Abstract-A collaborative study was conducted by the Subcommittee of the Japanese Society of Materials Science Committee on High Temperature Strength, in order to examine thermal fatigue properties of four Japanese high temperature alloys, and to determine short tensile, creep and isothermal fatigue properties.Thermal fatigue lives of SUS 304 and Hastelloy X were found to be shorter than isothermal fatigue lives at the maximum temperature of the thermal cycle. Intergranular cracks were observed more frequently in in-phase thermal fatigue than in isothermal fatigue at the maximum temperature. The lower bounds of out-of-phase thermal fatigue life and inphase thermal fatigue life were nearly equal to Manson's pc-type fatigue life and his cptype fatigue life, respectively. An empirical formula previously reported by the authors was found to be useful as an approximate life prediction of thermal fatigue based on isothermal fatigue lives.
In the experiment, length of crack, which had started from a notch hole, was measured by(1) During a strain controlled low cycle fatigue test at 5cpm, the rate of crack propagation to be opposite of that of tensile ductility. The period of crack initiation becomes shorter as a temperature increases.initiated in the vicinity of a main crack tip. The main crack propagates rather rapidly by coalescing these micro cracks.Temperature dependence of the rate of crack propagation is effectively influenced by the change of test frequency.(3) The increase in the rate of crack propagation due to coalescing process is estimated qualitatively with the assumption that the micro cracks prohibit the decrease of ductility of a material in the vicinity of a main crack tip.
Grain boundary sliding (GB sliding) and grain boundary cracking in both isothermal fatigue under saw-tooth wave strain cycling and thermal fatigue were investigated on 304 stainless steel, in order to elucidate the physical meanings of the damage acceleration under these types of fatigue conditions found in the previous papers.In isothermal fatigue under saw-tooth wave and thermal fatigue, GB sliding was found to be accumulated in nearly proportion to the number of strain cycles, contrary to the case of isothermal fatigue under triangular wave where the accumulation of GB sliding was negligibly small. The direction of GB sliding accumulation both in isothermal fatigue under slow-fast saw-tooth wave and in-phase thermal fatigue was characterized as to induce the tensile residual strain of the specimen, while the direction both in isothermal fatigue under fast-slow saw-tooth wave and out-of-phase thermal fatigue was contrary.Grain boundary cracking was frequently observed in the vicinity of triple points or ledges on the grain boundary which accommodated the strain concentration due to GB sliding, and the higher amount of accumulation of GB sliding caused the earlier initiation of grain boundary cracking. Based on these metallurgical findings, it is concluded that the acceleration of fatigue damage is closely related to the accumulation of GB sliding.
Low cycle fatiguetests at elevated temperatures under several types of saw-tooth wave strain cycling were carried out on two steels, a low carbon steel and an austenitic stainless steel, in order to investigate the effect of the difference in strain rate between tension going and compression going periods on the fatigue life.For both steels, the fatigue life was found to be longest under the strain cycling with equal tension going and compression going periods, and it became shorter as the difference between the two periods became larger. These results of fatigue life can be explained by the following damage concept.
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