Microcracking in coarse and fine grain marbles

This paper examines the effect of the initial crack density on the strength of coarse and fine grain Yuen Long marbles. Two different mechanisms of microcracking are observed: (1) in fine grain marbles the uniaxial strength drops with both initial crack density ɛ o and grain size; (2) in coarse grain marbles the strength is roughly constant with both crack density and grain size if ɛ o >0.1. This observation contradicts the common belief that peak strength decreases with ɛ o ( Bieniawski 1967 ) and grain size ( Brace 1961 ). In an attempt to study the underlying reason, Ashby-Hallam (1986 ) model is employed and it predicts that for small ɛ 0 , the peak strength decreases with both initial crack length and ɛ o , but when ɛ o increases beyond a critical value, peak strength is only controlled by the length of shear crack. For Yuen Long marbles, the model predicts that the critical ɛ o equals approximately 0.12, which agree very well with our observed value of 0.1 mentioned above.