Characterisation of rate-dependent fracture in adhesive joints

For adhesive joints, several authors questioned the applicability of LEFM when the combination between the ductility of the adhesive, the stiffness of the adherents and the fracture mode leads to a relatively large process zone during crack propagation. For such cases, nonlinear fracture mechanics (NLFM) and the use of the critical value, 𝐽𝑐, of the J integral have been recommended. This was until it was recently showed, at least for the case of mode-I crack propagation along a thin interface, that the difference between 𝐺𝑐 and 𝐽𝑐is found to be equal to the partial derivative with respect to the increase in crack length, 𝑎, of the energy dissipated in front of the crack tip. The fact that this difference is extremely small for double-cantilever-beam (DCB) specimens, together with a rigorous assessment of the accuracy entailed by the use of the equivalent-crack-length concept, were used to develop an accurate and practically effective experimental/numerical procedure to assess the rate dependence of fracture resistance in adhesive joints made of aluminium arms bonded with an epoxy adhesive. The effectiveness is the result of using simple DCB tests without the need to measure the crack length. Numerical simulations were conducted using a bespoke finite-element code tailored for DCB tests based on Timoshenko beam theory and a cohesive-zone model based on fractional viscoelasticity.