Effect of strain rate on the failure of bimrocks using the combined finite-discrete element method

The block-in-matrix rocks (bimrocks) are a complex type of rock in which hard blocks are bonded in a weak matrix. Despite the widespread distribution of bimrocks in nature, they are usually ignored in the design and analysis stages, resulting in underestimation or overestimating critical design parameters. Moreover, in studies regarding the failure behaviour of the bimrocks, strain rate has been ignored and no study has focused on the influence of strain rate on the failure mechanism of bimrocks. Therefore, this study aims to numerically study the impact of the strain rate increase on the bimrock rectangular specimens with different volumetric bloc proportions (VBP). The combined finite-discrete element method (FDEM) is employed as the numerical tool to analyze the study objectives. FDEM has proved to be a suitable candidate, having extraordinary capabilities in modeling fracture development in brittle rock under complex loading types. Before the analysis of the bimrocks, a verification example is explained to demonstrate the capability of this method in modeling an existing problem. Six strain rates of 0.046/s, 0.092/s, 0.18/s, 0.74/s, 1.85/s, and 5.55/s are the considered strain rates chosen according to a detailed literature search. This study first discusses the influence of the strain rate on the response of the intact specimens composed of pure matrix or pure block properties. Then, four VBP of 25%, 50%, 75%, and 90% are numerically built and put under the chosen strain rates. The simulation results show that the strain rate considerably impacts the failure pattern, peak stress, and post-peak behavior of the bimrocks. Notably, it is observed that the low to medium strain rate (0.046/s, 0.092/s, 0.18/s s) have a similar influence on the failure of the bimrocks, while the loadings higher than these amounts cause complex failure (mainly multiple fracturing or axial splitting) and rapid increase in the peak stress. Furthermore, these observations are somehow similar in the bimrocks having a VBP of equal or less than 75%. The bimrock with a VBP of 90% behaves differently, and the rate of change in loading has a minor influence on the failure type (all are of axial splitting). Also, the change in the rate of loading also has a slight impact on the peak stress, and this parameter is observed to have a little change even under the very high strain rate of 5.55/s.