Fracture process zone in crystalline rock: effect of specimen size and shape

Abstract

Ability to predict fracture properties of granular materials based on their size and loading conditions needs rigorous experimental evidence. Charcoal granite specimens of various sizes and geometries are tested under three- and four-point bending to investigate the effect on the fracture toughness of rock and size of the fracture process zone calculated using Digital Image Correlation (DIC), with validation provided by Acoustic Emission (AE). Two different methods to evaluate the length of the fracture process zone using DIC are adopted: the first one assumes that a traction free crack does not propagate in the pre-peak regime, while the second one hypothesizes that a cohesionless crack may exist at peak load before unstable crack growth. The effects of specimen size, geometry, and loading conditions are found to be more prominent for the specimen sizes used in the laboratory but are predicted to become less significant and eventually negligible as the specimen size increases. DIC results are in general agreement with localization of AE events, but the former technique is suggested due to greater resolution and ability of continuous measurements of the fracture process zone dimensions.