Seminal Developments in the Durability and Damage Tolerance Assessment of Bonded Joints

Structural adhesives are widely used for joining composite components in many industries and crack growth in such materials is far more likely to occur when they are subjected to repeated cyclic loading than to monotonic loading. Whilst the Hartman- Schijve equation for fatigue crack growth (FCG) has been shown to hold for cohesive crack growth in adhesives under Mode I, Mode II and Mixed-Mode I/II loading, little attention has been paid to its ability to capture the effects of the thickness of the adhesive layer. The present paper examines the growth of fatigue cracks, that occurs cohesively through the adhesive layer, in two toughened epoxy adhesives typical of those used in the automotive and the aerospace industries. Firstly, it is established that when the crack growth rate, da/dN, curves are expressed as a function of Δ√G, or ΔG, where G is the energy release-rate, then the crack growth curves are a function of the thickness of the adhesive layer. It is then shown that this dependency vanishes when da/dN is expressed as a function of the crack-driving force, Δκ, as defined by the Hartman-Schijve equation. Therefore, it is suggested that the parameter Δκ appears to be a valid similitude parameter.