The propagation of a crack in a double-cantilever beam (DCB) geometry where there is extensive remote plastic flow both preceding and accompanying fracture is analysed. Experiments show that there is an appreciable path dependence in load-deflection-crack length behaviour because of the remote residual plastic zones left in the wake of the crack front. The deflection for a propagated crack is greater than the deflection predicted by nonlinear fracture mechanics since in real plasticity the plastic deformations cannot be recovered and their ‘energy’ released back into the system. A Griffith energy approach is employed to uncouple the work increments of elastic strain energy, the remote plastic work and the essential crack-tip fracture work. For geometries other than the DCB these components cannot be easily uncoupled. Analyses are given for elastic perfectly plastic solids and for elastic power-law work-hardening materials. There is good agreement with experiments on side-grooved double cantilever beam specimens made from 6082-TF aluminium alloy (which is almost elastic perfectly plastic) and from annealed α-brass (which work hardens appreciably). Varying degrees of elastoplasticity during propagation are obtained by altering the height of the beam arms; globally elastic fracture results are obtained with adequately deep arms.
It is found that the load-deflection curves can be predicted by assuming the essential work of fracture at the crack tip is constant, at initiation and propagation, for both these materials. In contrast the JR curves calculated from the load-deflection diagram by the conventional method are dependent on the specimen size because they contain non-recoverable global plastic work.