Effects of grain size, non-uniformity, discontinuity on tensile properties of Eastern Himalaya marble

The grain size distribution of crystalline rocks significantly influences their macro-strength and crack behavior, particularly in tensile fracture. However, the effects of grain size nonuniformity and discontinuity, two common phenomena in natural crystalline rocks, on tensile mechanical characteristics have rarely been reported. In this study, tensile tests were conducted on three marbles with distinct grain size distributions but similar mineral compositions, using an innovative visual micro-tensile testing instrument. The direct tensile test results show that grain size nonuniformity and discontinuity significantly affect tensile strength, crack critical stress, and the strain field. Marble with a non-uniform and discontinuous grain size distribution exhibits lower tensile strength, reduced crack propagation resistance, and a complex distribution of high-strain regions. Nonuniformity has a stronger influence on peak strength, while discontinuity governs crack initiation. In marble with pronounced grain size nonuniformity and discontinuity, microcrack observations revealed that cracks propagate along boundaries between large crystal grains and clusters of small grains (BLS). High stress concentration, a persistent propagation path, and lower connectivity along BLS make it the primary crack path, leading to lower macroscopic tensile strength. These findings help establish correlations between grain-scale properties and macroscopic strength, thereby enabling more accurate assessment of strength parameters for rock engineering applications.