Experimental studies of the change of spatial correlation length of acoustic emission events during rock fracture process

Changes in spatial correlation length of acoustic emission (AE) events during fracture process of granite and marble specimens under uniaxial compression are acquired with the single-link cluster (SLC) method. The results show that the 3-D location of AE events directly reflects the process of initiation, propagation and evolution of micro-cracks and the stress re-distribution in the specimens. Two factors may influence the change in spatial correlation length: stress release, which leads to decrease in correlation length, and stress re-distribution, which leads to increase in correlation length. Three types of changes of spatial correlation length of AE events are identified. For those specimens show no significant plastic deformation before failure, the spatial correlation length shows a sharp decrease at the early stage of loading, followed by a continuous increase until reaches its peak value at failure in a power-law relation. However, for specimens that show marked plastic phase before failure, their spatial correlation length can either increase or decrease depending on the combination of the above-mentioned two factors. In one case the specimens show clear cracking localization, which leads to clustering of events, hence reduction in spatial correlation length in zones of cracks localization. However, spatial correlation length shows an increasing trend in the whole process of failure. This increase reflects stress re-distribution and transfer in the specimens. The different characteristics of spatial correlation length during elastic and plastic phases may be used as an indicator for predicting failure of rock.