Buckling and postbuckling behavior of square compression-loaded graphite-epoxy plates with circular cutouts

Results are presented for unidirectional (0, 10)(sub s) and (90,10)(sub s) plates, ((0/90)(sub 5)(sub s)) plates, and for aluminum plates. Results are also presented for ((+/- theta)(sub 6)(sub s)) angle-ply plates for values of theta = 30, 45, and 60 degrees. The results indicate that the change in axial stiffness of a plate at buckling is strongly dependent upon cutout size and plate orthotropy. The presence of a cutout gives rise to an internal load distribution that changes, sometimes dramatically, as a function of cutout size coupled with the plate orthotropy. In the buckled state, the role of orthotropy becomes more significant since bending in addition to membrane orthotropy is present. Most of the plates with cutouts exhibited less postbuckling stiffness than the corresponding plate without a cutout, and the postbuckling stiffness decreased with increasing cutout size. However, some of the highly orthotropic plates with cutouts exhibited more postbuckling stiffness than the corresponding plate without a cutout. These results suggest the possibility of tailoring the cutout size and the stacking sequence of a composite plate to optimize postbuckling stiffness. It was found that plates with large radius cutouts do exhibit some postbuckling strength. The results also indicate that a cutout can influence modal interaction in a plate. Specifically, results are presented that show a plate with a relatively small cutout buckling at a higher load than the corresponding plate without a cutout, due to modal interaction. Other results are presented that indicate the presence of nonlinear prebuckling deformations, due to material nonlinearity, in the angle-ply plates with theta = 45 and 60 degrees. The nonlinear prebuckling deformations are more pronounced in the plates with theta = 45 degrees and become even more pronounced as the cutout size increases. Results are also presented that show how load-path eccentricity due to improper machining of the test specimens affects the buckling behavior. Some of the plates with cutouts and eccentricity exhibited a snap-through type of buckling behavior.