Non-linear strength criterion and direct tensile strength characteristics of rocks with confining pressures

The evolution of tensile stresses is closely related to the mechanical response of in situ rock masses in engineering applications. Understanding rock deformation and failure characteristics under triaxial tension is essential for assessing the stability of tunnels, underground caverns and other rock engineering structures. In this paper, a non-linear strength criterion for rocks was developed based on the theory of microscopic failure mechanisms, and a triaxial direct tensile testing method was established through a compression-to-tension load conversion. Direct tensile tests were conducted on four types of rock specimens under different confining pressures to investigate their mechanical responses and acoustic emission (AE) characteristics. The results show that tensile strength increases significantly with confining pressure within a certain range but stabilizes or even decreases beyond a critical threshold. The failure mode transitions from tensile to mixed, and eventually to shear failure, as confining pressure increases. Fracture surface roughness and inclination further confirm this transition. Acoustic emission analyses reveal the evolution of microcracking and stress redistribution during failure. The proposed non-linear strength criterion accurately fits the experimental data, with correlation coefficients of 0.977, 0.998, 0.991 and 0.998 for the four rock types. These findings enhance the understanding of rock mechanical behaviour under complex stress states and provide a foundation for safe and reliable geotechnical design.