Numerical simulation on the effect of cold shock on the initiation and propagation of microcracks in granite with different grain sizes

To clarify how grain size affects the thermal damage behavior of granite subjected to cold shock, the hybrid finite-discrete element method (HFDEM) was applied to establish granite numerical models with varying average grain sizes ranging from 1.5 to 4.5 mm. The role of grain size in the development of microcracks generated by thermal stress under cold shock was systematically studied. The results show that enlarging grain size from 1.5 mm to 4.5 mm lowers the threshold temperature for significant strength degradation by approximately 100℃; the average length of thermally induced microcracks exhibits a linear increase with grain size, while microcrack count and crack density are jointly affected by the initial temperature and grain size, reaching a peak when the average grain size is about 3 mm. In addition, the damage caused by cold shock exhibits obvious zoning characteristics: cracks are concentrated in the rapid temperature change zone near the low-temperature boundary, where the crack density is governed primarily by the initial temperature and shows minimal sensitivity to grain size; the maximum ratio of crack density between the internal gradual change zone and the boundary rapid change zone is about 0.65. • Reveals the effect law of particle size on the deterioration temperature threshold of granite’s cold shock resistance. • Quantifies the variations of microcrack length, count, and density with average particle size and initial temperature. • Clarifies the "abrupt–gradual change zone" partitioning mechanism of temperature gradients under cold shock, and determines the upper limit of their crack density ratio.