Novel insights into grain size effect of stressed crystalline rock using weakened grain boundary model

The grain size effect of rocks is crucial for engineering applications, including mining, tunneling, and oil extraction. Previous studies on the effect of grain size revealed discrepancies between experimental results and numerical simulations, hindering accurate predictions of rock strength and deformation. Initially, a parallel bond model was applied to describe the grain boundary contacts and investigate the effect of the grain size under initial stress conditions. The results of this model indicated that the uniaxial compressive strength (UCS) and Young's modulus increased with an increasing grain size, thus contradicting the experimental findings. Under the initial stress, the sample strength exhibited an irregular variation with grain size in this model. By analyzing the differences between the experimental and numerical results, a novel weakening grain boundary model (WGBM) that considers the strength and Young's modulus is proposed. The introduction of a grain boundary degradation factor provides a practical method by which to simulate the reductions in the strength and Young's modulus at grain boundaries as the grain size increases, which thereby enables a more realistic representation of the rock grain boundary behavior. The WGBM captured the experimental trends more accurately, and the UCS and Young's modulus decreased with an increasing grain size. Without the initial stress, the ratio of intergranular to intragranular cracks exceeded 2.5. The initial stress suppressed the propagation of intergranular cracks, and the intergranular and intragranular cracks were concentrated in one or more shear bands. Under initial stress conditions, the strength fluctuated slightly with an increasing grain size but showed an overall decreasing trend. The results and methods provide new insights into rock mechanics and related applications.