Longitudinal compressive behavior and failure mechanism of three-dimensional five-directional carbon/phenolic braided composites at high strain rates

The uniaxial compressive stress-strain response of three-dimensional (3D) five-directional carbon/phenolic braided composites are experimentally investigated at high strain rates from 350 to 1600 s^-1 using the split Hopkinson pressure bar technique. The compressive loads are applied in a longitudinal direction (along the braiding direction). The macro- and micro-fracture morphology examinations are conducted using a scanning electron microscope (SEM) to understand the deformation and dynamic failure mechanism. The results show that the dynamic stress vs. strain curves exhibit a linear response followed by a load-drop at the on-set of matrix failure. The composites clearly demonstrate the strain rate strengthening effects and dynamic toughness phenomenon. The fracture morphology of the composites is varied under different loading rates. The damage is in the form of matrix cracking yielding and falling off, the interface debonding, and the migration, local buckling and shear fracture of the fibers. In addition, the ideal aspect ratio for the composites under dynamic compression is in the range of 0.75-1.25.