Use of split Hopkinson pressure bar for testing off-axis composites

This work attempted to characterize the high strain rate behavior of fiber composites using off-axis composite specimens and the split Hopkinson pressure bar (SHPB). Various factors affecting SHPB analysis, such as specimen–bar interface friction, extension–shear coupling and the rise time of the loading pulse were numerically and experimentally investigated. A numerical analysis was employed to qualitatively study the effects of extension–shear coupling and interface friction. It was also found experimentally, that strong interface friction existed for various off-axis angles of AS4/3501-6, especially for small angles. This results in bending waves in the bars and non-uniform stresses and strains in the specimen, which violate the assumptions of the SHPB analysis. Improving the interface conditions by lapping and lubricating the composite contacting surfaces was investigated as a means to achieve a nearly frictionless interface condition. A second numerical analysis of the SHPB experimental set up studied the effect of the rise time of the loading pulse on the early part of the dynamic stress–strain curve. Pulse shaping using copper tabs at the point of impact of the striker bar and the incident bar was found to generate gently rising pulses, which helped in generating more accurate dynamic stress–strain curves. These modifications were used to produce dynamic stress–strain curves of various off-axis S2/8552 glass–epoxy specimens.