Experimental investigation into grain size effect on failure behavior and nonlinear time-varying acoustic emission series of granite

This study aims to investigate the grain size effect on mechanical properties and failure mechanisms of hard rock. Thus, a series of uniaxial compression experiments were carried out on granites with various grain sizes, and acoustic emission (AE) signals were synchronously collected in the experimental process for further nonlinear analysis. The results show that the four characteristic stresses of granite (a cc , a ci , a cd , and a c ) significantly decrease with the increase of grain size. The ratio of a cd /a c basically remains a constant of 0.8; the ratio of a ci /a c increases gradually from 0.6 to about 0.7; the ratio of a cc /a c also shows a rising trend overall. With the increase of grain size, the AE energy level rises; the proportion of high-frequency tensile signals is reduced; the multi-fractal parameters Delta a (non-uniformity of signals) and Delta f (frequency difference between minimum and maximum signals) decrease. During the loading process, the time-varying value of Delta alpha changes between 0.0 and 0.35, and that of Delta f changes within the range from -0.2 to 0.5, showing a 'rise-fall-rise-fall' mode. The grain size effect on the influence of crack evolution is further discussed based on the nonlinear time-varying AE analysis. All these results reflect the mechanisms of grain size effect on granite. Large grain size weakens grain boundaries, facilitating shear slip along weak planes between crystal particles. In addition, the enhanced local stress concentration and initial defects in rock with large grain size could activate the crack coalescences of intra-crystalline cracks and weak boundary planes more easily in a relatively lowstress state.