austenitic steel (316L), together with weld (308L) and buttering material (309L/308L), is conducted. The behavior of the crack front at the load level, at which crack initiation is observed in the test, such as stress intensities (KI, KII, KIII), J-integrals and other relevant parameters along the crack front, are computed. Crack initiation assessments are thereafter made using three alternatives: (1) Mode I cracking; (2) Mixed mode cracking; (3) An empirical approach suggested for accounting the mixed mode effect using a so-called R6-method. The results confirm our earlier observation: For cases when mixed mode loading conditions are significant, (i) the fracture initiation predicted by using J-integral based mixed mode cracking criteria can approximately be achieved by using the R6-empirical approach for the mixed mode cracking; (ii) it is not conservative to use a purely mode-I based criterion for the evaluation of the fracture failure assessment for typical problems of mixed mode driven cracking.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A026. doi:10.1115/PVP2015-45933.

High responsibility components operating under cyclic loading can have their resistance against initiation and growth of fatigue cracks highly influenced by previous thermomechanical processing. Within the interest of the present work, different manufacturing processes and installation techniques incorporate cold plastic straining to engineering structures; two typical examples on the oil and gas fields are: i) the offshore pipelines installation method called reeling; ii) the fabrication of pipes using the UOE method and pressure vessels through calendering. Within this scenario, this work investigates the effects of plastic prestrain on the fatigue crack growth rates (da/dN vs. ΔK) of a hot-rolled ASTM A36 steel. Different from previous results from the literature, in which prestrains were applied directly to machined samples, in this work uniform prestraining was imposed to steel strips (1/2” thick) and specimens were then extracted to avoid (or minimize) residual stress effects. Prestrain levels were around 4, 8 and 14% and C(T) specimens were machined from original and prestrained materials according to ASTM E647 standard. Fatigue crack growth tests were carried out under load control in an MTS 810 (250 kN) equipment using R = 0.1. Results revealed that plastic prestraining considerably reduced crack growth rates for the studied material, which was expected based on the literature and hardening behavior of the studied material. However, results also revealed two interesting trends: i) the larger is the imposed prestrain, the greater is the growth rate reduction in a nonlinear asymptotic relationship; ii) the larger is imposed ΔK, the more pronounced is the effect of prestraining. Crack closure effects were also investigated, but revealed no influence on the obtained mechanical properties. Consequently, results could be critically discussed based on effective crack driving forces and elastic-plastic mechanical properties, in special those related to flow and hardening. The conclusions and success of the employed methods encourage further efforts to incorporate plastic prestrain effects on structural integrity assessments.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A027. doi:10.1115/PVP2015-45935.

A typical multipurpose canister (MPC) is made of austenitic stainless steel and is loaded with spent nuclear fuel assemblies. The canister may be subject to service-induced degradation when it is exposed to aggressive atmospheric environments during a possibly long-term storage period if the permanent repository is yet to be identified and readied. Because heat treatment for stress relief is not required for the construction of an MPC, stress corrosion cracking may be initiated on the canister surface in the welds or in the heat affected zone. An acceptance criteria methodology is being developed for flaw disposition should the crack-like defects be detected by periodic Inservice Inspection. The first-order instability flaw sizes has been determined with bounding flaw configurations, that is, through-wall axial or circumferential cracks, and part-through-wall long axial flaw or 360° circumferential crack. The procedure recommended by the American Petroleum Institute (API) 579 Fitness-for-Service code (Second Edition) is used to estimate the instability crack length or depth by implementing the failure assessment diagram (FAD) methodology. The welding residual stresses are mostly unknown and are therefore estimated with the API 579 procedure. It is demonstrated in this paper that the residual stress has significant impact on the instability length or depth of the crack. The findings will limit the applicability of the flaw tolerance obtained from limit load approach where residual stress is ignored and only ligament yielding is considered.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A028. doi:10.1115/PVP2015-45943.

Stress intensity factors for a three-point bend specimen under dynamic loadings are obtained through the displacement fields using digital image correlation (DIC) method. The dynamic loads are generated with a drop weight to impact the specimen while the crack remains stationary in this study. It shows that the stress intensity factor oscillates as a function of time. The experimental details and the comparison with theoretical solutions are presented.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A029. doi:10.1115/PVP2015-45948.

The paper presents T-stress solutions developed to characterize constraint levels in large-scale cracked pipes and elbows. Stress intensity factor, KI, solutions for pipes and elbows are normalised by material fracture toughness to define the Kr parameter in fitness-for-service procedures, such as R6. Adding knowledge on levels of T-stress allows more advanced analysis through a normalised constraint parameter βT.

The paper presents analyses for 6 pipes and 8 elbows. Values of the normalised constraint parameter βT are calculated for each pipe and elbow at the experimentally measured crack initiation point. Comparison of constraint levels in the pipes and elbows with those in various types of fracture toughness specimen are used to predict the initiation loads using the R6 method and to provide guidelines for transferability.

Topics: Pipes
Commentary by Dr. Valentin Fuster
2015;():V06AT06A030. doi:10.1115/PVP2015-45966.

This paper presents an investigation concerning the ultra-low-cycle fatigue (ULCF) characterization of large-scale elbows produced from line pipes subjected to hot bending process. Two distinct pipes were used in this process: a 16” (w.t. 9.5 mm) X60 and an 8 5/8” (w.t. 5.59 mm) X65 pipes that were bent to 45 and 90° elbows (8 tests). Cyclic external loading was applied to the elbows, combined with internal pressure, until failure was observed. The failure was preceded by a local plastic instability (bucking) and resulted due to intense cyclic plastic deformation. In general, the number of cycles to failure was lower than 100 cycles which typifies this failure mechanism as ultra-low-cycle fatigue. Besides the full-size tests, the plain material was investigated under ULCF conditions using both smooth and notched specimens. The thermal process used in the hot bending manufacturing process was also accounted for in the material testing in order to understand the effect of this process on pipe material. Non-linear finite element models of the elbows were constructed to simulate the cyclic behaviour of the elbows using the actual loading histories applied to the elbows. Damage models (e.g. Coffin-Manson, Xue) identified using material test data are applied to simulate the failure cycles of the tested elbows. Besides the use of damage models available in the literature and identified with generated materials experimental data, current ASME VIII Div.2 procedures are also used to compute the failure cycles of the elbows to allow an assessment of these existing procedures.

Topics: Fatigue , Cycles
Commentary by Dr. Valentin Fuster
2015;():V06AT06A031. doi:10.1115/PVP2015-45990.

One of the most significant approaches for predicting formability is the use of forming limit diagrams (FLDs). The development of the generalized model integrates other models. The first model is based on Von-Misses yield criterion (traditionally used for isotropic material) and power law constitutive equation considering the strain hardening exponent. The second model is also based on Von-Misses yield criterion but uses a power law constitutive equation that considers the effect of strain rate sensitivity factor. The third model is based on the modified Hill’s yield criterion (for anisotropic materials) and a power law constitutive equation that considers the strain hardening exponent. The current developed model is a generalized model which is formulated on the basis of the modified Hill yield criterion and a power law constitutive equation considering the effect of strain rate. A new controlling parameter (γ) for the limit strains was exploited. This parameter presents the rate of change of strain rate with respect to strain. As γ increases the level of the FLD raises indicating a better formability of the material.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A032. doi:10.1115/PVP2015-46000.

The core of a CANDU®1 (CANada Deuterium Uranium) pressurized heavy water reactor includes horizontal Zr-2.5Nb alloy pressure tubes that contain the fuel. Pressure-temperature limits are used in CANDU® reactors for normal operation heat-up and cool-down conditions to maintain margins against fracture. The pressure-temperature limits are determined by postulating a 20 mm long axial through-wall crack in the pressure tube and using a fracture toughness-based calculation procedure. Due to a corrosion reaction with the heavy water coolant, pressure tubes absorb deuterium isotope in service, resulting in an increase in hydrogen equivalent concentration. Experiments have shown that high hydrogen equivalent concentration reduces the fracture toughness of pressure tube material at low temperatures during reactor heat-up and cool-down from normal operating temperatures. New fracture toughness curves that are applicable to material with high hydrogen equivalent concentration have been developed to address this issue. These curves are being used to develop new pressure-temperature limits for fracture protection of CANDU® pressure tubes. The methodology for deriving the pressure-temperature limits for a CANDU® Zr-2.5Nb pressure tube using the new fracture toughness curves is presented in this paper. Preliminary results of pressure-temperature limits for a CANDU® reactor are also provided.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Asian Programs in Structural Integrity

2015;():V06AT06A033. doi:10.1115/PVP2015-45577.

Pressurized thermal shock (PTS) is a potential major threat to the structural integrity of the reactor pressure vessel (RPV) in a nuclear power plant. A comprehensive structural integrity analysis of the Chinese Qinshan 300-MWe RPV subjected to PTS events including the small break loss-of-coolant accident (SB-LOCA) and large break loss-of-coolant accident (LB-LOCA) transients was performed by Shanghai nuclear engineering and design institute (SNERDI). The J-integral values at the deepest and the near cladding-base interface points of the crack were calculated with the linear elastic material model. And the RTPTS values were determined by the tangent approach. In the case that the RTNDT at or beyond the RPV design life may exceed the RTPTS according to the previous analysis procedure, the objective of this paper is to apply the Master Curve method to the re-evaluation of the integrity of this RPV, taking account of constraint and crack length effects. The over-conservatism in the previous evaluation is identified by comparing the new calculation with the previous one. The new RTPTS values are increased to varied extents for the different loading transients.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A034. doi:10.1115/PVP2015-45747.

Both Charpy V-notch (CVN) impact energy and fracture toughness are parameters reflecting toughness of the material. Charpy tests are however easy to perform compared to standard fracture toughness tests, especially when the material is irradiated and quantity is limited. Correlations between the two parameters are therefore of great significance, especially for reactor pressure vessel (RPV) structural integrity assessment. In this paper, correlations between CVN impact energy and fracture toughness of three commonly used RPV steels, namely Chinese A508-3 steel, USA A533B steel, Euro 20MnMoNi55 steel, are investigated with two methods. One method applies a direct conversion using empirical formulas and the other adopts the Master Curve method. It is found that when the empirical formula is used, the difference between the predicted fracture toughness (from the CVN impact energy) and actual test data is relatively small in upper shelf, lower shelf and the bottom of transition region, while relatively large in other parts of the transition region. When the Master Curve method is adopted, whether the reference temperature T0 is estimated through temperature at 28J or 41J CVN impact energy, the predicted fracture toughness values of the three steels are consistent with actual test data. The reference temperature T0 is also estimated through the IGC-parameter correlation and through a combination of empirical formula and multi-temperature method. Both procedures show excellent agreement with test results. The mean value of T0 estimated from T28J, T41J, IGC-parameters and the combination method is denoted by TQ-ave and is then used as the final reference temperature T0 for the Master Curve determination. Accuracy of TQ-ave (and therefore the Master Curve method) is demonstrated by comparison with actual test data of the three RPV steels. It is concluded that Master Curve method provides a reliable procedure for predicting fracture toughness in the transition region utilizing limited CVN impact energy data from open literature.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A035. doi:10.1115/PVP2015-45963.

The inner surface of a reactor pressure vessel (RPV) is assumed to be subjected to pressurized thermal shocks (PTSs) caused by the downstream of emergency cooling water. The downstream is not homogeneous but typically in a plume shape coming from the inlet nozzles.

In this paper, both deterministic and probabilistic methods are used to assess the integrity of a model RPV subjected to PTS. The FAVOR code is used to calculate the probabilities for crack initiation and failure of the RPV considering crack distributions based on cracks observed in the Shoreham and PVRUF RPVs.

The study shows that peak KI of the cracks inside the plume increases about 33% compared with that outside. The conditional probability inside the plume is more than eight orders of magnitude higher than outside the plume. In order to be conservative, it is necessary to consider the plume effect in the integrity assessment.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Code Fatigue Design Criteria and Environmental Effects

2015;():V06AT06A036. doi:10.1115/PVP2015-45022.

Consideration of Environmentally Assisted Fatigue (EAF) has been introduced to the German Regulatory Framework with the November 2013 revision of the German Safety Standard KTA 3201.2. Therefore, so called threshold of attention values by means of the cumulative usage factor have been included in the German KTA Program of Standards for the design of primary circuit components (KTA 3201.2: components of the reactor coolant pressure boundary of light water reactors, part 2: design and analysis) and for secondary side piping components (KTA 3211.2: pressure and activity retaining components of systems outside the primary circuit; part 2: design and analysis). Possible measures are linked to the KTA 3201.4 in-service inspection and operational monitoring regulations. Additionally, consideration of EAF, respectively the introduction of threshold values (temperature, strain rate and amplitude) is currently being discussed in the revision process of KTA 3204 for reactor pressure vessel internals. Note that threshold of attention values refer to the cumulative usage factor CUF and limit (threshold) values (temperature, strain rate and amplitude) refer to the activation of the penalty factor Fen.

In contrast to international procedures (like e.g. US-NRC Regulatory Guide 1.207, ASME Code CC N-792), EAF has to be considered for German plants below the design life of 40 years as well. More precisely, in the framework of ASME EAF has to be taken into account only for plant life extension, license-renewals or new builds while in Germany an evaluation is compulsory for all operating NPPs. Based on this fact an engineering approach is needed which is realized by introducing so called threshold of attention values. If the cumulative usage factor is higher than the threshold of attention value, additional measures have to be accomplished by the operator. According to the regulatory framework these measures are additional calculations (e. g. Fen calculation), NDT including fracture mechanical calculations or real component testing.

The calculation of the threshold of attention values is based on representative specified and operational measured temperature transients. Based on this information a comprehensive component specific picture of operational loading can be drawn. According to the latest revision of NUREG/CR-6909 (currently Rev. 1, draft for comments) the Fen formula comprises dedicated threshold values for the relevant temperatures, strain rates and strain amplitudes. Having these boundary conditions in mind, detailed plant transient information can be analyzed according to its relevance. Based on the comprehensive set of information threshold of attention values for KTA 3201.2 / 3211.2 were determined to be 0.4, both for ferritic and austenitic materials of BWR and PWR components. In the framework of KTA 3204, technical evaluation yields to the assumption, that Fen threshold values are likely not to be exceeded for pressurized water reactor internals.

In the framework of the paper the technical background for the evaluation of threshold of attention values in the context of EAF will be described in detail.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A037. doi:10.1115/PVP2015-45295.

In the wake of numerous experimental tests carried out in air and also in a PWR environment, both abroad and in France, an update of the current fatigue codification is underway. Proposals are currently being formulated in France [1] [2] and discussions are taking place in the frame of a French working group involving EDF, AREVA and CEA.

In parallel with these worldwide modification efforts, it is necessary to evaluate their impact on the NSSS components. In the USA, many such evaluations have already been implemented for license renewal to operate power plants beyond their initial 40 years of operation. In order to reduce the scope of the calculations to perform, a preliminary screening was carried out on the various areas of the primary loop: this screening is detailed in an EPRI report [3]. The output of this screening process is a list of locations that are most prone to EAF degradation process and it is on these zones only that detailed EAF calculations are carried out.

In France, with the approaching fourth decennial inspection of the 900 MWe (VD4 900 MWe) power plants, EDF needs also to map out the impact of these updates to the RCC-M code before initiating detailed calculation efforts.

The EPRI report was not applicable as such to the French plants due to domestic specificities and more particularly, a need for a more detailed Fen estimation. A method was therefore developed by EDF, peer-reviewed by SI with the main innovation being the introduction of correlations enabling the calculation of Fen on the basis of the geometrical dimensions and the information available in the transient document.

This paper presents how these correlations were built and proposes to benchmark them with some existing sample case problems.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A038. doi:10.1115/PVP2015-45694.

In USA there are approximately 100 operating light water reactors (LWR) consisting fleet of both pressurized water reactors (PWR) and boiling water reactors (BWR). Most of these reactors were built before 1970 and the design lives of most of these reactors are 40 years. It is expected that by 2030, even those reactors that have received 20 year life extension license from the US nuclear regulatory commission (NRC) will begin to reach the end of their licensed periods of operation. For economical reason it is be beneficial to extend the license beyond 60 to perhaps 80 years that would enable existing plants to continue providing safe, clean and economic electricity without significant green house gas emissions. However, environmental fatigue is one of the major aging related issues for these reactors, and may create hurdles in long term sustainability of these reactors. To address some of the environmental fatigue related issues, Argonne National Laboratory (ANL) with the sponsorship of Department of Energy’s Light Water Reactor Sustainability (LWRS) program trying to develop mechanistic approach for more accurate life estimation of LWR components. In this context ANL conducted many fatigue experiments under different test and environment conditions on 316 stainless steel (316SS) material that is or similar grade steels are widely used in US reactors. Contrary to the conventional S∼N curve based empirical fatigue life estimation approach, the aim of the present DOE sponsored work is to understand material ageing more mechanistically (e.g. time dependent hardening and softening) under different test and environmental conditions. Better mechanistic understanding will help to develop computer based advanced modeling tools to better extrapolate stress-strain evolution of reactor component under multi-axial stress states and hence to help predicting their fatigue life more accurately. In this paper (part-I) the fatigue experiments under different test and environment conditions and related stress-strain results for 316 SS are discussed. In another paper (part-II) the related evolutionary cyclic plasticity material modeling techniques and results are discussed.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Creep and Creep-Fatigue Interaction

2015;():V06AT06A039. doi:10.1115/PVP2015-45108.

Hastelloy X is widely used in the pressure vessel and piping (PVP) industries, specifically in nuclear and chemical reactors, pipes and valves applications. Hastelloy X is favored for its resistance to extreme environments, although it exhibits a rate-dependent mechanical behavior. Numerous unified viscoplastic models proposed in literature claim to have the ability to describe the inelastic behavior of superalloys subjected to a variety of boundary conditions; typically limited experimental data is used to validate their performance. In this paper, two unified viscoplastic models (Miller and Walker) were experimentally validated for Hastelloy X creep behavior. Both constitutive models are coded into ANSYS Mechanical as user programmable features (UPF). Creep behavior is simulated at a broad range of stress levels. The results are compared to an exhaustive database of experimental data to fully validate the capabilities and performance of these models. Material constants are calculated using the recently developed Material Constant Heuristic Optimizer (MACHO) software. This software uses the simulated annealing algorithm to determine the optimal material constants by using an extensive database of experimental data. A qualitative and quantitative discussion is presented to determine the most suitable model for Hastelloy X PVP components.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A040. doi:10.1115/PVP2015-45120.

Remaining life prediction model of ethylene cracking tubes (ECT) suffered joint damage of carburizing and creep have been set up in this study. Materials employed in this work were cut from Cr35Ni45-type radiation section of ECT. Layered structure, including oxide layer, carburized layer and carburizing transition layer, have been found from inside to outside of the tubes in scanning electron microscopy (SEM) images. However, no abrupt transition occurs between carburized layer and the base material. Both carburizing related layers caused the damage of the tubes, together with oxide layer. In order to facilitate the life prediction accuracy, carburizing transition layer was considered as a part of the damage layer. The remaining life of ECT was investigated by review of the microstructure and stress-rupture tests. Stress-rupture tests have been finished to obtained the rupture life of the tubes at 1000°C, 1040°C, 1080°C and 1125°C under six loading stresses (10MPa, 15MPa, 17MPa, 20MPa, 25MPa and 30MPa, respectively). In the results of stress-rupture tests, the combination of testing temperature at 1040°C and loading stress with 15MPa got the highest rupture life of 830h. Finally, a modified Larson-Miller remaining life prediction model of ECT, considering the comprehensive effect of carburizing and creep damage, has been established.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A041. doi:10.1115/PVP2015-45206.

Main steam pipelines work in the environment of high temperature and pressure steam and withstand double damage between oxidation and creep load for a long time. Creep-oxidation interrupt tests were used in high temperature steam conditions at different stress load to get P92 steel mechanochemical behavior date in the present study. Weight gain method was used to get the oxidation kinetics under different applied loading. Scanning electron microscopy (SEM) micro observation techniques was applied to obtain growth characteristic in the interaction between steam oxidation and creep loading.

Topics: Creep , Stress , Pipes , oxidation , Steam
Commentary by Dr. Valentin Fuster
2015;():V06AT06A042. doi:10.1115/PVP2015-45222.

Two novel fracture mechanics specimens were developed to enable creep crack growth (CCG) studies, under secondary and combined loading, to be carried out in laboratory controlled environments. Test samples were produced by the insertion of wedges into the mouths of C(T) specimens such that the geometrical misfit of the wedge induced on effective residual stresses. It is shown that the extent of crack tip plasticity generated during wedge insertion was limited which made these specimens ideal for CCG testing under secondary loading. Samples were also made by strategically placing electron beam welds in C(T) specimens, such that the weld induced residual stresses provided the crack driving force in subsequent CCG tests.

These specimens were used to perform crack growth studies under secondary and combined loading conditions in Type 316H stainless steel at 550°C for up to 1,300 h, where crack extensions of up to ≈5 mm occurred. The experimental results are presented and compared to relevant CCG tests published in literature. It is shown that CCG testing under secondary and combined loading conditions requires the creep ductility of the material to be sufficiently low to ensure crack initiation occurs. A low creep ductility was achieved in this study by uniformly pre-compressing the material to 8% plastic strain.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A043. doi:10.1115/PVP2015-45252.

Creep strain equations of Grade 92 steel which is used in boilers and piping systems of ultra-supercritical (USC) thermal power plants were developed based on the results of creep tests on smooth round bar specimens of three kinds of Grade 92 steels. In these equations, primary creep behavior was represented by a power-law and tertiary creep behavior was described by an exponential function. Creep parameters were determined as a function of creep rupture times which were calculated from stress and absolute temperature. Additionally, generalized creep failure criteria considering the multiaxial stress were established on the basis of results of creep tests on circumferentially notched round bar specimens. These creep strain equations and creep failure criteria were incorporated into finite element analysis software. Then, creep failure analyses were carried out and the resulting deformation behavior and rupture times were compared with the experimental results. Creep rupture lives were predicted with a good accuracy, within a factor of two in most cases.

Topics: Creep , Steel , Simulation , Rupture
Commentary by Dr. Valentin Fuster
2015;():V06AT06A044. doi:10.1115/PVP2015-45297.

One of the most common methods for estimating crack extension in the laboratory is electrical potential drop (PD). A key limitation of this technique is that it is sensitive to strains at the crack tip as well as crack extension. When producing J-R curves the onset of crack growth may be identified from a point of inflection on a plot of PD vs. CMOD. For creep crack growth (CCG) tests however, the effects of strain are often ignored. This paper investigates whether a similar method may be applied to CCG testing.

A single CCG test was performed on type 316H stainless steel and a point of inflection, similar to that observed during J-R curve testing was identified. A finite element (FE) based approach was used to investigate this phenomenon further. A 3D sequentially-coupled structural-electrical FE model was used to reproduce the experimental PD vs. CMOD plot up to the point of inflection. The model was capable of predicting the general relationship between strain and PD. It predicted the magnitude of the change in PD to within 30%. A simplified 2D FE model was then used to perform a parametric study to investigate whether a similar trend may be expected for a range of materials. Power law tensile and creep properties were investigated with stress exponents of 1, 3 and 10. The results confirm that a point of inflection should be observable for the range of material properties considered.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A045. doi:10.1115/PVP2015-45353.

Low cycle fatigue tests of original ferritic P92 steel at high temperatures and different strain amplitudes were conducted to investigate its cyclic softening behavior and fracture behavior. LCF tests of strain amplitudes ranging from ±0.2% to ±0.8% were performed in fully reversed manner with constant strain rate at 600 °C and 650 °C. In order to represent the different hysteresis stress-strain curves and the cyclic softening behavior of P92 steel, a cyclic plastic material model was used. In the model, improved nonlinear isotropic hardening parameter was proposed to make better simulation of the cyclic softening behavior. Based on the simulated stress-strain hysteresis loops, an energy-based life prediction model was used to predict the low cycle fatigue life. When compared with experimental responses, the simulations and predicted life were found to be quite reasonable. Low cycle fatigue fractography of the P92 steel was also observed, and it was found to be associated with the different strain amplitudes imposed on the specimen, the larger strain amplitude the more amounts of crack initiation sites could be found.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A046. doi:10.1115/PVP2015-45354.

The centrifugal casting 25Cr35Ni-Nb ethylene pyrolysis furnace tubes with different contents of Pb were selected to study the effects of Pb content on the creep and fracture properties. Both the high temperature rupture test at 1100 °C /17 MPa and the slow straining test at 850 °C with different loading rates show that the increase of Pb content significantly degrades the high temperature creep properties of centrifugal casting 25Cr35Ni-Nb alloy furnace tubes. The fractographic observation and the chemical composition analysis show that Pb segregates to the grain boundary at high temperature. It shows that the segregation of Pb to grain boundary occurs even when the content of Pb is several ppm, which leads to the reduction of grain boundary surface energy at high temperature. The initiation of creep cavities becomes easier. The creep cavities coalesce together under the local stress, forming cracks and finally causing the early failure of furnace tubes.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A047. doi:10.1115/PVP2015-45380.

It is customary to study the creep deformation of materials at high temperatures and the incubation and growth of creep cracks using constant load test machines. However, this highly idealised loading condition does not accurately reflect the practical circumstances that occur when operating high temperature plants. Real loading conditions often lie between load and displacement control and correspond to situations where there is elastic follow-up, with low values relating to near displacement control and high values near to load control.

This paper explains a series of experiments where pre-cracked martensitic P92 steel compact tension specimens are loaded and tested for different values of structural elastic follow-up, ranging from constant load to near fixed displacement. It is found that the degree of elastic follow-up significantly changes the time taken for creep crack incubation. This is a consequence of the relaxation of the load applied to the specimens. Elastic-plastic creep finite element simulations are used to reveal the underlying mechanical behaviour of the specimens. The simulations were confined to 2D analyses for plane stress and plane strain conditions. It is observed, that, irrespective of the initial loading and boundary conditions, the predicted mechanical response for plane stress and plane strain lies either side of the experimental results.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A048. doi:10.1115/PVP2015-45384.

Estimation of failure lives for welded joints in piping made of creep strength enhanced ferritic steel is of great importance for ensuring safe and sound operation of ultra-supercritical fossil power plants using this type of material. As a part of the various efforts for developing the procedure meeting this need, a large scale pipe test was performed on grade 122 pipe with circumferential welds, in order to grasp its fracture behavior as well as its predictability. Finite element analysis as well as damage calculation were performed on this test. By using creep strain equations, as well as failure criterion newly developed for this material and welded joint, both creep deformation and failure life were successfully predicted with reasonable accuracy.

Topics: Pressure , Creep , Steel , Pipes , Failure
Commentary by Dr. Valentin Fuster
2015;():V06AT06A049. doi:10.1115/PVP2015-45423.

In this study, an inner pressure/bending creep test on a circumferentially welded, large diameter pipe of grade 122 steel was carried out under conditions where the test temperature was 923 K and the expected rupture time was 8,700 h to investigate creep damage mechanisms and the applicability of nondestructive evaluation techniques for the detection of creep damage. Deformation of the specimen rapidly increased at 7,894 h, and the creep test was interrupted. Surface cracks were observed on the lower part of the specimen along the circumferentially welded line where axial tensile stress was applied. As a result of phased array ultrasonic inspection, indications of internal defects were detected along welded lines where no indication was detected in previous inspection at 6,520 h, and it was predicted that cracks propagated along the fine-grained heat-affected zone (HAZ) from within the specimen to near the inner and outer surfaces. The indications of internal defects were in good agreement with observations from sectional views of the welded joints.

Topics: Pressure , Creep , Steel , Pipes
Commentary by Dr. Valentin Fuster
2015;():V06AT06A050. doi:10.1115/PVP2015-45510.

In order to operate a thermal power plant safely, it is necessary to establish the prediction methods of fracture life for heat-resistant steels which is used in thermal power plant. For the establishment of prediction methods, it is necessary to consider the effects of creep damage and creep-fatigue interaction on the fracture life.

Therefore, in this paper, creep-fatigue tests were conducted on notched specimens of W-added 9-12Cr steels for various temperatures and load frequencies. From these results, the influence of load frequency on temperature dependence of crack growth life for 9-12Cr steels was determined.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A051. doi:10.1115/PVP2015-45513.

The most important failure mode to be prevented is creep-fatigue at elevated temperatures in fast reactors. 316FR stainless steel is a candidate material for the reactor vessel and internal structures. A method to evaluate creep-fatigue life, based on the time fraction rule, has been already developed in base metal of 316FR stainless steel. Development of procedure in evaluating creep-fatigue life is also necessary for the weldment of 316FR stainless steel by similar fillers or 16-8-2 fillers. Compared between mechanical properties of weldment and those of base metal, strength-reduction factors for weldment have been evaluated. Strength-reduction factor for fatigue has been proposed. It is considered that strength-reduction factor for creep strength is not necessary. Creepfatigue life could be evaluated in the same way for weldments of similar fillers and 16-8-2 fillers, because a difference in mechanical properties between both filler metals is negligible. Creep-fatigue life by the time fraction rule using analytical relaxation curve for weldments were compared with experimental data, and a method to evaluate creep-fatigue life for the weldments of 316FR stainless steel has been proposed.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A052. doi:10.1115/PVP2015-45823.

The creep behavior of structural materials is often measured using uniaxial tension creep rupture tests. Unfortunately, the time required for austenitic steel samples to rupture under ideal (i.e. elastic stress) conditions is prohibitive. To accelerate creep rupture in these samples, a tensile stress in excess of the material yield strength is often applied and the post-load deformation is assumed to be largely creep-based. There is currently no method of measuring the creep deformation separately from the yield-induced plastic flow that may occur during such accelerated tests.

Using validated finite element models, the effects of creep and yield-induced plastic strain have been decoupled for a series of accelerated creep tests using 316H austenitic steel. The influence of continued yielding after the initial sample loading was predicted to be significant, which suggests the diffuse necking in the samples due to creep is responsible for stress intensification and further yield through the tests. These results suggest the initial plastic loading in accelerated creep tests may significantly influence the measured creep rupture time in these samples.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Development of Stress Intensity Factors

2015;():V06AT06A053. doi:10.1115/PVP2015-45032.

The extended finite element method (XFEM) is an extension of the conventional finite element method based on the concept of partition of unity. In this method, the presence of a crack is ensured by the special enriched functions in conjunction with additional degrees of freedom. This approach also removes the requirement for explicitly defining the crack front or specifying the virtual crack extension direction when evaluating the contour integral.

In this paper, stress intensity factors (SIF) for various crack types in plates and pipes were calculated using the XFEM embedded in ABAQUS. These results were compared against handbook solutions, results from conventional finite element method, and results obtained from finite element alternating method (FEAM). Based on these results, applicability of the ABAQUS XFEM to stress intensity factor calculations was investigated. Discussions are provided on the advantages and limitations of the XFEM.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A054. doi:10.1115/PVP2015-45086.

Prior probabilistic fracture mechanics (PFM) analysis of reactor pressure vessels (RPVs) subjected to normal cool-down transients has shown that shallow, internal surface-breaking flaws dominate the RPV failure probability. This outcome is caused by the additional crack driving force generated near the clad interface due to the mismatch in coefficient of thermal expansion (CTE) between the cladding and base material, which elevates the thermally induced stresses. The CTE contribution decreases rapidly away from the cladding, making this effect negligible for deeper flaws. The probabilistic fracture mechanics code FAVOR (Fracture Analysis of Vessels, Oak Ridge) uses a stress-free temperature model to account for residual stresses in the RPV wall due to the cladding application process. This paper uses finite element analysis to compare the stresses and stress intensity factor during a cool-down transient for two cases: (1) the existing stress-free temperature model adopted for use in FAVOR, and (2) directly applied RPV residual stresses obtained from empirical measurements made at room temperature. It was found that for a linear elastic fracture mechanics analysis, the application of measured room temperature stresses resulted in a 10% decrease in the peak stress intensity factor during a cool-down transient as compared to the stress-free temperature model.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A055. doi:10.1115/PVP2015-45457.

The stress intensity factor (SIF) is the major fracture mechanics parameter in LEFM concept. Since the SIF can be used for not only calculation of J-integral based on the GE/EPRI and reference stress method but also evaluation of fatigue crack growth, an accurate estimation of the SIF is an important issue for the piping in nuclear power plant. Recently, there is a need to develop the SIF solution which can cover wide geometric variables since there are on-going efforts that are developing next generation reactors in Korea, which is designed to thin-walled structures. For the through-wall cracked straight pipes, many researchers have proposed the SIF solutions which can cover wide range of wall thickness. However, since only limited solutions have been proposed yet for the through-wall cracked elbows, a research related to the SIF estimation for the elbows with wide geometric variables should be performed. In this study, the extended SIF solution for circumferential through-wall cracked elbows subjected to in-plane bending is proposed as the tabulated form through the finite element (FE) analyses. Wide elbow geometries are selected to range between 5 and 50 of Rm/t and range between 2∼20 of Rb/Rm. The existing solutions are then reviewed by comparing with the FE results. Furthermore, effects of geometric variables on the SIF are addressed through systematic investigation of FE based SIF results. These investigated results are expected to contribute to the development of closed form solution for the circumferential through-wall cracked elbows subjected to in-plane bending.

Topics: Stress
Commentary by Dr. Valentin Fuster
2015;():V06AT06A056. doi:10.1115/PVP2015-45462.

A complex crack is one of severe crack that can occur at the dissimilar metal weld of nuclear piping. A relevant fracture mechanics assessment for a pipe with a complex crack has become interested in structural integrity of nuclear piping. A stress intensity factor is not only an important parameter in the linear elastic fracture mechanics to predict the stress state at the crack tip, but also one of variables to calculate the J-integral in the elastic plastic fracture mechanics. The accurate calculation of stress intensity factor is required for integrity assessment of nuclear piping system based on Leak-Before-Break concept.

In the present paper, stress intensity factors of complex-cracked pipes were calculated by using detailed 3-dimensional finite element analysis. As loading conditions, global bending, axial tension and internal pressure were considered. Based on the present FE works, the values of shape factors for stress intensity factor of complex-cracked pipes are suggested according to a variables change of complex crack geometries and pipes size. Furthermore, the closed-form expressions based on correction factor are newly suggested as a function of geometric variables. These new solutions can be used to Leak-Before-Break evaluation for complex-cracked pipes in the step of elastic J calculation.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: European Programs in Structural Integrity

2015;():V06AT06A057. doi:10.1115/PVP2015-45103.

Numerous experimental and numerical studies have been conducted in France to demonstrate the beneficial effect of warm pre-stress (WPS) on the brittle fracture resistance of reactor pressure vessel (RPV) steels and take into account this effect in French RPV integrity assessment. A large panel of experimental data is available, obtained on small, medium and large scale specimens. These data have been included in a specific EDF WPS experimental database.

An analytical criterion — ACE criterion — is proposed by French organizations (AREVA, CEA and EDF) for analytical evaluation of warm pre-stress effect on the brittle resistance of RPV steels.

After a description of ACE criterion and the EDF WPS database, the validation of the criterion is shown — based on this database — by comparison between experimental results and predictions of the model.

Topics: Stress
Commentary by Dr. Valentin Fuster
2015;():V06AT06A058. doi:10.1115/PVP2015-45145.

The purpose of this paper is to disseminate the results of an EURATOM project MULTI-METAL focusing on the structural integrity assessment of dissimilar metal welds. The project started in February 2012 and ended in February 2015. The project is coordinated by VTT with 10 partner organizations from Europe : Technical Research Centre of Finland, Finland (VTT) – Coordinator, AREVA NP, France and Germany (ANP), Commissariat à l’Énergie Atomique et aux energies alternatives, France (CEA), Joint Research Centre of the European Commission, Belgium (JRC), EdF-Energy, United Kingdom (BE), Bay Zoltán Foundation for Applied Research, Hungary (BZF), Electricité de France, France (EDF), TECNATOM, Spain (TEC), Jožef Stefan Institute, Slovenia (JSI), Studsvik Nuclear AB, Sweden (STU).

The underlying aim of the project is to provide recommendations for a good practice approach for the integrity assessment (especially testing) of tough dissimilar metal welds as part of overall ductile integrity analyses; this has been presented in the project overview [1].

Experience on typical DMWs concerning manufacturing, residual stresses, flaw assessment and testing have been reviewed. The specimens were taken from mock-ups of welded plates. Three DMWs design variants have been covered: narrow gap DMW with Ni-52, DMW with austenitic steel buttering and a DMW with Nienriched austenitic steel buttering. Mechanical characterization and fracture mechanics testing (CT, SEN(B) and SEN(T) specimens) have been performed. Interpretation of the test has required numerical analysis since the standard ASTM E1820 [2] (CT, SEN(B)) and guidelines dealing with SEN(T) [3][4] are not directly intended to cover DMW.

The motivation of the project and its results are generally presented and discussed.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A059. doi:10.1115/PVP2015-45179.

Within the framework of European project MULTIMETAL (Structural performance of multi-metal component), several fracture tests on different types of multi-material specimens have been performed. Present fracture toughness standard methods, e.g. ASTM E 1820 are not directly intended for Dissimilar Metal Weld (DMW). Therefore further investigations are needed in order to define the best practice in fracture mechanical tests and their analysis for DMWs.

Specimens are taken from welded plates: a narrow gap Inconel DMW junction between ferritic and austenitic stainless steels, designed and delivered by AREVA France. The aim of this work is to provide guidelines for the determination of DMW fracture properties. For that purpose, fracture specimen needs to be modelled by FE. The first task, which is the purpose of that paper, is the determination of the mechanical properties in terms of stress-strain curve of all DMW constitutive materials: austenitic stainless steel, ferritic steel, heat affected zone of the ferritic steel zone and Nickel alloy zone properties.

Conventional techniques for tensile test are not able to provide the tensile curve of the different materials constituting a weld joint. Image correlation techniques are well suitable but imply too long and difficult work for the images analysis. Therefore CEA has developed an intermediate solution based on laser sensors which provides a complete profile of the specimen during the tensile test. Using Bridgman equations, the stress and strain can be deduced from the measurement of the shape of the specimen (reduction of section but not only…). This innovative device has been used with new developments using local Bridgman equations in the post-processing of measurements. This allows to access to the material behaviour of several materials with only one specimen.

Numerical interpretation using FE methods is presented and confirms the material behaviour determined from the experimental work using Bridgman equations assumptions.

Finally, this combined experimental and numerical work has provided material data relative to all constitutive materials of the DMW junction. A hardened area in stainless steel material due to the welding process has been pointed out, and the heat affected zones of the ferritic material have been characterized in terms of stress-strain curves. The next stage of the project is to carry out tests on fracture specimens and to model these multi-materials specimens by FE. The gradient of elasto-plastic properties is now available for this next step.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A060. doi:10.1115/PVP2015-45180.

The integrity of a reactor pressure vessel (RPV) has to be ensured throughout its entire life in accordance with the applicable regulations. Typically an assessment of the RPV against brittle failure needs to be conducted by taking into account all possible loading cases. One of the most severe loading cases, which can potentially occur during the operating time, is the loss-of-coolant accident (LOCA), where cold water is injected into the RPV at operating conditions. High pressure in combination with a thermal shock of the ferritic pressure vessel wall caused by the injection of cold water leads to severe loading conditions at the beltline area known as Pressurized Thermal Shock (PTS).

Usually the assessment against brittle failure is based on a deterministic fracture mechanics analysis, in which common parameters like the J-integral or stress intensity factor are employed to calculate the load path during the PTS event for an assumed (postulated) flaw. Subsequently the results of the fracture mechanics analysis are compared with material properties obtained from the irradiation surveillance program of the RPV to demonstrate the exclusion of brittle fracture initiation.

In this paper an alternative novel method for the calculation of the crack initiation and possible crack propagation by means of the eXtended Finite Element Method (XFEM) will be introduced and compared with results of the standard PTS analysis.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A061. doi:10.1115/PVP2015-45181.

Within the framework of European project MULTIMETAL (Structural performance of multi-metal component), several fracture tests on different types of multi-material specimens have been performed. Present fracture toughness standard methods, e.g. ASTM E 1820 are not directly intended for Dissimilar Metal Welds (DMWs). Therefore further investigations are needed in order to define the best practice in fracture mechanical tests and their analysis for DMWs.

Specimens were taken from welded plates: a narrow gap Inconel DMW junction between ferritic and austenitic stainless steels, designed and delivered by AREVA France. The work is focused on the nickel alloy - ferritic steel interface which is the weakest area of such welded pipes regarding ductile tearing. This paper is focusing on the experimental methodology in this particular case of multi-material fracture test specimens.

The first difficulty consists in machining specimens with a location of crack plane at the interface or close to the interface. Physically, the interface between the nickel alloy weld and the ferritic steel is not a straight plane. Some normalized specimen - geometries like CT (Compact Tension specimen), SE(B) (Single Edge Bend specimen), or SE(T) (Single Edge Traction specimen) - have been machined in the DMW junction. Some recommendations are discussed about this first step of work.

The second difficulty is the procedure of fracture test itself. The present fracture toughness standard method uses the method of compliance for the determination of J-Δa curves. The plasticity, in ferritic and stainless steel parts of specimens, has induced unsymmetrical response. Locally the path of the crack has been influenced by the gradient of properties of the multi-material medium (weld, heat affected zone). The J values, determined at crack initiation, are dependent of the location of crack plane. Results between specimens are compared and discussed.

Based on the current standard, this article highlights some shortcomings of this standard when used in heterogeneous specimens. The well-known problem of transferability is also pointed out (different values of J for crack initiation depending of the geometry and size of specimen).

In conclusion further numerical works are needed to assess experimental results and provide intrinsic material data, in terms of fracture properties of the Nickel alloy-ferritic steel interface.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A062. doi:10.1115/PVP2015-45217.

NUGENIA, an international non-profit association founded under Belgian legislation and launched in March 2012, is dedicated to nuclear research and development (R&D) with a focus on Generation II and III power plants. NUGENIA is the integrated framework between industry, research and safety organisations for safe, reliable and competitive nuclear power production, and is aimed at running an open innovation marketplace, to promote the emergence of joint research and to facilitate the implementation and dissemination of R&D results. The technical scope of NUGENIA consists of eight technical areas. One of these areas, Technical Area 4, is associated with the structural integrity assessment of systems, structures and components.

A brief overview of recent NUGENIA activities in general is provided in this paper and a specific focus is given on developments in relation to Technical Area 4.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A063. doi:10.1115/PVP2015-45558.

The phenomenon of thermal ageing of low alloy steels comes more into focus in terms of long term operation of nuclear power plants (NPP). Safety-relevant components such as the RPV or the pressurizer have to bear the respective loads at elevated temperatures for longer times. However the mechanical properties of the applied materials might experience certain degradations such as a decrease of the impact energy levels and a shift in the ductile to brittle transition temperature (e.g. T41) leading to higher ductile-brittle reference temperatures and a reduction of material toughness. In terms of a safe long term operation it is important to understand in how far thermal ageing alone, meaning for the RPV without the cumulative damaging effects through neutron irradiation, has detrimental influences on the respective materials of interest.

First of all an overview is provided of the current state of the art with respect to thermal ageing by describing influencing mechanisms, its implementation into different nuclear codes, standards and selected experimental investigations in this field. Following this, the test results of the thermal surveillance sets from three German PWRs are presented and discussed. The tested Charpy-V specimens, taken from representative RPV base and weld metals (22NiMoCr3-7 / NiCrMo1UP) as well as their heat affected zones, were exposed to ∼290°C for ∼30 years on the cold leg of the according plants’ main coolant loops. The obtained results are compared with the existing thermal aging data base (baseline and ∼7 years data) of the materials concerned. Finally, the role of thermal ageing particularly with respect to RPV irradiation surveillance will be assessed.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Fatigue and Fracture of Welds and Heat Affected Zones

2015;():V06AT06A064. doi:10.1115/PVP2015-45583.

This study provides the application of damage model to complex cracked pipes which can be found especially in weld overlay region. From the perspective of structural integrity, enough basic and large-scale tests are required to accurately evaluate the components containing a crack-like defect. In this case, damage model using finite element (FE) method can be effectively used for the assessment of full-scale cracked pipes with minimum basic experiments data. The proposed method in this research is based on the stress-modified fracture strain damage model with stress reduction technique.

In this paper, Battelle full-scale complex cracked pipe tests are simulated by the proposed damage model with reasonable procedure. FE simulation is conducted for basic experiments to determine failure criteria with calibrations. Then, crack initiation and maximum loads are predicted to characterize the fracture behavior of full-scale complex cracked pipes. Damage model is applied to both of carbon and stainless steel materials and verification with comparing to test data is conducted.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A065. doi:10.1115/PVP2015-45748.

Weld residual stress (WRS) in dissimilar metal welds (DMWs) has been identified as an important driver for primary water stress corrosion cracking, which is observed in nuclear power plant safety-related components. In this work, a newly developed dynamic strain hardening rule is implemented in finite element (FE) thermal-mechanical model to simulate the residual stress distribution in a dissimilar metal weld studied in a recent NRC/EPRI Round Robin study. This new dynamic strain hardening constitutive rule takes into account the effect of dynamic recovery and recrystallization at elevated temperatures on the strain hardening behavior during welding. Weld residual stresses calculated using the new dynamic strain hardening rule are compared to those with the conventional strain hardening ones (isotropic and kinematic), as well as the experimental measurement data. The new dynamic strain hardening rule results in improvements in WRS prediction.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A066. doi:10.1115/PVP2015-45834.

Japanese nuclear power plants have recently experienced several large earthquakes beyond the previous design basis ground motion. In addition, cracks resulting from long-term operation have been detected in piping lines. Therefore, it is very important to establish a crack growth evaluation method for cracked pipes that are subjected to large seismic cyclic response loading. In our previous study, we proposed an evaluation method for crack growth during large earthquakes through experimental study using small specimens. In the present study, crack growth tests were conducted on pipes with a circumferential through-wall crack, considering large seismic cyclic response loading with complex wave forms. The predicted crack growth values are in good agreement with the experimental results for both stainless and carbon steel pipe specimens and the applicability of the proposed method was confirmed.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A067. doi:10.1115/PVP2015-46011.

This study proposes a complete set of integrated experimental procedures to assess the risk of Hydrogen Induced Cracking (HIC) using implant test. The proposed experimental procedures assess HIC susceptibility in base metals using two measures: the implant static fatigue limit stress (σimp); and Heat Affected Zone (HAZ) maximum hardness (HV10MAX). The base metal susceptibility to HIC was evaluated by examining the effect of three welding factors: the critical cooling time between 800 °C and 500 °C (t800/500); the base metal carbon equivalent (CE); and the diffusible Hydrogen content (H). A 3-D mapping technique was used to demonstrate the interactive integrated relationships among the three examined welding factors (i.e. t800/500, CE and H) and the susceptibility of the base metal to HIC. Using the 2-D projection of the developed 3-D mapping, it was proven that the diffusible hydrogen content (H) had more effect on the HIC susceptibility of High Strength Low Alloy (HSLA) steel compared to the effect of H on the HIC susceptibility of Carbon-Manganese (C-Mn) steel.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Fitness for Service and Failure Assessment

2015;():V06AT06A068. doi:10.1115/PVP2015-45247.

A trayed column at an operating facility suffered a loss of containment incident and was shut down. The damage to the steel is thought to have been caused by loose tray components which rattled around and eventually wore through the shell. An initial repair plan involving welded repairs was proposed. This plan would entail a field post weld heat treat (PWHT) due to the process environment of the vessel. Upon further developing the PWHT plan it was determined that this approach was costly and would have excessive lost profit opportunity (LPO) due to the time it would take to execute and the criticality of this vessel to plant operations. Instead, a second approach involving no field welding was devised, vetted, fabricated, and implemented. The facility was able to restart the process, saving several days of production. A permanent repair or replacement will be planned and implemented at the next planned shutdown.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A069. doi:10.1115/PVP2015-45630.

It has become apparent with the development of creep strength enhanced ferritic steels, the mandatory ASME B31.1 Chapter VII and the non-mandatory ASME B31.1 Appendix V guidelines require a more rigorous method to manage the Grade 91 piping integrity at Genesee Unit 3. Given the relatively young age of Genesee Unit 3, three questions have been asked: 1) when do the examinations start, 2) what locations should be examined first, and 3) how often should the same location be reexamined? To ensure that the best value is obtained from the reexamination budget, a five-step process can be effectively used to define and categorize the scope of each set of reexaminations in the girth weld integrity management program. The five processes are performing the following analyses: 1) an evaluation of the historical information, 2) piping system hot and cold walkdowns, 3) as-designed and as-found piping stress analyses, 4) creep life consumption evaluations, including elastic and inelastic axial and radial stress redistributions, and 5) creep crack growth curve analyses. Reexaminations of the few critical lead-the-fleet weldments are performed with lower examination costs and higher confidence.

Evaluations of the Genesee Unit 3 main steam (MS) piping system revealed that the applicable weldment stress is probably the most significant parameter in determining the Grade 91 girth weld critical reexamination locations and intervals. ASME B31.1 piping stress analyses of the MS piping system have sustained load stress variations of more than 100% among the girth welds. The lower bound American Petroleum Institute (API) 579 creep rupture equation for Grade 91 operating at 1,060°F (571°C) indicates that the creep life is a function of stress to the power of 8.9; consequently, a 15% stress increase results in about 2/3 reduction of creep rupture life. Creep crack growth analyses of several of the MS piping system weldments revealed that the creep crack growth time to grow from 1/8 inch to through-wall is a function of stress to the power of 8.8; consequently, a 15% stress increase results in about 2/3 reduction of time for a 1/8-inch crack to grow through-wall.

This evaluation reveals that a few critical lead-the-fleet locations should be reexamined most frequently and justification can be provided for much longer reexamination intervals of the remaining girth welds with much lower applied stresses.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A070. doi:10.1115/PVP2015-45733.

This paper will focus on the gradual change of corrosion behaviors among different zone metal of 13MnNiMoR steel weld joint in the EO reactor service environment. Metallographic analysis and electrochemical corrosion tests in boiler water have been carried out on cylindrical specimens, taken from the 13MnNiMoR steel welded joints at the direction across the weld. Metallographic structures, values of linear polarization resistance (Rp) and potentiodynamic polarization curves on sections normal to the axis of the specimens have been obtained. The results showed that the Rp values and potentiodynamic polarization curves of the weld joint were significantly varied with the microstructure among different zones. Results indicated that the overheating zone in HAZ shows the minimum value of Rp and the maximum value of corrosion current density (icorr), which make it the weakest part of the joint from a corrosion stand point.

Topics: Steel , Corrosion
Commentary by Dr. Valentin Fuster
2015;():V06AT06A071. doi:10.1115/PVP2015-45807.

The effect of residual stress on potential crack growth and fracture in welded structures is usually assessed through its contribution to the stress intensity factor (SIF) for the crack size and shape of interest. The idea of defining bounding residual SIF profiles for surface breaking circumferential cracks in pipe butt welds was presented at ASME PVP2013. The limiting profiles were based on through-thickness residual stress measurements for eight pipe girth welds. This paper presents new axial residual stress measurements made using the contour method for an Esshete 1250 stainless steel pipe girth weld. A wide variation in the through-wall distribution of axial residual stress around the circumference of the pipe is observed which has a significant effect on calculated values of SIF for postulated surface breaking circumferential cracks. Nonetheless, SIFs based on all of the new measurements (a total of 14 profiles) are comfortably bounded by the simple SIF prescriptions previously published.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A072. doi:10.1115/PVP2015-45926.

This paper reviews the basic elements of a facility integrity management program and describes the process used to assess risk conditions related to a facility. The policies, goals and objectives of the program should be defined before implementing it. The location and details of the facility and all its equipment must be described and the information should be recorded in a computerized database. Important triggers for change management and the minimum features of change management are reviewed. Ensuring the competency and training of personnel responsible for integrity management is essential. The integrity management team must identify hazards associated and ways of controlling them. Once hazards are identified, risk assessment is performed and options for reducing risk are considered. Results of the risk assessment are then used to plan and execute activities of the integrity management program. Needed repairs or replacements are identified, planned and completed. Finally, the integrity management program should incorporate a continuous improvement process and information from investigations of incidents at the facility, at other company locations, and within the industry.

Topics: Risk
Commentary by Dr. Valentin Fuster

Materials and Fabrication: Integrity Issues in SCC and Corrosion Fatigue

2015;():V06AT06A073. doi:10.1115/PVP2015-45093.

Arbitrary 3-D crack growth behavior due to stress corrosion cracking under residual stress was evaluated by finite element analyses. The analytical evaluation enables consideration of complicated stress distributions, such as weld residual stress, and expression of arbitrary 3-D crack shapes. The estimated weld residual stress for non-cracked components was distributed on the crack surface on the basis of the superposition principle to calculate stress intensity factors. Arbitrary 3-D crack shapes due to the crack propagation were expressed by FINAS/CRACK software. Moreover, stress corrosion cracking (SCC) crack growth evaluation under weld residual stress in a butt-welded large diameter pipe was conducted. The weld residual stress was estimated by finite element analyses considering a moving heat source during welding. An initial crack was postulated in a heat affected zone. The 3-D no planar crack growth behavior due to the complicated weld residual stress was observed by the analytical evaluation. To verify the applicability of flaw growth procedure in a fitness-for-service code on the SCC crack growth considering 3-D shape, SCC crack growth evaluation on the basis of the code procedure using a planar semi-elliptical crack shape was conducted and compared with the analytical results.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A074. doi:10.1115/PVP2015-45273.

This study carried out burst tests on steam generator (SG) tube specimens containing multiple axial part-through-wall (PTW) flaws at room temperature (RT). The specimens were machined from SG tube of Alloy 690TT with an outer diameter of 19.05mm and a thickness of 1.067mm. The flaws were made by electro-discharge machining (EDM) method on the outer surface of specimen. In the experiment, six types of multiple PTW flaws with a constant depth of 50% of wall thickness and single flaw with four different lengths were considered. The results showed that the interaction effect for collinear axial PTW flaws diminished with increasing ligament length between flaws. The ligament length had less influence on the interaction effect for longer flaws than shorter flaws. For non-aligned axial PTW flaws, however, the interaction effect was increased and remained with increase in circumferential ligament length. For parallel axial PTW flaws, the positive interaction effect appeared when the ligament between flaws was less than 2mm. However, the failure pressure decreased with increasing circumferential ligament length between flaws and reached the minimum when they were separated by about 90-degree in circumferential direction. Interaction effect was enhanced as number of flaws for collinear and non-aligned axial PTW flaws increased. Regardless of flaw configurations and ligament lengths, multiple PTW flaws of shorter length were failed by rupture at bottom of flaws followed by coalescence and unstable tearing. But, the flaws of length longer than 25.4mm were failed by rupture at bottom of one of multiple axial flaws.

Topics: Boilers
Commentary by Dr. Valentin Fuster
2015;():V06AT06A075. doi:10.1115/PVP2015-45323.

This paper presents irradiation-assisted stress corrosion cracking (IASCC) disposition curves developed in a multi-year international data collection, data review and modeling project. More than 800 IASCC crack growth rate (CGR) data points were collected from six laboratories worldwide, and an international panel of experts reviewed and ranked the data. The better-ranked data were used to calibrate empirical models for IASCC CGR in boiling water reactor (BWR) normal water chemistry (NWC) and hydrogen water chemistry (HWC) environments and in pressurized water reactor (PWR) primary water environments. The mean models were shifted upward to the 75th percentile of the calibration data for use as crack disposition curves. The disposition curves are presented in this paper and compared with data used for fitting and data not used for fitting, including field data from BWR core shrouds and additional laboratory data. The paper is intended as a basis document for possibly incorporating the new disposition curves in the ASME code.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A076. doi:10.1115/PVP2015-45417.

With the extension of pressurized water reactor’s design life or continued operation, more careful study on the integrity of the internal structures needs to be pursued. In this study, warm-rolling and heat-treatment were applied to 316L stainless steel, in order to simulate the effect of radiation damage such as hardening and radiation-induced grain boundary segregation. And, the crack growth rate testing under constant load condition was performed in the primary water conditions of a pressurized water reactor. Also, in order to investigate the effect of dissolved hydrogen on the crack growth, the dissolved hydrogen concentration was varied between 30 to 50 cc/kg in simulated primary water condition of a pressurized water reactor. The warm-rolled specimens showed the higher crack growth rate than as-received one. Also, the crack growth rate increased as the dissolved hydrogen concentration increases.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A077. doi:10.1115/PVP2015-45759.

This paper presents an applicability of elastic-plastic fracture mechanics parameters for evaluating a crack growth rate of stress corrosion cracking (SCC). Currently linear fracture mechanical approaches have been applied for the SCC crack growth evaluation, even though some cracks due to SCC are found in plastic deformation zones near welding where linear fracture mechanics is no longer applicable. In this paper, the authors have proposed an elastic-plastic parameter “equivalent stress intensity factor KJ” for evaluating the SCC crack growth rate based on the J-integral value, which is valid in both elastic and plastic stress fields. In order to verify the applicability of the evaluation by KJ, SCC crack growth tests were carried out in a simulated boiling water reactor (BWR) water. When the SCC crack growth rate was evaluated by the stress intensity factor K, no linear relationship between the K values and the crack growth rates was observed in the high K-value region, where a small-scale yielding condition was not met. The crack growth rates increased exponentially according to increasing the stress intensity factor to exceed the linear relationship. On the other hand, when the crack growth rate was evaluated by the elastic-plastic parameter KJ, a linear correlation between the KJ values and the crack growth rates was confirmed regardless the specimen size and the stress condition. This result suggests that by applying the elastic-plastic parameter KJ, the SCC crack growth rates in a wider range could be estimated easily with using a smaller specimen.

Commentary by Dr. Valentin Fuster

Materials and Fabrication: Leak Before Break

2015;():V06AT06A078. doi:10.1115/PVP2015-45156.

The thermal-hydraulics computer code, RELAP (Reactor Excursion and Leak Analysis Program) is used to analyze loss of coolant accidents (LOCAs) and system transients in PWRs and BWRs. However, RELAP requires the knowledge of break-opening area versus time history for Double-Ended Guillotine Break (DEGB) of a pipe fracture event as an input to calculate pressure drops at critical locations in the primary pipe loop. Previously authors conducted a detailed dynamic FE analyses to determine the condition for DEGB that provided moment versus rotation of the cracked-pipe and time histories for DEGB under beyond design basis seismic loading.

In this paper, crack-opening area was calculated using the moment-rotation-time history obtained from dynamic FE analyses. In the LBB.ENG2 J-estimation scheme for circumferentially cracked pipe, the rotation at the cracked-pipe cross-sectional location (rotation due to the crack) is uniquely related to the total crack length and crack-opening displacement at the center of the crack. However, the relationship is only valid when the moment versus rotation from the FE analyses corresponds to the ductile tearing curve from the LBB.ENG2 ductile fracture analysis. During any unloading (and reloading) parts of the applied seismic history, the rotation can drop down from the upper-envelope for the tearing resistance of the cracked pipe in an elastic unloading manner from the seismic/cyclic unloading. During this part of the seismic time-history, the crack length remains constant but the center-crack-opening displacement decreases, i.e., there is crack closure with a constant crack length which needs to be included in predicting crack-opening area. Based on a number of past cyclic pipe fracture tests with large amounts of ductile tearing, a procedure was developed to predict the crack-opening area that included crack closure during cyclic loading of the seismic event. The resulting opening-area versus time history then becomes the input to the RELAP analysis for determination of emergency core cooling/safety processes.

Topics: Design
Commentary by Dr. Valentin Fuster
2015;():V06AT06A079. doi:10.1115/PVP2015-45404.

Current crack opening displacement (COD) solutions for leak-before-break (LBB) analyses assume the ends of the cracked pipe, which is subjected to remote bending and internal pressure, are free to rotate. However, in plant piping systems, the pressure induced bending and imposed rotations are restrained, because the ends of the pipe are constrained by the rest of the piping system and other components. Hence, existing evaluation procedures, theoretically overestimate the COD values of a circumferential through-wall crack (TWC) in a piping system. These overestimations comprise one of the uncertainties in an LBB analysis, as it leads to an under-prediction of the leakage-size-crack length of a postulated leaking TWC for a prescribed leakage detection limit in a plant, and thus, results in a non-conservative estimation of the crack stability from an LBB perspective.

Historical efforts on the effects of restraint on COD have focused on a restraint distance from the crack to restrain the rotation of the pipe. This study provides a fundamentally different approach in that the underlying theory develops a relationship between the apparent rotational stiffness of a pipe with unrestrained ends and the material modulus as a function of crack length and pipe geometry. Thus, the local system stiffness from a plant structural model can be used to modify the unrestrained value of COD.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A080. doi:10.1115/PVP2015-45461.

Leak-before-break (LBB) is an important concept that could confirm design and integrity evaluation of nuclear power plant piping. For the LBB analysis, the detective leakage rate should be calculated for a through-wall cracked pipes. For this calculation, the crack opening displacement (COD) calculation is essential.

Recently, sodium faster reactor (SFR) which has thin-walled pipes with Rm/t ranged 30–40 was introduced and then the investigation of these thin walled pipes and elbows has received great attention in the LBB evaluation. In this context, the three-dimensional finite element (FE) analyses for thin elbows with circumferential crack under in-plane bending are carried out to investigate the elastic COD values.

Finally, the solution for elastic COD which can cover sufficiently thin elbow is successfully addressed.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A081. doi:10.1115/PVP2015-45468.

The geometry of a crack is a fundamental consideration when calculating leakage rates for Leak-before-Break assessments. Carrying out fluid mechanics calculations does not give any additional benefit if there is not enough information on the crack shape. To address this issue, work is being carried out under the R6 development programme to derive a model that couples fluid mechanics and solid mechanics. The aim is to combine complex crack shapes with relatively simple fluid mechanics models and compare with experimental data. Then, the model can be extended to examine various stress distributions, and give indications as to how conservative are the current models. The model is a development of the one presented in a previous PVP paper (Reference 1), and a special case of isothermal gas flow is considered, where the equations reduce to an Ordinary Differential Equation (ODE). This is solved using a Runge-Kutta integration scheme in MATHCAD. A test case is presented based on the crack geometries considered in experiments, and upon comparison with numerical results; it is clear that choosing the correct crack shape is crucial in obtaining accurate predictions of leak rate. The assumed crack openings are rectangular, diamond or elliptical. In addition to this, weld residual stress profiles are postulated, based on experience of welds in piping components. Comparing the numerical simulations with the simplified DAFTCAT model indicates that the more precise ODE method can reduce conservatism in calculation of leak rates.

Commentary by Dr. Valentin Fuster
2015;():V06AT06A082. doi:10.1115/PVP2015-45515.

In light water reactor designs, the concept of leak-before-break (LBB) can be applied for piping systems to exclude the dynamic effects of pipe rupture. The crack opening displacement (COD) and the J integral are important parameters in the LBB design. From preceding researches, it was revealed that when the restraint of pressure induced bending (PIB) is not considered in LBB evaluation, COD can be overestimated, resulting in a decrease in conservatism of the LBB design. If the pipe restraint is not considered, however, applied moment can also be overestimated. Thus, to take the pipe restraint effect into account when conducting the LBB analysis, the decrease of COD and the decrease of the applied moment must be considered simultaneously. In this regard, the authors have developed the restraint coefficient to account for the pipe restraint effect on both COD and the applied moment at the cracked section in earlier research. In this paper, the restraint coefficient was validated by comparing with an elastic-plastic FEA model including restraint boundary conditions. The effects of the restrained COD and the effective applied moment on the LBB evaluation were then investigated using the piping evaluation diagram. As a result, it was confirmed that the decrease in applied moment had greater influence on LBB evaluation than the decrease in COD. Therefore, the current practice of LBB evaluation which assumes the pipe is free to rotate can provide more conservative results than the case in which the pipe restraint is considered.

Commentary by Dr. Valentin Fuster

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