Automatic near-threshold fatigue crack growth rate measurements at liquid helium temperature |
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Authors: | RL Tobler YW Cheng |
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Affiliation: | Fracture and Deformation Division, US Department of Commerce, National Bureau of Standards, 325, Broadway, Boulder, CO 80303, USA |
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Abstract: | The development of a fully automated test apparatus for near-threshold fatigue crack growth rate measurements in a liquid helium environment is described, and some initial results for AISI 300 series stainless steels are presented. The experimental apparatus consists of a servohydraulic test machine and a cryostat, complete with a minicomputer, a programmable arbituary waveform generator, a programmable digital oscilloscope and a fully automatic liquid helium refill system. The technique uses 6.4 mm thick compact specimens subjected to systematically decreasing loads, with 24 h operation at 40 Hz, the crack growth being continuously monitored by specimen compliance measurements. The results presented in this study include da/dN vs ΔK curves and threshold fatigue stress intensity factors, ΔKth, at 4 K for AISI 304L, 304LN and 316 stainless steels. The near-threshold fatigue behaviours of these materials are similar, and the fatigue crack growth rate trends at intermediate ΔK levels nearly agree with published results. |
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Keywords: | fatigue low temperature cryogenic equipment computerized control fatigue crack growth rates threshold stress intensity factors austenitic steels |
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