Closed-form analysis of interlaminar crack initiation in angle-ply laminates

Abstract

The layerwise discontinuous stiffness properties in composite laminates may cause highly localized, singular concentrations of interlaminar stresses. These stress singularities occur in the vicinity of free edges and may induce interfacial cracks, often referred to as delaminations. Since conventional design processes, based on classical laminate plate theory, only consider inplane stress components, they do not cover the so-called free-edge effect and the associated delamination onset and, therefore, disregard premature laminate failure. In the present study, interlaminar crack initiation in symmetric angle-ply laminates under tensile loading is investigated assuming the spontaneous formation of delaminations of finite length. A coupled stress and energy criterion is used requiring only two fundamental material parameters, the strength and the fracture toughness. The mechanical behaviour of the laminate is modeled by a highly efficient analytical closed-form approach providing results for the interlaminar shear stress in the interface and the energy release rates of potential cracks. The challenge of unknown interface fracture toughness is addressed by a parameter fit within physically reasonable limits. The resulting effective failure loads are compared to experimental data and show the high potential of the coupled criterion on the one hand and of the proposed analytical model on the other hand.