Acoustic emission from polygranular graphites

Abstract

Acoustic emission (AE) was monitored from six industrial graphites of widely different grain size subjected to three-point bend loading. The AE is characteristic of graphite type and microstructure. The development of AE with applied load is shown to be associated with the micromechanical events that cause nonlinear stress-strain behaviour in graphites, and postfracture AE is indicative of the crack propagation mode at fracture. For a given graphite, the total number of AE and the proportion of AE events of large amplitude both increase with load. For different graphites, the total number of AE at fracture and the proportion of AE events of small amplitude tend to increase with grain size. The changing pattern of AE with load and graphite type is explained by ascribing AE events of small amplitude mainly to easy cleavage of favourably oriented basal planes. AE events of large amplitude are ascribed mainly to extension of microcracks into less favourably oriented basal planes and into the binder phase. The lower proportion of AE events of small amplitude found for fine-grained graphites is related to the effects of size reduction of filler particles by grinding.