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

The unusual characteristics of mixed-mode delamination fracture in the pres ence of large bridging zones of through-thickness reinforcement are explain ed through an idealization of the mixed-mode bending specimen. With simple but realistic constitutive laws assumed for the bridging mechanisms, beam t heory offers insightful analytical solutions and illuminates the length sca les that control the crack characteristics. Of particular significance is t hat the mode I energy release rate can vanish after a period of crack growt h, due to the bridging effect of the through-thickness reinforcement. This crack tip closure phenomenon is one manifestation of bridging-induced spati al oscillations that sometimes appears in the crack displacement function, which can be understood qualitatively through the similarity of the fractur e problem to the classical problem of a beam on a Winkler foundation. The p ossibility of crack closure and consequent crack arrest has important conse quences for designing certifying tests for laminates with through-thickness reinforcement and predicting strength and lifetime.