Unusual characteristics of mixed mode delamination fracture in the presence of large scale bridging

The unusual characteristics of mixed-mode delamination fracture in the presence of large bridging zones of through-thickness reinforcement are explained through an idealization of the mixed-mode bending specimen. With simple but realistic constitutive laws assumed for the bridging mechanisms, beam theory offers insightful analytical solutions and illuminates the length scales that control the crack characteristics. Of particular significance is that the mode I energy release rate can vanish after a period of crack growth, due to the bridging effect of the through-thickness reinforcement. This crack tip closure phenomenon is one manifestation of bridging-induced spatial oscillations that sometimes appears in the crack displacement function, which can be understood qualitatively through the similarity of the fracture problem to the classical problem of a beam on a Winkler foundation. The possibility of crack closure and consequent crack arrest has important consequences for designing certifying tests for laminates with through-thickness reinforcement and predicting strength and lifetime.