MICROPLASTICITY OF ALUMINA IN FORERUNNING PROCESS OF FRACTURE

Mechanical hysteresis in alumina with microcracks has been investigated by a loading-unloading test in the microstrain range around 10(-4) and examined in comparison with the strain amplitude dependence of internal friction. While there remains a permanent strain after the initial loading, successive cyclic loading stabilizes the mechanical response, resulting in a single closed hysteresis loop with a symmetrical shape. Such a stabilized hysteresis loop is responsible for the internal friction based on the friction mechanism and can be attributed to the microplasticity in the forerunning process of fracture. With increasing strain amplitude, the area enclosed by the stabilized hysteresis loops increases remarkably. Since the loop area determines the energy loss per cycle, its strain amplitude dependence is considered to arise from the same origin as that of internal friction. The internal friction data have also been analyzed on the basis of the theory of microplasticity. The stress-strain responses thus obtained show that the microplastic strain of the order of 10(-9) increases nonlinearly with increasing effective stress. The variation in the microplastic flow stress corresponds well to the decrease in the macroscopic fracture strength resulting from the formation of microcracks and crack propagation.