Crack Deflection at an Interface Between Two Orthotopic Media

To achieve toughness in many brittle composites, crack deflection at interfaces is essential. For this, it is necessary to establish crack deflection criteria by employing the principles of interfacial fracture mechanics applicable to anisotropic pair of materials. Such an analysis, with two aligned orthotropic media, is considered here. The stress and deformation fields derived for such cases are shown to depend on material parameters λ and p for the two media and on the two so-called Dundur constants α and β. For β = 0, the dependence on λ and λ₂ collapses to Λ = (λ₁,/λ₂)^1/4. The delamination criterion is insensitive to λ, p, Λ, and β over practical ranges of these material parameters. Thus, generalized delamination charts become possible as a function of the bi-material constant a alone, which characterizes the elastic dissimilarity between the two media. Using these charts, it is possible to determine the desired level of the interface strength required in composite manufacturing in order to enhance the overall toughness of a composite. Furthermore, such charts can be used for the interfaces between fiber/coating, fiber /matrix, or matrix/coating, depending on which interface is of critical interest for the crack deflection. It is shown how these charts can be used to identify composite systems where it is possible to maximize both the transverse strength and the longitudinal toughness.