| In this investigation we performed plate impact experiments to study the evolution of damage in ceramics and ceramic composites by subjecting the specimen to stress pulses which are large enough in amplitude to initiate microcracks, but short enough in duration to prevent their coalescence into macrocracks. In the "soft-recovery" experiment, a star-shaped flyer plate impacts a square specimen which has a square impedance-matching 'momentum trap' behind it. The size and shape of these three plates is such that the central portion of the specimen can be subjected to compressive and tensile stress pulses, which are controllable in magnitude and duration, and which allow the specimen to be recovered for microstructural analyses. Through the use of a laser interferometer, these pulses are monitored and used to determine the validity of proposed models. The recovered specimens are examined using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to determine the pattern of microcracks for comparison with the predictions of the assumed models.; In spite of many contributions to the field of damage induced by compressive waves, lack of consensus on the mechanisms responsible for the measured velocity profiles still remains. Our observations seem to unequivocally point to deformation in the intergranular glassy phase as a major mechanism of inelasticity in ceramics subjected to compressive dynamic loading. The possibility of compressive cracking, an alternative candidate mechanism, is ruled out by the observed absence of retardation of waves travelling through regions of the specimen previously deformed in compression. In addition, the recorded compressive pulse profiles strongly suggest a deformation by 'areal slip', i.e., by interfacial sliding over an expanding area. A detailed finite element analysis of the glass/grain-junction system has revealed that the stress concentration is sufficient to induce plastic flow in the glass under the conditions of our experiments. The numerical simulations demonstrate the ability of the areal slip model to reproduce all the salient features of the experimental record over a wide range of impact velocities.; In tension, microcracking is identified as the principal inelastic mechanism. Microcrack growth is modeled using standard relations of dynamic fracture mechanics. Because dynamic crack-tip equations of motion are differential equations of first order in time, the overall response under dynamic tensile loading is of the viscoplastic type. Our simulations demonstrate that the dilute approximation is valid over a wide range of the material response. |
