Failure mechanism of composite laminates with different size multiple delaminations subjected to compressive load

Postbuckling behavior of rectangular composite plates with multiple circular delaminations is studied experimentally and numerically. The plate is fixed at its loading edges and simply-supported at its side edges. Embedded circular delaminations are placed at regular intervals in the thickness direction at the plate center. The diameter of the delamination increases from the top surface to the other surface as usually observed in the impact damage in composite laminates. The effect of size difference of the delaminations on the compressive behavior is investigated. The three-dimensional block element is adopted in order to accurately obtain distributions of each component of energy release rates along the delamination fronts. The contact problem of the delaminated surface is approximately considered by placing tensionless spring elements between the nodes at the upper and lower surfaces of delaminations. The numerical results are consistent with the experiment. The buckling load obtained is quite low owing to the existence of the large size delamination near the surface. The overall structural instability of the plate does not appear until the small size delaminated portions deforms significantly. The complex compressive behavior consists of three parts, instability of the delamination of large diameter which does not directly relate to the plate failure, the local instability of the whole delaminated portion, and global instability of the plate, both of which can be a trigger of the failure.