DELAMINATION AND BUCKLING IN 3D COMPOSITES

Elementary results are applied to the buckling of stitched laminates and woven composites with three-dimensional (3D) reinforcement that contain delamination cracks. The through-thickness fibers are assumed to provide continuous, linear restoring tractions opposing the deflection of the delaminated layer adjacent to the crack. With the boundary condition that the ends of the delaminated layer are clamped and with deflections permitted in one direction only, there exists a characteristic length a(o) for buckling: if the length, 2a, of the delamination crack exceeds 2a(o), then, when buckling occurs, it will consist of waves of period 2a(o) and will usually not span the whole delamination. A simple expression is derived for the minimum density of through thickness reinforcement required to suppress buckling of the delamination layer prior to failure by other mechanisms. It is shown that for typical densities of through-thickness reinforcement in current stitched laminates and 3D woven composites, the length 2a(o) will rarely exceed the thick ness of the delaminated layer by as much as an order of magnitude. Furthermore, the current generation of these materials are probably considerably overdesigned in the quantity of through-thickness reinforcement they contain for suppressing buckling.