| Summary: | 碩士 === 國立中央大學 === 機械工程研究所 === 87 === This study investigated the effects of mean stress on the low-cycle fatigue (LCF) strength of Custom 450 stainless steels in different tempers. Uniaxial LCF tests were conducted under strain control with three strain ratios R=-1, 0 and 0.5 at several strain amplitudes in cyclic loading. LCF specimens were prepared in three different tempers, namely solution-annealed (SA), peak-aged (H900), and overaged (H1150) conditions. The effects of aging treatment on the LCF behavior are also discussed. Fractography analysis with scanning electron microscopy (SEM) was applied to determine the LCF failure mechanisms and fracture initiation sites.
Results show that LCF specimens in these three tempers all exhibited cyclic softening at high strain amplitudes under a strain ratio of R=-1. At low strain amplitudes, the cyclic softening is less evident for SA and H900 temper while H1150 temper exhibits cyclic hardening. Under a strain ratio of R=-1, Custom 450 in H900 temper has longer LCF life than those in SA and H1150 tempers. However, this advantage for H900 over SA and H1150 tempers disappeared at higher load ratios (R=0 and 0.5) due to the greater sensitivity to mean stress effect in H900 temper. For a given temper at high strain amplitudes, the LCF lives among the three applied strain ratios did not show significant differences as a result of the mean stress relaxation effect. However, at low strain amplitudes, cyclic loading at R=-1 generated the lowest mean stress levels and longest LCF lives as compared to R=0 and 0.5. The LCF life data obtained for this precipitation-hardening stainless steel under various combinations of heat treatment conditions and strain ratios could be well correlated by a modified Manson and Halford approach and a modified SWT approach.
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