Extending near-threshold fatigue crack growth mesoscopic model-application to case of high vacuum

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In this research, we assessed the contribution of the environment on the decrease in threshold stress intensity factor (SIF) range ΔKth due to high maximum SIF Kmax," that is observed for several materials in the Kmax-constant fatigue crack growth (FCG) tests, under i) closure-free and non-corrosive atmosphere conditions are realized. We ran the following tests for 0.55% carbon steel JIS S55C, which is known to experience the phenomena. The tests were: i) static sustained load cracking (SLC) test (load held constant at Kmax, after running Kmax-constant ΔKth test) in air, dynamic SLC tests (constant ΔK below ΔKth with constant Kmax) in air, and iii) Kmax-constant ΔKth test in high vacuum. No crack growth was observed in tests and The decrease in ΔKth due to high Kmax in vacuum was negligible. The results of the Charpy impact tests indicated that the material was in the brittle-ductile transition range at room temperature. From these results, we concluded that the decrease in ΔKth due to high Kmax was mainly due to the brittleness of the material, which can be excluded in high vacuum. The results support the validity of our model for the phenomena, that microscopic crack growth in a grain due to static mode failure (brittle) accelerated the expected cyclic crack growth, considering that high vacuum excluded the possibility of static failure mode.