Abstract
In order to investigate the fracture mechanics law of macroscopic crack propagation in thermal fatigue, strain-controlled thermal fatigue tests on Cr-Mo-V rotor steel and SUS304 stainless steel were carried out by using a notched specimen. A smooth specimen was also employed for thermal fatigue tests under the same conditions as the crack propagation tests to discuss the applicability of the fatigue life law (i.e., the strain-energy-parameter approach) derived from the macroscopic crack propagation law to thermal fatigue life evaluation.The results obtained were summarized as follows:(1) The characteristics of thermal fatigue crack propagation could be classified into two types (i.e., cycle-dependent and time-dependent) by applying the transition criterion between the types.(2) The fracture modes in the cycle-and time-dependent thermal fatigue were of transgranular and intergranular types, respectively.(3) The fatigue J-integral range, ΔJf, was a fracture mechanics parameter controlling the crack propagation rate, dl/dN, in the cycle-dependent thermal fatigue, while the creep J-integral range, ΔJc, in the time-dependent thermal fatigue. In this case, rapid straining could be successfully applied to the partition of the inelastic strain into plastic and creep components for evaluating the ΔJc-value. The crack propagation law (i.e., the dl/dN-ΔJf and dl/dN-ΔJc relations) in thermal fatigue was identical with that obtained in isothermal fatigue.(4) The thermal fatigue failure life of the smooth specimen was correlated with the strain-energy parameter, and the correlation agreed with that in isothermal fatigue. Also, it could be predicted from the crack propagation characteristics obtained by using the notched specimen.
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