###### Abstract  
Contact breakage of particles makes a large difference in the strength of coarse-grained soils, and exploring the characteristics within the process of the breakage is of great significance. Ignoring the influence of particle shape, the micromechanism of two spherical particles breaking under normal-tangential contact conditions was investigated theoretically and experimentally. Through theoretical analysis, the breakage form, the shape and size of the conical core, and the relationship between the normal and tangential forces at crushing were predicted. Particle contact tests of two gypsum spheres were carried out, in which the breakage forms, features of the conical cores and the normal and tangential forces at crushing were recorded for comparison with the predicted values. The test results and the theoretical predictions showed good agreement. Both the analysis and test demonstrate that the presence of tangential forces causes the conical core to assume the shape of an oblique cone, and the breakage form to change. Moreover, with increasing normal contact force, the tangential force needed for crushing increases gradually first and then decreases suddenly. particle contact test; coarse-grained soil; normal-tangential contact; conical core +
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many factors that influence the crushing characteristics of particles, such as the particle shape [34], material [35], contact form [30; 31; 32] and force state [34; 36], in which the effect of particle shape has a strong uncertainty due to its randomness. The research on the features of a single particle in the process of contact breakage is still in the initial stages, and some scholars start from the simplest spherical particles to explore the micromechanism of particle breakage. For instance, using a statistical approach, [PERSON] [35] investigated the spherical particle crushing patterns of three materials, sandstone, limestone and slate, and derived empirical equations for the conical core size and the critical normal force at overall crushing. Through a combination of experiments and numerical simulations, [PERSON] [37] studied the micromechanism of ball-plane normal contact breakage of spherical particles. Current studies on particle contact breakage have mainly focused on experimental studies and numerical simulations, with relatively little theoretical research on this problem. Theoretical analysis of the internal stress state of spherical rock particles subjected to external forces has been carried out since the 1950s [3; 38; 39; 40; 41]. Based on the Hertz contact model, many scholars have proposed various models to solve different contact problems, such as [PERSON] and [PERSON] [42]. One of the most popular tests is the compression test of spheres between a pair of diametrical concentrated loads, and the mechanical model proposed by [PERSON] and [PERSON] [43] has been applied by various authors [44; 40; 45; 46; 47]. To date, most of the experimental studies on the breakage mechanism of brittle spherical rock materials subjected to external forces have been based on this model. It assumed the brittle rock sphere as an isotropic elastic material and equated loads acting on both ends of the sphere as uniform loads acting on a small area pointing to the center of the sphere, which does not change with the loading. However, during the contact crushing of two spherical particles, local compaction occurs at the contact point due to the stress concentration, resulting in the emergence of a circular plane, which causes the boundary conditions to change as the contact process progresses. The model of a sphere squeezed by a symmetrical pair of concentrated loads cannot be applied to the conditions of contact crushing between two spherical particles since the mechanism of the whole crushing process cannot be revealed.