Microcracking and grain size effect in Yuen Long marbles

This paper examines the effect of the initial crack density and grain size on the uniaxial compressive strength of fine- and coarse-grain Yuen Long marbles. We found experimentally that the peak strength decreases with the inverse square root of the mean grain size, and this agrees with the previous experimental observations by Olsson (1974), Brace (1961, 1964) and Fredrich et al. (1990). However, two different regimes of microcracking dependence on the peak strength are observed: (1) in fine-grain marbles the uniaxial strength drops rapidly with the initial crack density ϵ0; (2) in coarse-grain marbles the strength is roughly constant with the crack density if ϵ0 > 0.025. This observation contradicts the common belief that peak strength decreases monotonically with ϵ0. In an attempt to study the underlying reason for this nonlinear dependence of the crack density, the Ashby-Hallam (1986) model is employed and it predicts that for small ϵ0, the peak strength decreases rapidly with the initial crack density, but when ϵ0 increases beyond a critical value, peak strength becomes relatively insensitive to the crack density. For Yuen Long marbles, the model predicts that the critical ϵ0, which separates the rapidly-decreasing and the roughly-constant regimes, is approximately 0.025, which agrees well with our observed value.