Strain-amplitude-dependent internal friction and microplasticity in alumina with microcracks

Abstract

Internal friction in polycrystalline alumina subjected to thermal shock has been measured as a function of strain amplitude by means of the free-decay method of flexural vibration. The curves of the amplitude dependence show a more rapid rise with increasing thermal shock damage. The data have been analysed on the basis of the phenomenological theory of microplasticity assuming a friction-type hysteresis. Thus the internal friction in alumina with microcracks is transformed into a microplastic strain of the order of 10^—9. The stress-strain responses show that the microplastic strain increases nonlinearly with increasing stress under conditions where macroscopic plastic flow can never be observed. 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.