Wide-plate Crack-arrest Tests Research Articles

Heavy-Section Steel Technology and Irradiation Programs—Retrospective and Prospective Views

In 1965, the Atomic Energy Commission, at the advice of the Advisory Committee on Reactor Safeguards, initiated the process that resulted in the establishment of the Heavy Section Steel Technology (HSST) program at Oak Ridge National Laboratory. In 1989, the Heavy-Section Steel Irradiation (HSSI) program, formerly the HSST task on irradiation effects, was formed as a separate program, and in 2007, the HSST/HSSI programs, sponsored by the U.S. Nuclear Regulatory Commission (USNRC), celebrated 40 years of continuous research oriented toward the safety of light-water nuclear reactor pressure vessels (RPVs). This paper presents a summary of results from those programs with a view to future activities. The HSST program was established in 1967 and initially included extensive investigations of heavy-section low-alloy steel plates, forgings, and welds, including metallurgical studies, mechanical properties, fracture toughness (quasi-static and dynamic), fatigue crack-growth, and crack-arrest toughness. Also included were irradiation effects studies, thermal shock analyses, testing of thick-section tensile and fracture specimens, and nondestructive testing. In the subsequent decades, the HSST Program conducted extensive large-scale experiments with intermediate-size vessels (with varying size flaws) pressurized to failure, similar experiments under conditions of thermal shock and even pressurized thermal shock (PTS), wide-plate crack-arrest tests, and biaxial tests with cruciform-shaped specimens. Extensive analytical and numerical studies accompanied these experiments, including the development of computer codes such as the recent Fracture Analysis of Vessels Oak Ridge code currently being used for PTS evaluations. In the absence of radiation damage to the RPVs, fracture of the vessel is improbable. However, it was recognized that exposure to high energy neutrons can result in embrittlement of radiation-sensitive RPV materials. The HSSI Program conducted a series of experiments to assess the effects of neutron irradiation on RPV material behavior, especially fracture toughness. These studies included RPV plates and welds, varying chemical compositions, and fracture toughness specimens up to 101.6 mm (4 in.) thickness. The results of these investigations, in conjunction with results from commercial reactor surveillance programs, are used to develop a methodology for the prediction of radiation effects on RPV materials. Results from the HSST and HSSI programs are used by the USNRC in the evaluation of RPV integrity and regulation of overall nuclear plant safety.

Wide-plate crack-arrest tests utilizing prototypical and degraded (simulated) pressure vessel steels

Sixteen wide-plate crack-arrest tests have been completed, ten utilizing specimens fabricated from A533B class 1 material and six fabricated from a low-upper-shelf base material. 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 ( K 1a) with temperature, with the rate of increase becoming greater as the temperature increases. When the wide-plate test results are compared with other large-specimen results, the data show a consistent trend in which the K 1a data extend above the limit provided in ASME Section XI.

HSST wide-plate test results and analysis

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.

High-temperature crack-arrest behavior of prototypical and degraded (simulated) reactor pressure vessel steels

Sixteen wide-plate crack-arrest tests have been completed, ten utilizing specimens fabricated from A533B Class 1 material and six fabricated from a low-upper-shelf base material. 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 ( K Ia ) with temperature, with the rate of increase becoming greater as the temperature increases. When the wide-plate test results are compared with other large-specimen results, the data show a consistent trend in which the K Ia data extend above the limit provided in ASME Section XI.

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.

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.

Wide plate crack arrest testing

Five wide-plate crack arrest tests have been performed between September 1984 and December 1985 on an ASTM A533B quenched and tempered steel. These tests were done in the 26 MN Universal Testing Machine located at the National Bureau of Standards. The specimens were fractured in a thermal gradient to make the crack initiate at a preexisting notch in a cold, brittle region, and arrest in a hot, tough region. To obtain a great deal of information from these tests, each specimen was thoroughly instrumented with thermocouples, strain gages, crack-mouth-opening displacement gages, timing wires, and/or acoustic emission transducers. Fast data response was obtained from these sensors during the run-arrest events, which generally lasted for less than 10 ms. Estimates of two types of important data were obtained: (1) crack velocity as a function of time, and (2) initiation and arrest toughness. Thus, a successful data collecting system was developed for acquiring the basic data required for the prediction of the behavior of nuclear pressure vessels subjected to pressurized thermal shock.

Elastodynamic fracture analysis of large crack-arrest experiments

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.