Investigation into the effects of grain size on strength and failure behaviors of granites using a breakable polygonal grain–based model

摘要

The microstructure of rock is one of the most important factors that affect its mechanical behaviors. In order to study the effects of grain size on the strength and failure behaviors of granites, a breakable polygonal grain-based discrete-element model is developed to simulate the microstructure of rocks. In the model, mineral grain types, grain size distributions, and mineralogical composition are incorporated, and the intergranular and intragranular cracks are captured in the failure process. Using this method, six kinds of specimen with varying grain size are established, and the effects of grain size on peak strength, deformation properties, microcrack distributions, and macroscopic failure pattern are studied through conducting a series of compressive loading tests. The results indicate that rock strength is inversely proportional to the grain size, while a decreasing tendency in peak strain, an increasing tendency in Young's modulus, and Poisson's ratio with the grain size are observed. Meanwhile, an increase of grain size also results in a dramatic rise in crack initiation stress, ratios of shear cracks/intragranular cracks to generated total cracks. Under uniaxial compression tests, rocks consisted of small grains are trend to fracture with a single shear band, while rocks consisted of large grains are trend to fracture with an axial splitting. The failure pattern of rocks under high confining pressure is mainly double shear in a "X" fashion, and the width of fractures increases as the grain size increases. The proposed model provides a reliable method for investigating the influences of grain size on the rock mechanical behaviors in microscopic scale, and the results can be used to predict the macroscopic behaviors of brittle rocks.