THREE DIMENSIONAL EFFECTS IN COMPOSITE PLATES

In this study, a finite element investigation of the three dimensional nature of stress fields in the near tip region of a cracked orthotropic plate was conducted. Two and three dimensional finite element analyses were used to investigate the relative extent of regions of three dimensional to two dimensional (plane stress or plane strain) deformation in the cracked plate. The material properties used in the simulations corresponded to those of a graphite/epoxy composite. A three point bend loading geometry, with the fiber directions either parallel or perpendicular to the crack, was simulated. In analogy to isotropic materials, it was observed that a plane stress K-dominant region does not arise arbitrarily close to the crack tip because of the existence of a three dimensional zone. However, it was seen that the shape and the size of this three dimensional zone in the cracked composite plate is substantially different from that of an isotropic plate, and depends intimately on material properties. For a crack parallel to the fiber direction the three dimensional zone extends to 0.46h (h = specimen thickness) ahead of the crack tip but only to 0.27h at 30°. Fibers perpendicular to the crack produce a highly elongated three dimensional zone in the direction of the fibers (up to 0.78h). The zone is also sensitive to the variations in the Poisson’s ratio’s of the orthotropic solid.