C.E. Pugh

Some trends in constitutive equation model development for high-temperature behavior of fast-reactor structural alloys

Abstract Constitutive equations for representing the inelastic behavior (elastic-plastic and creep) of fast-reactor structural alloys are discussed with an emphasis on 2 1 4 Cr-1 Mo steel. Equations that are recommended for use in current design activities are outlined, and background information to their selection is discussed. Some results from ongoing efforts to establish improved methods are also described. Sample experimental data illustrate specific features of material behavior, and a constitutive equation model that is qualitatively capable of representing these observations is described. The model is a flow potential formulation, and its relationship to the works of other investigators is identified.

Wide-plate crack-arrest tests utilizing a prototypical pressure vessel steel

Wide-plate crack-arrest tests are being performed at the National Bureau of Standards (Gaithersburg, MD) under the Heavy-Section Steel Technology (HSST) Program and are designed to provide fracture-toughness measurements at temperatures approaching or above the onset of the upper-shelf regime, in a rising toughness region and with increasing driving force. The test specimens are 1 × 1 × 0·1 m and possess a single-edge notch (crack) that initiates in cleavage propagation at low temperature and arrests in a region of increased fracture toughness. The toughness is achieved through a linear transverse temperature profile across the plate. Results obtained using a prototypical reactor pressure vessel steel (A533 grade B class 1 material) exhibit a significant increase in toughness at temperatures near and above the onset of Charpy upper shelf. Additionally, cleavage crack propagation and arrest at temperatures above the onset of Charpy upper shelf have been demonstrated.

Overview of the International Comparative Assessment Study of Pressurized Thermal-Shock in Reactor Pressure Vessels (RPV PTS ICAS)

Abstract This paper summarizes the recently completed International Comparative Assessment Study of Pressurized Thermal-Shock in Reactor Pressure Vessels (RPV PTS ICAS). The ICAS project brought together international experts to perform comparative evaluations of methodologies employed in the assessment of RPV integrity under prototypical PTS conditions. ICAS grew out of a strong interest expressed by the participants to proceed with further evaluations after completion of the earlier FALSIRE II project. FALSIRE focused on evaluation of structural analysis and fracture assessment methods on the basis of experimental and analytical results. The ICAS problem statement defined transient thermal–mechanical conditions postulated to result from loss-of-coolant accidents in a Western type four-loop RPV with cladding on the inner surface. The primary focus was on the behavior of relatively shallow cracks located under and through the cladding. The assessment activities were divided into three tasks: Deterministic Fracture Mechanics (DFM), Probabilistic Fracture Mechanics (PFM) and Thermal-Hydraulic Mixing (THM). The results showed that a best-estimate methodology for RPV integrity assessment could benefit from a reduction of the uncertainties in each phase of the process. Within the DFM task, where account was taken of material properties and boundary conditions, reasonable agreement was obtained in linear-elastic and elastic–plastic analyses. Results from linear-elastic and J -estimation analyses were shown to provide conservative estimates of peak crack driving force when compared with those from complex 3D finite element analyses. For the PFM task, linear-elastic solutions were again shown to be conservative with respect to elastic–plastic solutions (by a factor of 2 to 4). Scatter in solutions obtained using the same computer code was generally attributable to differences in input parameters, e.g. standard deviations for the initial value of RT NDT , as well as for nickel and copper content. In the THM task, while there was a high degree of scatter during the early part of the transient, reasonable agreement was obtained during the latter part of the transient. Generally, the scatter was due to differences in analytical approaches used by the participants, which included correlation-based engineering methods, system codes and three-dimensional computational fluids dynamics codes. Based on concluding discussions from ICAS participants, a project is being organized to develop a computer software tool named QUAMET (acronym for QUAlification METhodology) for future use in qualifying codes and analysts engaged in the structural integrity assessment of RPVs.

Fracture analyses of heavy-section steel technology wide-plate crack-arrest experiments

A series of six wide-plate crack-arrest tests was recently completed by the Heavy-Section Steel Technology program at the National Bureau of Standards, Gaithersburg, MD, using tensile-loaded specimens of A533 Grade B Class 1 steel. Crack-arrest data were obtained at temperatures in the transition range and above the onset of the Charpy upper shelf, thereby providing a basis for the development and evaluation of improved fracture-analysis methods. The 1 by 1 by 0.102-m single-edge-notched (SEN) specimens were welded to long straight pull tabs and subjected to a transverse linear temperature gradient before loading. The crack tips were sharpened by hydrogen-charging an electron-beam weld. The tests were designed to obtain crack arrest near the middle of the specimen where the temperature would produce a high-toughness level in the upper transition region of the material. The specimens were instrumented with strain gages and thermocouples. Initial static design calculations were made using textbook formulas. Additional calculations, using an assumed set of K I D versus a and T relations and an effective stress wave concept, confirmed the reasonableness of tentative design parameters. Pretest and posttest dynamic finite-element calculations were performed for each test. Computed results are compared with transient data for crack-line strains, crack speed, crack-opening displacement, arrest location, and postarrest tearing. Results from both application-mode and generation-mode dynamic analyses are presented. The arrest toughness values calculated from the test data are summarized for temperatures ranging from the transition into the Charpy upper-shelf range.

Some advances in fracture studies under the heavy-section steel technology program☆

Recent results are summarized from HSST studies in three major areas that relate to assessing nuclear reactor pressure vessel integrity under pressurized-thermal-shock (PTS) conditions. These areas are irradiation effects on the fracture properties of stainless steel cladding, crack run-arrest behavior under non-isothermal conditions, and fracture behavior of a thick-wall vessel under combined thermal and pressure loadings. Since a layer of tough stainless steel weld overlay cladding on the interior of a pressure vessel could assist in limiting surface crack extension under PTS conditions, its resistance to radiation embrittlement was examined. A stainless steel overlay cladding, applied by a submerged arc, single-wire, oscillating-electrode method, was irradiated to 2 × 1023 neutrons/m2 (> 1 MeV) at 288°C. Yield strength increases up to 27% and a slight increase in ductility were observed. Charpy V-Notch data showed a ductile-to-brittle transition behavior caused by temperature-dependent failure of the 8-ferrite phase. The type 308 cladding, microstructurally typical of that in reactor pressure vessels, showed very little degradation in either upper-shelf energy or transition temperature due to irradiation. Crack-arrest behavior of A533 grade B class 1 steel was examined for temperatures extending above the onset of Charpy upper-shelf. Crack-arrest experiments that use wide-plate specimens have shown crack arrest occurring prior to transition to tearing or tensile instability. High values of crack-arrest toughness have been recorded (static values above 400 MPa m that are well above the maximum value that safety assessment criteria assume such materials can exhibit. A validation experiment was performed by exposing an intentionally flawed HSST intermediate test vessel to combined pressure and thermal transients. The experiment addressed warm-prestressing phenomena, crack propagation from brittle to ductile regions, and crack stabilization in ductile regions. Test and analysis results are summarized.

Behavior of annealed type 316 stainless steel under monotonic and cyclic biaxial loading at room temperature

Abstract This paper addresses the elastic-plastic behavior of type 316 stainless steel, one of the major structural alloys used in liquid-metal fast breeder reactor components. The study was part of a continuing program to develop a structural design technology applicable to advanced reactor systems. Here, the behavior of solution annealed material was examined through biaxial stress experiments conducted at room temperature under radial loadings (√3τ = σ) in tension-torsion stress space. The effects of both stress limited monotonic loading and strain limited cyclic loading were determined on the size, shape, and position of yield loci corresponding to a small offset strain (10 microstrain) definition of yield. In the present work, the aim was to determine the extent to which the constitutive laws previously recommended for type 304 stainless steel are applicable to type 316 stainless steel. It was concluded that for the conditions investigated, the inelastic behavior of the two materials are qualitatively similar. Specifically, the von Mises yield criterion provides a reasonable approximation of initial yield behavior and the subsequent hardening behavior, at least under small offset definitions of yield, is to the first order kinematic in nature.

Late-event viscoplasticity in wide-plate crack-arrest tests

A primary objective of the crack-arrest studies being conducted by the Heavy-Section Steel Technology (HSST) program is to understand pressure vessel conditions that would initiate growth of an existing crack and conditions that would lead to arrest of a moving crack. In meeting this objective, the HSST program is generating crack-arrest data over an expanded temperature range through tests involving large cylinders, pressure vessels and wide-plate specimens. The role of nonlinear rate-dependent effects in the interpretation of crack run-arrest events in ductile material is being investigated by the HSST program through development and applications of viscoplastic-dynamic finite element analysis techniques. This paper describes the portion of those studies that relate to the installation of two viscoplastic constitutive models and several proposed fracture criteria into the ADINA finite element program. The formulations of the Bodner-Partom and the Perzyna constitutive models that were installed in ADINA are presented, including a compilation of material parameters for these models obtained from dynamic stress-strain data for A533 grade B class 1 steel. This is followed by a description of path-area integrals J′, T∗ (from Nishioka and Atluri) and J (from Kishimoto) and of the function γ (representing energy flow to the crack tip) installed in the ADINA program. Finally, the combined predictive capabilities of these techniques are applied to the analysis of one of the recently completed HSST wide-plate crack-arrest tests.

Elastodynamic fracture analysis of large crack-arrest experiments

Abstract Recent elastodynamic fracture analysis results are summarized from Heavy-Section Steel Technology (HSST) studies in two major areas that related to assessing nuclear reactor pressure vessel integrity under pressurized-thermal-shock (PTS) conditions. These areas are crack run-arrest behavior in wide plates under nonisothermal conditions and fracture behavior of a thick-wall vessel under combined thermal and pressure loadings. The WP-1 series of HSST wide-plate crack-arrest tests are being performed at the National Bureau of Standards (NBS), Gaithersburg, MD, using specimens from HSST Plate 13A of A533 grade B class 1 steel. The six tests in the WP-1 series are aimed at providing crack-arrest data at temperatures up to and above that corresponding to the onset of the Charpy upper-shelf, as well as providing information on dynamic fracture (run and arrest) processes for use in evaluating improved fracture analysis methods. Elastodynamic analyses have been completed for the actual test conditions of the four tests, WP-1.1 through WP-1.4, conducted thus far in the WP-1 series. In this paper, the computed results are compared with data for crackline strain-time response, crack-propagation speed, arrest location and post-arrest tearing. The paper includes a summary of the arrest toughness calculations compiled in the four tests at temperatures that range from transition to upper-shelf values for the wide-plate material. These same elastodynamic fracture analysis techniques have been applied to the analysis of the first pressurized-thermal-shock experiment (PTSE-1) performed at ORNL. The experiment addressed warm-prestressing phenomena, crack propagation from brittle to ductile regions, and crack stabilization in ductile regions. Test and analysis results are summarized in the paper.

On establishing constitutive equations for use in design of high-temperature fast-reactor structures☆☆☆

Abstract The presentation describes the approach being used to establish constitutive equations for wide use in the design of fast breeder reactor (FBR) components in the US. The approach combines exploratory experiments, constitutive model studies, studies of computational techniques, and tests of simple structural configurations. Short-time (elastic-plastic) behavior, long-time (creep) behavior, and their interactions are considered, and some of the background to equations now identified for use in current FBR design applications involving three structural alloys is discussed. Comments are also given on current efforts aimed at identifying improved constitutive equations for these alloys and on properties data required for design applications. References are cited which have addressed the status of the process at various times.

Applications of adina to viscoplastic-dynamic fracture mechanics analysis

Nonlinear rate-dependent effects in crack run-arrest events in ductile materials are being investigated through development and applications of viscoplastic-dynamic finite element analysis techniques. This paper describes a portion of these studies wherein various viscoplastic constitutive models, dynamic crack-propagation algorithms, and proposed nonlinear fracture criteria have been installed in the general purpose ADINA finite element computer program. The predictive capabilities of the nonlinear techniques are evaluated through analyses of crack-arrest tests of nonisothermal wide-plate specimens. Values of fracture parameters calculated by elastodynamic and viscoplastic dynamic techniques are compared to assess the impact of including viscoplastic effects in the computational models. Mesh refinement studies are presented that examine whether the proposed fracture parameters converge to nonzero values in viscoplastic dynamic analyses or whether they are controlled by element dimensions. Plans are reviewed for additional computational studies to assess the utility of viscoplastic analysis techniques in constructing a dynamic inelastic fracture mechanics model for ductile steels. 26 refs., 8 figs., 4 tabs.