Ti-6Al-4VにおけるDwell fatigue効果を考慮した疲労寿命設計に向けたき裂進展加速機構の解明

抄録

Dwell sensitive fatigue is a problematic issue when titanium alloys are used as aero engine parts. It has been found that displacement/load holding accelerates local strain evolution, crack initiation, and crack growth, which causes deterioration of fatigue resistance. The present study focused on elucidating the crack growth acceleration mechanism during dwell fatigue. The dwell fatigue test was performed under load control with a triangular waveform, which included 10-min displacement holding at the peak load for only one cycle at ΔK = 25, 30, 40, 50 MPa√m in the Ti-6Al-4V alloy with a bimodal microstructure. Digital image correlation-based strain maps of the in-situ fatigue test showed the strain evolution near the crack tip during 1-cycle dwell. Moreover, locally extended striations corresponding to 1-cycle dwell were observed on the fracture surface. From the two results, it was proposed that the crack propagation during dwell fatigue occurs with crack tip blunting that is originated from dislocation emission assisted by thermally activated process during the stress holding. Another important finding was that the substructure beneath the fracture surface consisted of planar dislocations bundles and a locally thick bundle was observed in the region corresponding to 1-cycle dwell.