Branching and softening of loading path during onset of crack at elastic-brittle interface

The load response during onset and reformation of an interface crack is investigated experimentally and theoretically using linear-elastic analysis. A number of double cantilever beam experiments is carried out under the prescribed displacement and the prescribed displacement rate loading conditions. During the onset of the interface crack with the initially straight front, the crack grows from the centre of the specimen before adopting a semi-elliptic shape. The shape of the crack front eventually becomes self-similar (sizes of the crack front process zone and the ratio between the crack front depth and its width become invariant) when reaching the free edges of the specimen. As a consequence, during the crack reformation the incremental crack area cannot be attributed to the growth along a single direction. Finite element calculations are implemented in which the recorded crack front morphologies are imposed as the crack front boundary conditions. It is concluded that the load response curve can become non-linear upon the geometrical reconfiguration of the crack front and quantitatively is associated with gradual transition from the plane strain to the plane stress conditions experienced by the adherend during the crack front reformation.