Fatigue crack growth in diffusion-bonded Ti-6Al-4V laminate with unbonded zones

Abstract

Fatigue crack growth (FCG) tests were conducted to investigate the behavior and laws of crack growth in diffusion-bonded laminates of Ti-6Al-4V (2mm+2mm +2mm+2mm) with three unbonded zones, the initial crack being a semi-elliptical surface crack. Post-test fractographic analyses indicate the crack growth in the laminate characterized by four distinct stages. The crack initially propagates in the first layer as a semi-elliptical surface crack (Stage I). After the surface crack penetrates the first layer, the crack becomes a through crack, which continues propagating in the first layer (Stage II). When the through crack arrives at the boundary of the first unbonded zone, it evolves into a part-through crack, during which crack growth rate keeps decreasing (Stage III). After that, the crack propagates as a part-through crack until the laminate fractures (Stage IV). It also indicates that the crack-growth-deceleration effect of the first unbonded zone is of more significance compared with the other unbonded zones. With the marker bands on the fractography, FCG data was obtained. Then we used the finite element method to calculate stress intensity factors for the laminates and obtained the relationship between FCG rate (da/dN) and stress intensity factor range (ΔK). The analysis on the da/dN-ΔK curve shows that the decrease in ΔK is an important factor, although not the sole factor, contributing to the crack growth deceleration near the boundary of the unbonded zone.