Symmetric arrangements of cracks with perturbed symmetry: Extremal properties of perturbed configurations

Perturbations of symmetric crack configurations produce mixed-mode conditions at crack tips, in cases of pure mode I (or II) conditions in unperturbed configurations. This may lead to the increase of stress intensity factors and energy release rates, so that their maximums are reached in somewhat perturbed configurations (even if the perturbations increase spacing between crack tips) thus making them energetically preferable. The effect of perturbations on the direction of crack kinking is also examined, in particular, whether the kinking drives the crack system farther away from the original configuration or back towards it. These considerations may be related to the experimentally observed tendency of cracks to "avoid each other" Melin (1983). They also shed some light on the crack coalescence problems. Closed-form approximate expression for the maximal energy release rate at the crack kink tip is presented. The results of Panasiuk et al (1976), Cotterell and Rice (1980), and Amestoy and Leblond (1992) are also discussed.