Ti–6Al–4V合金のギガサイクル疲労特性における応力比の影響

Abstract

The effect of stress ratio on giga-cycle fatigue properties was investigated for three heats of 900-MPa-class Ti–6Al–4V alloy. Fatigue tests were carried out using ultrasonic fatigue testing at 20 kHz and conventional fatigue testing at 120 Hz. The ultrasonic and conventional fatigue testing used 3 and 6 mm specimens, respectively. These fatigue tests were conducted under stress ratio of R=0 and 0.3 and under the condition fixing the maximum stress at the yield stress. As the result, internal fracture occurred in all heats, and in that case, frequency effects were negligible. This result meant that the specimen size effect was also negligible. In these tests, no specimen failed at over 109 cycles, which meant that the Ti–6Al–4V alloy probably revealed fatigue limits in a giga-cycle region in spite of occurrence of internal fracture. The fracture sites of the internal fracture revealed no inclusion but a cluster of several facets. The size of a facet was almost equal to the α grain size and the size of the cluster was 100~300 μm. In comparison with steels, the Ti–6Al–4V alloy revealed lower fatigue strength under R=0 in spite of almost equal under R=−1. Hence, the fatigue limits determined at 1010 cycles were below a modified Goodman line under R=0 and 0.3. In comparing the fatigue limits with ΔKth, the dependency of the fatigue limits on the stress ratios was similar to that of ΔKth.