Bond thickness effect in mixed-mode fracture and its significance to delamination resistance

The fracture behavior of adhesively bonded joints is of interest in a variety of industrial, natural and biological applications. This effort re-examines literature data with an emphasis on thin bonds and their relation to delamination resistance. Brittle, ductile and particulate epoxy adhesives were considered with the bond thickness t varying from 1 to 1000 μm. The results shed new light on some basic morphological and fracture characteristics. The fracture energy is affected by numerous factors including adhesive material, bond thickness and loading mode. Shear fracture generally begins with growth of tensile microcracks ahead of the crack tip and continues with coalescence of these cracks along the interface in a process accompanied by extensive plasticity. With G_IIC ≈ G_IIIC, mixed-mode fracture can be described by tensile (G_I) vs. shearing (G_SC) ERR. For thin bonds (t < ≈ 10 μm), G_IC ≈ constant, G_SC increases virtually linearly with t, and G_SC ≈ G_IC = G₀ when t → 0. These traits result in a closed-form expression for G_C given in terms of basic material properties. The thin-bond results provide cost-effective means for predicting interlaminar fracture energy G_IC or G_IIC and elucidating an interleaf thickness for optimal delamination resistance. A detailed examination of published data reveals that within certain limitations, the mixed-mode fracture envelope is independent of t or material type. This finding may help develop analytical fracture models for practical use.