Investigation on the Small Fatigue Crack Propagation Behavior of TC4‐ELI Titanium Alloy With Different Microstructures Under Various Stress Ratios

ABSTRACT This study investigates small fatigue crack propagation in titanium alloys with equiaxed, bimodal, and lamellar microstructures under various stress ratios. Whereas fatigue life differences were negligible at negative stress ratios ( R < 0), the lamellar microstructure exhibited superior performance at R > 0. Increasing the stress ratio generally reduced propagation rates due to enhanced crack arrest. Mechanistically, the lamellar structure induced significant path deflection through α ‐colony orientation, acting as a more effective barrier than the equiaxed α phase in the other microstructures. A transition from microstructure‐sensitive deflection to load‐dominated transgranular propagation was observed as cracks grew. Finally, although both the Santus and reference stress ratio–modified models yielded accurate life predictions, the modified model offered greater reliability for engineering applications by avoiding the nonconservative tendencies of the Santus model.