Effect of interface region on fatigue crack growth in diffusion-bonded laminate of Ti-6Al-4V

This paper aims to illustrate the effect of the bonding interface on fatigue crack growth (FCG) in the diffusion-bonded (DB) laminate of Ti-6Al-4V (Ti64). To achieve this end, we performed FCG rate measurements for the DB laminate of Ti64 (2 mm + 2 mm + 2 mm + 2 mm) using a semi-elliptical surface crack, the size of which was determined by marker bands. The FCG behavior and laws along the surface direction and the thickness direction were investigated through fractographic analyses. It is found that there are interface regions in the DB laminate of Ti64, significantly influencing the FCG rate along the thickness direction (da/dN). When the crack front approaches the interface region from the base material, da/dN decreases with crack growth and reaches a local minimum when the crack front enters the interface region. With crack propagation in the interface region, da/dN increases regularly until the crack front approaches the base material, after which crack growth accelerates until the crack extends to a specific point in the base material. To illustrate the mechanism of the interface region influencing da/dN, we obtained the microtexture and the elastic/plastic properties of the base material and the interface region. The results show that although there is no apparent difference between the base material and the interface region in terms of elasticity modulus and yield strength, their differences in microtexture are significant, resulting in nonlinear variation in log(da/dN) with the logarithmic stress intensity factor range near the interface region.