DIC-based studies of the overloading effects on the fatigue crack propagation behavior of Ti-6Al-4V ELI alloy

•Crack propagation behavior under four typical fatigue overloads was obtained.•The crack-tip strain fields under four overloading cases were measured using DIC.•The virtual extensometer based on crack-tip DIC measurement was used to determine COD.•The overloading effects on the crack growth behavior were analyzed from three aspects. In this paper experimental correlations between da/dN and ΔK and between a vs. N of an extra-low-interstitial titanium (Ti-6Al-4V ELI) alloy under four typical overload conditions (OL, UL, OL-UL and UL-OL) in an otherwise constant amplitude loading were obtained. By means of the digital image correlation (DIC) technique, the crack-tip deformation fields as well as the crack opening process corresponding to the four overloading patterns were measured. The results shown that both the crack propagation behaviors and the crack-tip strain distributions of the Ti-alloy were greatly influenced by the overloading conditions applied. The effect of the overloading pattern (sequence) on the fatigue crack propagation behavior of the Ti-alloy was then analyzed by combining the crack-tip deformation measurements with the fatigue crack propagation results from three possible mechanisms, i.e., the crack-tip blunting effect, the crack closure effect and crack-tip plastic zone effect. It was found that for the present investigated Ti-6Al-4V ELI alloy, the above three mechanisms made contributions in different extents to the fatigue crack growth retardation behavior.