Dynamic punch shear behavior of AS4/3501-6

Punch shear experiments are conducted on a 24 ply graphite/epoxy AS4/3501-6 quasi-isotropic layup over a wide range of loading rates. Low and medium rate experiments are performed using a hydraulic testing machine with a specially designed punch shear fixture. High rate experiments are performed using a punch shear version of the split Hopkinson bar apparatus. Optical microscopy of sectioned samples of partially punched specimens is used to construct a sequence of the damage process. Cracking initiates in the shear zone of the specimen side contacted by the punch bar. This is followed by delamination in the plug on the lower third of the specimen and finally after significant rotation of the material, tensile fiber failure results. The peak punch load and displacement at the peak are found to be relatively insensitive to the punching speed. It is postulated that the values of the peak are determined principally by the tensile fiber failure, which is known to be rate-independent for graphite fibers. The lower portion of the curve, however, which is thought to reflect the matrix cracking response, is seen to exhibit substantial rate-dependence. Since matrix and delamination cracking are extremely important in assessing damage in composite structures subjected to low velocity impact loading, this rate-dependence is considered to be quite significant.