Crack front morphology near the free edges in double and single cantilever beam fracture experiments

We analyse edge effects in two common adhesive joint fracture configurations, namely the single and the double cantilever beam tests. Several experiments were performed studying the crack front vicinity for deflection and crack front morphology using light scanning and microscopy techniques. To understand the findings a finite element model is used to determine the energy release rate at the crack front, particularly in the vicinity of free edges. Two approaches - an inverse procedure and a direct method - are compared. Using the inverse procedure, the local values of energy release rate along the experimentally obtained crack front are investigated. With this approach, the fracture criterion is however not necessarily fulfilled at each node along the crack front. In the direct method, a mixed-mode fracture criterion is used to determine the shape of the crack front at steady-state propagation. Qualitatively, the direct method stays in good agreement with experimental findings, showing that the free edges are likely to affect crack front morphology. A full match between the experimental results and numerical simulations can be achieved by using the Poisson's ratio as a fitting parameter.