MICROCRACK TOUGHENING IN 2-PHASE MULTILAYERED MEDIA

This work presents a numerical model of the toughening behavior exhibi ted when a Mode I macrocrack propagates through alternating tough and brittle layers of a composite. The toughening occurs due to microcrack ing in brittle layers. Microcracks form at periodic flaw sites in brit tle layers when the stress at a site is sufficient to propagate a crac k within the layer. An established microcrack is modeled by a dislocat ion dipole, with Burgers vector chosen to render the normal component of stress equal to zero at the dipole midpoint. R-curves for layered m edia are predicted as a function of bi-layer thickness, volume fractio n, density of flaw sites, and inhomogeneity in fracture toughness and elastic modulus. A critical combination of properties for the onset of R-curve behavior is identified, and features which optimize the upper plateau of the R-curve are discussed in light of experimental observa tions. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science L td. All rights reserved.