Mixed-Mode I–II Fatigue Crack Propagation Behavior for Commercial Pure Titanium TA2 Under In-Plane Biaxial Nonproportional Loading

This study investigates Mixed-Mode I–II fatigue crack propagation behavior of commercial pure titanium TA2 under biaxial nonproportional loading conditions using an improved cruciform specimen (CS). Through in situ characterization technology, the experimental results show that when the biaxial load ratio λ = 1, the crack bifurcates due to the bimodal effect of circumferential stress, and the symmetric bifurcation is significantly controlled by the initial crack inclination angle (45° or 60°). Both increased phase difference φ and biaxial load ratios λ accelerate fatigue crack growth rates, whereas larger crack inclination suppresses the propagation. The crack initiation angle was predicted based on the maximum tangential stress criterion (MTS) theoretically and the solution by the finite element method (FEM). Microstructural analysis indicates that with the increase of biaxial load ratio and phase difference, the geometrically necessary dislocation (GND) density, Kernel average misorientation (KAM) value in the plastic deformation zone, and the proportion of small angle grain boundaries increase.