Postbuckling of flat unstiffened graphite-epoxy plates loaded in shear

An experimental and analytical study was conducted of the postbuckling response and failure characteristics of 8-, 16-, and 24-ply unstiffened graphite/epoxy plates under shear loading, including orthotropic, quasi-orthotropic, entirely +, -45 deg ply, and (0/90)s-class plates. Postbuckling stiffness is noted to be influenced by the stacking sequence and by the inplane boundary condition imposed on the test section by a picture frame test fixture. The ratio of failure load to buckling load was higher for specimens that had higher width-to-thickness ratios. It is noted that high displacement gradients may induce transverse shear stresses, and that delamination was the failure mechanism for many of the 8-ply specimens. An adhesive failure between the composite specimen and metal edge reinforcements initiated failure of the 16- and 24-ply specimens.