Characterization of crack nucleation in TA6V under fretting-fatigue loading using the potential drop technique

Crack initiation in a Ti-6Al-4V alloy was investigated experimentally in fretting fatigue conditions. A cylinder-plane geometry was used for the contact, and partial slip conditions were considered. The experiments were conducted on a new bi-actuator fretting fatigue machine and an original interrupted test expertise methodology was proposed. The potential drop technique is used to monitor crack nucleation. The careful calibration procedure used in this study and presented in this paper allowed the detection of cracks as small as 50 [mu]m. Three experimental campaigns were conducted: fretting tests (optical expertise), fretting fatigue tests up to a lifetime of 100,000 cycles (optical expertise) and fretting fatigue interrupted tests (potential drop technique). The experiments have shown that the addition of a fretting-fatigue to conventional fretting conditions can induce a drop off of 45% of the tangential threshold for crack nucleation. Two mechanisms were shown to promote this drop off: increasing the fatigue load leads to a decrease of the tangential force nucleation threshold whereas raising the fretting load leads to a decrease of the nucleation and fracture fatigue lifetime.