抄録
The effects of strain rate and strain wave form on the low-cycle fatigue behavior of 316 stainless steel at FBR temperature were studied in order to clarify the controlling factor of fatigue strength and fracture mechanism.
The following major results are obtained.
(1) Under symmetrical and asymmetrical straining (slow-fast and fast-slow wave) with the strain rate between 100%/sec to 10-3%/sec, the fatigue life decreases with a decrease of strain rate in tension going period. The fatigue life is affected only by the strain rate in tension going period, and is not affected by the strain rate in compression going period. Slow-fast wave is most damaging, but the effect of saw-tooth wave is not significant.
(2) The dependence of fatigue life on the strain rate in tension going period may not be due to the creep effect but due to the dynamic strain aging effect proper to FBR temperature. (500°C to 600°C)
(3) The fracture mode changes from transgranular cracking to intergranular cracking with a decrease of strain rate in tension going period. Slow-fast wave enhances the intergranular cracking, whereas fast-slow wave enhances the transgranular cracking.
(4) Thermal aging increases the fatigue life under symmetrical and asymmetrical straining, and the life reduction with the strain rate reduction in tension going period is not so significant for the thermally aged condition. This may be due to the disappearance of the dynamic strain aging effect by the thermal aging. Thermal aging enhances the precipitation of second-phase particles (mainly carbides) at the grain boundary, so that the dissolved carbon in the matrix which causes the dynamic strain aging decreases.
