Energy-driven failure mechanisms of rock and coal: A comparison between compressive and tensile loading

• A universal four-stage energy evolution pattern, based on elastic strain energy, is identified for both compressive and tensile failure modes. • A key dichotomy is revealed: compressive failure involves progressive pre-peak energy dissipation, while tensile failure is governed by abrupt post-peak energy release. • A sharp surge in energy dissipation rate is a precursor to failure; an anomaly index ( W i = 1 ) quantifies this as a robust failure criterion. Understanding the disparate failure mechanisms of rock and coal under compressive and tensile loading is crucial for stability assessment in underground engineering. This study presents a comparative investigation into the mechanical behavior, failure modes, and energy evolution characteristics of the mudstone, sandy mudstone, and coal. Uniaxial compression tests (UCTs) and Brazilian splitting tests (BSTs) were conducted to systematically quantify these differences from an energy perspective. The results show that the energy storage capacity of rocks and coal in UCT is 1-2 orders of magnitude higher than that in BST. This vast energy difference results in diverse, high-energy failure events (shear, mixed-mode, fragmentation) under compression, contrasting sharply with the uniform, single-fracture, and quiescent failure under tension. Energy evolution analysis revealed distinct dissipation mechanisms. Compressive failure is characterized by a progressive damage process with significant pre-peak energy dissipation due to widespread micro-cracking. Conversely, tensile failure is an abrupt event, with energy dissipation almost entirely concentrated in a post-peak instantaneous release. Furthermore, a failure criterion based on the dissipated energy anomaly index W i was validated, where W i reaching a critical value of 1 proved to be a robust indicator of catastrophic failure under both loading conditions. These findings clarify the energy-based differences between compressive and tensile failure mechanisms, providing a theoretical basis for predicting and preventing geological hazards in underground engineering.