Delamination in flat sheet geometries with material imperfections and thickness variations

In this paper the influence of material imperfections, in the form of small delamination damage, thickness variations and boundary conditions on the fracture behavior of singly and multiply delaminated plates subject to quasi-static out of plane loading is investigated. To simplify the problem and highlight relevant mechanisms, cylindrical bending deformation is assumed and the problem is solved using a semi-analytic model, based on Timoshenko beam theory, that includes relative rotations of the sublaminates at the delamination tips to account for near tip deformations. The results show that the presence of small delamination damage, consisting of an array of small parallel delaminations, induces phenomena of amplification and shielding of the energy release rate of a main delamination propagating through the damaged area. The damage also leads to modification of the mode ratio. Similar phenomena are observed in plates with geometric discontinuities, which are idealizations of stiffeners, patches and cross section variations. These behaviors are controlled by the boundary and loading conditions.