The current Section VIII Division 2 of ASME code permits only the method based on experience with comparable equipment of paragraph 5.5.2.2 to be used for the exemption from fatigue analysis when the design allowable stress is taken in the time dependent temperature range and the specified minimum tensile strength is greater than 552 MPa. In the case of 2.25Cr-1Mo-0.25V steels, the allowable stress of new Div.2 is higher than that of the old Div.2. Therefore, there are no experiences with comparable equipment. Code Case 2605-1 provides rules for fatigue evaluation of 2.25Cr-1Mo-0.25V steels at temperatures greater than 371 °C and equal to or less than 454 °C. An inelastic analysis including the effect of creep is required to be performed for all pressure parts according to Code Case 2605-1. There are six and more types of nozzles in each hydro-processing reactor. The inelastic analyses shall be performed for all types of nozzles. The simplified screening method for fatigue analyses and the simplified fatigue analysis procedure need to be developed because it takes much time to perform inelastic analyses. This paper provides sample analysis results for various types of nozzles and clarifies issue with respect to implementation of Code Case 2605-1. Then, a proposal for simplification and modification of Option 1 and Option 2 fatigue evaluation methods for nozzles of Code Case 2605-1 are proposed from these investigations.

The present paper presents results of numerical simulation for statistical evaluation of stress and strain at elevated temperature from view point of crystal plasticity level by employing a new Voronoi tessellation algorithm in the three dimensional geometry for general grain shape using first order tetrahedron element (four nodes). The elasticity tensors are assumed to include isotropic material and anisotropic material of FCC or BCC crystal using three material constant parameters. The employed finite element formulation is based on the updated Lagrange type expressed in the general form using trapezoidal integration rule in time domain, and the selective numerical integration scheme is used in the present analysis. The obtained numerical examples include the effects of employed finite elements, employed grain aggregate model, grain diameter size, and grain regularity on local stress. The statistical variation around mean value is investigated for the isotropic material and the anisotropic materials having different anisotropy ratio A in elastic range. The inelastic analysis at elevated temperature is also carried out for the anisotropic materials in order to investigate the statistical variation for the anisotropic materials in strain rate effect problem and creep strain program by introducing additional six cubic slip systems into the conventional 12 octahedral slip systems inelastic range.

A new robust method, called MARS (modulus adjustment and redistribution of stress), based on linear elastic finite element analyses has been proposed to evaluate inelastic strains in anisotropic bodies. The linearity of relaxation locus forms the basis of the method. A combination of modulus adjustment scheme and iterative strategy used in the MARS method satisfies the equilibrium and yield conditions, which in turn brings the static and kinematic distributions close to the actual distributions for a given load. Several notched bodies made of anisotropic material are analyzed using the MARS method and the inelastic strains evaluated are found to be in good agreement with those predicted using elastic-plastic finite element analysis.

Mechanical components and structures loaded into inelastic region can fail by low cycle fatigue (LCF). Evaluation of inelastic strain is an important stage in the LCF life prediction methodology. Different techniques, viz., experimental methods, elastic-plastic finite element analysis (FEA), and robust methods, can be used to predict inelastic strains. The state predicted by available robust methods does not correspond to equilibrium state of the component. A method called MARS (modulus adjustment and redistribution of stress) based on linear elastic FEA has been developed to obtain equilibrium and kinematic distributions close to the actual one. The proposed method uses an iterative strategy combined with a modulus reduction technique.

Within the last two decades, the use of elastic finite element analyses to demonstrate design compliance with the rules of the ASME Code has become a generally accepted engineering practice. Linearized stresses from these analyses are commonly used to evaluate primary stresses. For redundant structures or complex structural details, the use of such analyses, instead of simple equilibrium models, often results in significant overconservatism. Direct use of finite element results is often preferred because equilibrium solutions are not unique and effective equilibrium models are not easily constructed for complex three-dimensional structures. However, finite element analyses include secondary stresses, even for pressure, mechanical, and shock loading. For primary stress evaluation, the ASME Code allows the use of inelastic methods based on lower-bound solutions and plastic analysis. For primary stresses, the Code requires equilibrium to be satisfied without violating the yield strength of the material. The use of finite element inelastic analysis to partition mechanically induced stresses into the primary and secondary categories was introduced by Porowski et al. (1993). The latter provides a detailed discussion of the technical approach and the results for the axisymmetric junction between the plate and shell in a pressure vessel. This example was selected by the Session Organizer as a benchmark case to compare the efficiency of various analytical approaches presented at the Session. The authors have since used this approach to design more efficient structures. The practical application of this method to reduce the weight of complex redundant structures designed to meet primary stress limits is described herein for a more complex three-dimensional case. Plastic design utilizes the ability of actual materials to find the most efficient load distribution. A heat exchanger subjected to pressure, accelerations, and nozzle external loads is evaluated as a practical example. The results of elastic analyses are compared with those obtained by inelastic analyses. It is shown that inelastic analyses can be used effectively to reduce the weight of structures using only modern PCs for the engineering computations, as illustrated in this paper.

The Bree diagram has been incorporated in the ASME B&PV Code in the elevated temperature Code Case N47 as a design approach for limiting strain accumulation in cylinders subjected to cyclic thermal loadings under sustained primary stress. Since the Bree diagram is based upon uniaxial-stress model, it is pertinent to examine the influence of biaxial stresses on strain growth and cyclic stress-strain hysteresis response. The results of inelastic analyses presented in this paper showed that ratcheting and hysteresis behavior may also occur in the axial direction in addition to the hoop direction. Results of almost 100 load cases were presented to clarify the influence of biaxial membrane and thermal bending stresses on the structural behavior. A design approach for the assessment of this type of problem was suggested which utilizes these results.

Elastic follow-up behavior was estimated using simplified inelastic methods newly developed, where only a constitutive equation of creep and a triangle of elastic follow-up were employed in the analysis. Then, uniaxial cyclic elastic follow-up tests were performed on an austenitic stainless steel SUS 304 at 700°C, and test results were compared with analyses. Since most of the starting points of elastic follow-up tests in the present experiments were beyond the elastic limit of the test material, the triangle of elastic follow-up had to be modified in the analyses. As a consequence, experimental results of elastic follow-up behavior for the first cycle agreed well with those analytically determined. Moreover, the test results obtained in subsequent cycles were found conservative compared with an extension of analyses that used the data obtained in the first cycle of cyclic test.

The results of a long-term (approximately 2-1/2 yr), high-temperature failure test of a pressurized nozzle-to-spherical-shell model made of a well-characterized heat of type 304 stainless steel are presented and compared with inelastic deformation and failure predictions. The model, which was tested at 593°C (1100°F), was instrumented with capacitive strain gages in key locations. In addition to recording strains, the surface of the model in the junction region was periodically examined throughout the test for the formation and progression of the creep cracking that ultimately led to a leakage failure. The inelastic analysis predictions were based on guidelines developed and used for design of elevated-temperature nuclear components. Similarly, the failure model used was that upon which the ASME elevated-temperature Code Case N-47 for nuclear components is based. Thus, the reasonable agreement observed between experimental results and predictions adds confidence in the overall design methodology for elevated-temperature components. The results also demonstrated the leak-before-break nature of failure and the fact that pressure-induced stresses do not necessarily relax with time.

Towards assessing the creep-fatigue damage for the inner vessel of a pool-type fast reactor, 3-D elastic and axisymmetric inelastic analysis with classical kinematic hardening model, ORNL material model and more realistic CHABOCHE visco-plastic model have been performed. The material properties of stainless steel type 316 LN at various temperatures are used in the analysis. Using the results of this analysis, the creep-fatigue damage and ratcheting are assessed as per the rules of RCC-MR and results have been compared for the various constitutive models used. Inelastic analysis has been performed only at highly stressed local regions. Computations with the ORNL material model is found to be satisfactory in terms of conservatism and computer cost involved, and hence recommended for use in the design stage. Further, conservatism in estimating the creep fatigue-damage of the component associated with the elastic route of RCC-MR has been quantified. The inelastic and viscoplastic analysis tool has been validated using experimental benchmark problems.

The purpose of this paper is to identify the weld effects which are of primary importance in elevated temperature design. A full-scale Fast Flux Test Facility (FFTF) Intermediate Heat Exchanger (IHX) was tested at Westinghouse to investigate weld effects at elevated temperature. The IHX was subjected to two and a half times the design pressure. In addition, one of the four welded nozzles of the IHX was subjected to 26 severe thermal downshock transients, which were interspersed with 156 hr of creep hold time at 1100°F (593°C). At the end of testing, creep rupture cracks were observed in the weldments at the nozzle to cylinder intersections, whether or not they experienced downshock transients. Detailed three-dimensional inelastic analyses were performed to investigate the effects of welding on the creep-rupture strength of weldments. The analyses suggest that the weldment material property variation contributed to creep-rupture cracking at high primary pressure loading. The weld metal and heat-affected zone had higher yield strength, but lower creep ductility compared to the nozzle base material. The analytical predictions and metallurgical observations suggest that the role of residual stresses on creep-rupture cracking is of secondary importance, and need not be numerically simulated in the elevated temperature design of weldments.

The design of mechanical components that operate in elevated temperature environment where creep effects are significant usually requires creep-fatigue assessments. The ASME Code Case N-47 contains rules for these assessments based on both inelastic and elastic stress analysis. Although an inelastic stress analysis generally more accurately predicts effects from creep and plasticity, an elastic analysis is often preferred since it is much simpler and less costly. New creep-fatigue rules for use with elastic analysis results have recently been proposed to enhance the rules’ accuracy and usefulness. This paper describes such new methods and rules for creep-fatigue assessments. Ingredients of the new methods include elastic follow-up, ratcheting, multiaxiality, plasticity, creep, and relaxation considerations and associated adjustment factors. The basis for the adjustments and a comparison of results to those obtained using inelastic analysis are provided. The new methods will provide a wider range of practical application of elastic creep-fatigue rules than permitted by previous code methods in design of components for elevated temperature service.

High-level dynamic testing of a prototypic insulated small bore diameter pipe system is described. The pipe system was supported by a minimum number of supports, with several combinations of supports and support locations tested. Load input consisted of a dynamic multifrequency time history and sinusoidal loading at increasing acceleration levels until gross deformation of the system occurred. Very high level pipe damping was exhibited when plastic response occurred. Post-test analyses were performed to assess the structural loading and response of the pipe system under the dynamic load input. Several elastic and elasto-plastic analysis failure prediction methods were used, along with simplified inelastic analysis.

A mature design methodology, consisting of inelastic analysis methods provided in U.S. Department of Energy guidelines and failure criteria contained in ASME Code Case N-47, exists in the United States for high-temperature reactor components. The objective of this paper is to assess the adequacy of that overall methodology by comparing predicted inelastic deformations and lifetimes with observed results from structural failure tests and from an actual service failure. Comparisons are presented for three structural cases: 1) nozzle-to-spherical shell specimens, emphasizing stresses at structural discontinuities; 2) welded structures, emphasizing metallurgical discontinuities; and 3) thermally loaded cylinders and pipes, emphasizing thermal discontinuities. The comparisons between predicted and measured inelastic responses are generally reasonable; quantities are sometimes overpredicted somewhat and sometimes underpredicted. However, even seemingly small discrepancies in predicted stresses and strains can have a significant effect on life, which is thus not always as closely predicted. For a few cases, the lifetimes are substantially overpredicted, which raises questions regarding the methodology and/or the adequacy of the current design margins.

This paper compares theoretical and experimental results for full-scale, prototypical components tested at elevated-temperatures to provide validation for inelastic analysis methods, material models, and design limits. Results are discussed for piping elbow plastic and creep buckling, creep ratcheting, and creep relaxation; nozzle creep ratcheting and weld cracking; and thermal striping fatigue. Comparisons between theory and test confirm the adequacy of components to meet design requirements, but identify specific areas where life prediction methods could be made more precise.

This paper describes the stress analysis performed to assess structural adequacy of the Clinch River Breeder Reactor (CRBR) core removable shield assemblies. Removable shield assemblies are located in the peripheral region of the core (between blanket assemblies and the fixed radial shield), and are subjected to severe cross-sectional thermal gradients and seismic loads requiring a relatively complex duct load pad design. For cost-effectiveness, the analysis was conducted in two stages. First, an elasto-plastic seismic stress analysis was performed using a detailed nonlinear finite element model (with gaps) of the load pad configuration. Next, in order to determine the total strain accumulation and the creep-fatigue damage the maximum seismic stresses combined with the “worst” thermal stresses from a single assembly model were used to perform a simplified inelastic analysis using two sets of material properties to bound the changing material conditions during reactor operation. This work demonstrated the necessity and applicability of the two simplified analysis techniques in elevated temperature structural design, i.e., the treatment of time-dependent degradation of material properties due to temperature and nuclear irradiation, and the use of time-independent finite element stress analysis results to perform a simplified creep-fatigue analysis.

Inelastic analysis of a prototypic liquid metal fast breeder reactor (LMFBR) piping system is generally performed without explicit consideration of time-dependent seismic loading. This paper presents an approximate procedure to simulate dynamic seismic loading as an equivalent static load for inelastic analysis. This procedure utilizes the results of the available linear seismic response spectrum analysis to calculate external (statically equivalent) loads. These statically equivalent loads are selected such that they produce internal forces and moments comparable to those predicted by the linear response spectrum analysis.

This paper illustrates that in addition to the elastic stress intensity range it is necessary to evaluate the stress intensities (and corresponding material properties) at specific times during a postulated thermal transient to characterize inelastic response at a structural discontinuity. The specific case of an austenitic flued head operating in a temperature range of 600 to 1050°F (316 to 538°C) is used to calculate ratcheting strain accumulation due to cyclic primary and secondary loads. Detailed inelastic analyses were performed to evaluate two designs. Both designs have essentially the same stress intensity range; however, only the design with lower stress intensity during steady-state elevated temperature operation satisfied the ASME Code Case N-47 strain accumulation limits.

Explicit piping design procedures are not provided in theASME Code for structural evaluation of ovalization and fabrication effects at the butt-welded end of an elbow. This paper presents a conservative semi-analytical procedure that can be used with elastic as well as inelastic analysis to evaluate an elbow cross section welded to a straight pipe. The concept of carry-over factors is used to obtained ovalization stresses or strains at the elbow end. The stresses introduced by material and geometric nonuniformities in the fabrication process are then added to the ovalization stresses to complete structural evaluation of the girth butt-welded elbow joint.

A propagating buckle can be initiated in tubes under external pressure from localized imperfections. Once initiated a buckle will propagate with the result of flattening the tube behind it. The lowest pressure at which it can propagate is known as the propagation pressure, and it represents a characteristic pressure of the tube. The paper attempts to model quasistatic steady-state propagation through a plane strain large deflection, inelastic analysis of the collapse of a circular ring. An energy balance type of argument is used to obtain a lower-bound estimate of the propagation pressure from the ring response. The results are compared with experimental results obtained from stainless steel and aluminum tubes having diameter-to-thickness ratios ranging between 100 and 10. It was found that for these two materials the estimated value for the propagation pressure was in good agreement with experimental results for D / t > 20.

This paper discusses a simplified method which was proposed a few years ago by J. Zarka and J. Casier to obtain a straightforward evaluation of the total inelastic strain which accumulates in a structure under cyclic loadings when shakedown occurs for a linear kinematic hardening material. The theoretical basis is recalled and an example of the application of the method is given which underlines its efficiency.