Biaxial fatigue behavior of gradient structural purity titanium under in-phase and out-of-phase loading

Biaxial fatigue behavior of gradient structural pure titanium fabricated by surface nanocrystallization technique (SNC) was investigated under in-phase (IP) and out-of-phase (OP) loading. The fatigue life of SNC Ti under 90° OP loading is shorter than that under IP loading, which is attributed to the fact that both the maximum shear stress and maximum normal stress under 90° OP loading are larger than that under IP loading at the same cyclic equivalent stress amplitude. Under IP loading, the fatigue life increases as the stress ratio increases; Under OP loading, the fatigue life decreases as the phase angle increases. The fatigue microcracks nucleate and propagate preferentially along shear bands (SBs) under both IP and OP loading. The growing direction of the microcracks and macrocrack were determined experimentally and predicted using the critical plane approaches. The results indicate that the measured directions of macrocrack under IP and OP loading are closely consistent with the theoretical prediction. After biaxial fatigue tests, the stress-induced nanograins growth was observed on the top surface. As a result of rotation of maixmum shear stress plane uner 90° OP loading. The degree of nanograins growth under 90° OP loading is larger than that under IP loading. The corresponding dislocation substructure in coarse grained region changes from the parallel dislocation lines under IP loading to the dislocation tangles under 90° OP loading.