B.R. Bass

Fracture assessment of shallow-flaw cruciform beams tested under uniaxial and biaxial loading conditions

A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed for application to the safety assessment of RPVs containing postulated shallow surface flaws. Matrices of cruciform beam tests were developed to investigate and quantify the effects of temperature, biaxial loading, and specimen size on fracture initiation toughness of two-dimensional (constant depth), shallow, surface flaws. The cruciform beam specimens were developed at Oak Ridge National Laboratory (ORNL) to introduce a far-field, out-of-plane biaxial stress component in the test section that approximates the nonlinear stresses resulting from pressurized-thermal-shock or pressure–temperature loading of an RPV. Tests were conducted under biaxial load ratios ranging from uniaxial to equibiaxial. These tests demonstrated that biaxial loading can have a pronounced effect on shallow-flaw fracture toughness in the lower transition temperature region for an RPV material. The cruciform fracture toughness data were used to evaluate fracture methodologies for predicting the observed effects of biaxial loading on shallow-flaw fracture toughness. Initial emphasis was placed on assessment of stress-based methodologies, namely, the J–Q formulation, the Dodds–Anderson toughness scaling model, and the Weibull approach. Applications of these methodologies based on the hydrostatic stress fracture criterion indicated an effect of loading-biaxiality on fracture toughness; the conventional maximum principal stress criterion indicated no effect. A three-parameter Weibull model based on the hydrostatic stress criterion is shown to correlate with the experimentally observed biaxial effect on cleavage fracture toughness by providing a scaling mechanism between uniaxial and biaxial loading states.

The heavy-section steel technology pressurized-thermal-shock experiment, PTSE-1☆

Abstract A pressurized-thermal-shock (PTS) facility was developed in the Heavy-Section Steel Technology Program at Oak Ridge National Laboratory for performing experiments that challenge predictions of analytical methods applicable to full-scale reactor pressure vessels under combined loading. The first experiment (PTSE-1) was designed to address three principal issues: (1) warm-prestressing phenomena; (2) crack propagation from brittle to ductile regions; and (3) transient crack stabilization in ductile regions. The paper presents a description of the PTS facility at ORNL and a review of the objectives and results of the first test. Also included are elastodynamic finite-element analyses of the two crack run-arrest events that occurred in the second and third phases of the test. Finally, some conclusions and recommendations are presented based on the outcome of the first experiment.

Developments in Local Approach methodology with application to the analysis/re-analysis of the NESC-1 PTS benchmark experiment

Abstract Local Approach methods have received considerable attention in recent years as a complementary approach to structural integrity assessment. These approaches are based on the application of micro-mechanistic models of failure in which the stress, strain and ‘damage’ local to the crack-tip are related to the critical conditions required to initiate/propagate fracture. The models are calibrated in terms of material parameters that are deemed fully transferable and derived using a combination of reference test data and supporting stress analysis. Once calibrated, using small-scale test data, the models are assumed independent on geometry and loading configuration. For a given failure mechanism, the model parameters may be used in the assessment of a structure fabricated from the same material (for appropriate temperatures, loading rates, etc). The paper describes the work initially undertaken in relation to the NESC-1 (Network for Evaluating Steel Components) spinning cylinder test, in order to validate the application of Local Approach methods for the case of PTS loading. The predicted amount of pre-cleavage ductile tearing and the timing of the subsequent cleavage event are compared with the observed fracture behaviour of the defect. The paper then highlights several areas in which Local Approach methodology has been developed since the initial work on PTS. These include: • Calibration of the cleavage model across a range of temperatures and constraint states. • Treatment of 3D defects. • Reference to hydrostatic stress in cleavage fracture predictions. • Simplification and standardisation of analytical techniques for more routine use in integrity assessments. The paper concludes that results from large-scale structural experiments, such as the NESC-1 spinning cylinder test, will be of lasting value in validating developments in Local Approach methodology and other advanced methods of fracture assessment. This is particularly true in the authors’ current work that seeks to achieve an overall simplification in methodology, without sacrificing predictive accuracy.

HSST wide-plate test results and analysis

Abstract Fifteen wide-plate crack-arrest tests have been completed to date, ten utilizing specimens fabricated from A533B class 1 material (WP-1 and WP-CE series), and five fabricated from a low upper-shelf base material (WP-2 series). Each test utilized a single-edge notched specimen that was subjected to a linear thermal gradient along the plane of crack propagation. Test results exhibit an increase in crack-arrest toughness with temperature, with the rate of increase becoming greater as the temperature increases. When the wide-plate test results are combined with other large-specimen results the data show a consistent trend in which the K Ia data extends above the limit provided in ASME Section XI.

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.

Applications of Energy Release Rate Techniques to Part-Through Cracks in Experimental Pressure Vessels

In nonlinear applications of computational fracture mechanics, energy release rate techniques are used increasingly for computing stress intensity parameters of crack configurations. Recently, deLorenzi used the virtual-crack-extension method to derive an analytical expression for the energy release rate that is better suited for three-dimensional calculations than the well-known J-integral. Certain studies of fracture phenomena, such as pressurized-thermal-shock of cracked structures, require that crack tip parameters be determined for combined thermal and mechanical loads. A method is proposed here that modifies the isothermal formulation of deLorenzi to account for thermal strains in cracked bodies. This combined thermo-mechanical formulation of the energy release rate is valid for general fracture, including nonplanar fracture, and applies to thermo-elastic as well as deformation plasticity material models. Two applications of the technique are described here. In the first, semi-elliptical surface cracks in an experimental test vessel are analyzed under elastic-plastic conditions using the finite element method. The second application is a thick-walled test vessel subjected to combined pressure and thermal shock loadings.

Evaluating the NESC-I test and the integrated approach to structural integrity assessment

The NESC-I spinning cylinder test was designed to simulate selected conditions associated with an ageing reactor pressure vessel (RPV) subjected to severe pressurised thermal shock (PTS) loading and containing hypothetical flaws. It formed the focal point of the first project of the Network for Evaluation of Structural Components (NESC), with the objective of validating the combination of non-destructive inspection and structural mechanics assessment procedures for evaluating the integrity of such an aged structure containing postulated flaws. The huge amount of data generated over the seven-year project has been evaluated and is now available to the international community. The test demonstrated that, for the specific conditions considered, defects of up to 74 mm depth in material related to that of an ageing RPV would not propagate to cause catastrophic failure under a severe PTS-type thermal shock. This outcome was fully in line with the pre-test analysis forecasts, which combined the defect-size information supplied from blind inspections trials, a comprehensive materials data set, and a range of structural analysis tools.