間欠過大ひずみ下における低サイクル疲労特性と表面き裂進展挙動

抄録

Low-cycle fatigue properties and surface crack growth behavior were investigated under two-step intermittent over-straining (ISO) of various strain ratios λ on low-carbon steel S 15 C and austenitic stainless steel SUS 304 specimens. The results obtained were summarized as follows : (1) The plastic strain range Δε_p as a cyclic deformation property did not change after IOS for S15C, but theΔε_p decreased after IOS for SUS304 because of remarkable strain hardening. (2) A retardation phenomenon in surface crack growth at low values of λ after IOS and cycles to failure N_f were observed for both the S 15 C and the SUS 304 specimens. When the value of λ increased, the retardation effect decreased and crack growth curves for S 15 C were at the front or rear of the curve of a constant amplitude test, but for SUS 304, the N_f decreased more clearly than that of the constant amplitude test because of the remarkable acceleration phenomenon after ISO. (3) When we arranged the surface crack growth rate da/dN with J integral range J under the IOS condition, the data lay in the range of a factor of two for the da/dN-J relations of the constant amplitude tests, in spite of repeated retardation or acceleration of crack growth after IOS.