Mixed-mode fracture of layered plates subjected to in-plane bending

Layered architecture is being widely used in applications such as thermal protection systems, windshields, body and vehicle armor etc. In layered materials, properties vary in a discrete manner from layer to layer, leading to property jumps across the interfaces. The focus of this study is to understand the mixed mode fracture behavior of cracked layered plates in which elastic and fracture properties vary along the crack front. Layered plates were prepared by joining polymethylmethacrylate (PMMA) and Epoxy sheets using an Epoxy based adhesive (Araldite). Between the two materials, Epoxy has higher elastic modulus and lower fracture toughness. Single edge notched specimens were subjected to asymmetric four point bending. The failure was observed to be progressive in nature with the crack in Epoxy layer extending first followed by crack growth in PMMA layer. Inspection of fractured surfaces indicated twisting of crack during crack extension. Results of three-dimensional finite element analysis (FEA) indicated variation of SIF and mode mixity along the crack front and also presence of mode-III SIF. The load corresponding to extension of crack and angle of crack propagation in the Epoxy layer was reasonably predicted by the maximum tensile stress criteria. The interaction between the two layers once the crack in the Epoxy layer starts growing is explained using results of FEA.