Mechanical length scales and their link to fatigue crack growth kinetics in beta-annealed Ti-6Al-4V

Correlations among microstructure, crack-tip fields and fatigue fracture are studied in beta-annealed Ti-6Al-4V. By means of digital image correlation, the area integral of the opening strain (εint) over the cyclic plastic zone was obtained, which was proportional to ΔKm, where m is the Paris exponent, so that da/dN∝ εint. Analysis shows that m≅2(2+(1+n′)-1) where n′ is the cyclic hardening exponent (0 ⩽ n′ ⩽ 1). This predicts 5 ⩽ m ⩽ 6, in agreement with experiments, and strongly suggests that fatigue fracture of beta-annealed Ti-6Al-4V can be described using non-local strain parameters.