Post-irradiation fracture toughness of unalloyed molybdenum, ODS molybdenum, and TZM molybdenum following irradiation at 244 °C to 507 °C

This paper describes the fracture toughness characterization of annealed 304L and 316L stainless steels and precipitation hardened Alloy 718, performed at the Oak Ridge National Laboratory as a part of the experimental design and development for the Accelerator Production of Tritium (APT) target/blanket system. Materials were irradiated at 25 to 200C by high-energy protons and neutrons from an 800-MeV, 1-mA proton beam at the Los Alamos Neutron Science Center (LANSCE). The proton flux produced in LANSCE is nearly prototypic of anticipated conditions for significant portions of the APT target/blanket system. The objective of this testing program was to determine the change in crack-extension resistance of candidate APT materials from irradiation at prototypic APT temperatures and proton and neutron fluxes. J-integralresistance (J-R) curve toughness tests were conducted in general accordance with the American Society for Testing and Materials Standard Test Method for Measurement of Fracture Toughness, E 1820-99, with a computer-controlled test and data acquisition system. J-R curves were obtained from subsize disk-shaped compact tension specimens (12.5 mm in diameter) with thicknesses of 4 mm or 2 mm. Irradiation up to 12 dpa significantly reduced the fracture toughness of these materials. Alloy 718 had the lowest fracture toughness in both the unirradiated and irradiated conditions. All irradiated specimens of Alloy 718 failed by sudden unstable crack extension regardless of dose or test temperature. Type 304L and 316L stainless steels had a high level of fracture toughness in the unirradiated condition and exhibited reduction in fracture toughness to saturation levels of 65 to 100 MPa/m. The present reduction in fracture toughness is similar to changes reported from fission reactor studies. However, the currently observed losses in toughness appear to saturate at doses slightly lower than the dose required for saturation in reactor-irradiated steels. This difference might be attributed to the increased helium and hydrogen production associated with irradiation in the high-energy, mixed proton/neutron spectrum.

Fracture Toughness Characterization of 304L and 316L Austenitic Stainless Steels and Alloy 718 After Irradiation in High-Energy, Mixed Proton/Neutron Spectrum

This paper investigates the effect of load shedding steps on constant-ΔK testing. ASTM fatigue crack growth specimens have been used in constant-ΔK tests for many fatigue investigations. In the tests, load levels need to be gradually stepped down to keep the stress intensity factor constant. This work points out that care should be taken in choosing the length of the step in order to control ΔK variation. Formulas for ΔK as well as crack growth rate variability are derived. The experimental application of these techniques is also presented.

Load shedding effect on constant-ΔK testing

Recent advances in fracture toughness have led to employment of the Weibull statistic to model scatter of fracture toughness in the transition region of low-alloyed ferritic steels. This methodology uses a concept of the universal temperature dependence of fracture toughness in the transition region, the so-called “master curve”. The current physical background for this methodology suggests that it is applicable to a wide variety of ferritic bcc steels, including tempered ferritic-martensitic steels. Those steels are structural material candidates for fusion reactors. However, irradiation embrittlement (shift of the ductile-to-brittle transition region) of these steels is one of the main concerns for application purposes. Yet, the transition fracture toughness data for this class of steels are rather sparse. In this study, two types of fracture toughness specimens of a F82H steel were tested to verify the master curve concept. Specimens were tested at several temperatures in the transition region and at least four specimens were tested at each temperature allowing for application of the Weibull statistic/master curve analysis procedure. The largest specimens were 1T compact specimens. Broken halves of 1T specimens were later used to machine and test smaller, 0.4T, size specimens which could be more suitable for irradiation experiments. It was shown that scatter of fracture toughness of this material was rather high relative to scatter of low-alloyed steels but it was similar between larger and smaller specimens.

5276 - application of the master curve methodology to fracture toughness characterization of ferritic-martensitic steel

A computer-controlled system consisting of a servohydraulic test machine, an in-house designed test controller, and a desktop computer has been developed for performing automated fracture toughness and fatigue crack growth testing both in the laboratory and in hot cells for remote testing of irradiated specimens. Both unloading compliance and dc-potential drop can be used to monitor crack growth. The test controller includes a dc-current supply programmer, a function generator for driving the servohydraulic test machine to required test outputs, five measurement channels (each consisting of low-pass filter, track/hold amplifier, and 16-bit analog-to-digital converter), and digital logic for various control and data multiplexing functions. The test controller connects to the computer via a 16-bit wide photo-isolated bidirectional bus. The computer, a Hewlett-Packard series 200/300, inputs specimen and test parameters from the operator, configures the test controller, stores test data from the test controller in memory, does preliminary analysis during the test, and records sensor calibrations, specimen and test parameters, and test data on flexible diskette for later recall and analysis with measured initial and final crack length information. During the test, the operator can change test parameters as necessary. 24 refs., 6 figs.

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D.J. Alexander

Papers

A computer-controlled automated test system for fatigue and fracture testing