Fatigue crack growth behavior and mechanisms of diffusion-bonded TC4/TC21 laminates

Metallic laminates offer exceptional combined mechanical properties and design flexibility, making them highly attractive for engineering applications. In this paper, fatigue crack growth (FCG) tests were performed on central-hole specimens of TC4/TC21 diffusion-bonded laminates and their constituent monolithic plates (TC4 and TC21) using the marker load method. Fractographic analyses revealed a retardation in the FCG rate as the crack front approached the diffusion-bonded interface along the thickness direction. While standard stress intensity factor (SIF) calculations yielded direction-dependent d a /d N vs. Δ K curves, the incorporation of T -stress corrections effectively accounted for crack-tip constraint effects, establishing a unified, direction-independent FCG relation for the monolithic alloys. However, for the laminates, mechanical driving forces alone were insufficient to explain the growth behavior across the interface. Combined fractographic and electron back-scattered diffraction (EBSD) analyses demonstrated that microtexture heterogeneity—specifically the variation in grain orientation and size near the interface—induced a tortuous crack path. This microstructural barrier acts as the primary mechanism governing the complex FCG retardation observed in dissimilar titanium alloy laminates.