THE FAILURE OF FIBER-REINFORCED CERAMIC-MATRIX COMPOSITES UNDER DYNAMIC LOADING

High-strain-rate compressive failure mechanisms in fiber-reinforced ceramic-matrix composite materials have been characterized. These are contrasted with composite damage development at low strain rates and with the dynamic failure of monolithic ceramics. It is possible to derive significant strain-rate strengthening benefits if a major fraction of the fiber reinforcement is aligned with the load axis. This effect considerably exceeds the inertial microfracture strengthening observed in monolithic ceramics and nonaligned composites. Its basis is shown to be the transspecimen propagation time period for heterogeneously-nucleated, high-strain kink bands. For high-strain-rate tensile loading conditions, it is found that behavior is not correctly described by the current matrix fracture/fiber pullout models. This is a consequence of the rapid and extreme frictional heating produced at the fiber-matrix interface by sliding velocities on the order of 100 m/s. At rapid loading rates, the near-interface matrix appears to virtually melt, and the frictional interface shear resistance is reduced to the point that the fibers debond throughout the specimen, and pull out without failing. This suggests that for sufficiently rapid loading, the stress to fail the composite will approach that merely to create the initial matrix crack (i.e., a stress level well below the ultimate strength normally attainable under quasistatic conditions).