## 1 Introduction
[22] carried out triaxial CLU experiments on rock samples subjected to high temperatures to analyze the effects of confining pressure and temperature on the patterns of energy evolution and distribution. [PERSON] et al. [1], through triaxial tests on halite, detailed how energy dissipation correlates with damage severity across various confining pressures. [PERSON] et al. [23] explored the mechanical properties and energy behavior of marble, considering the impact of different levels of thermal damage and confining pressures. [PERSON] et al. [24] conducted standard triaxial compression experiments to uncover how energy transformation is linked to the failure of coal and rock masses. Additionally, [PERSON] et al. [25] examined the deformation behaviors and failure mechanisms of sandstone when subjected to varying confining pressures. [PERSON] et al. [26; 27] studied the energy evolution of rocks under CLU conditions. (3) Investigations on the failure traits of rocks include the following: [PERSON] et al. [11] focused on quantifying rock damage by studying how energy is dissipated and converted during the rock's failure phase under CLU. [PERSON] et al. [28], on the other hand, calculated a damage variable for rocks through triaxial creep experiments on coal and rock masses, applying the principles of energy conservation and minimal energy expenditure to develop a nonlinear model for creep damage. [PERSON] et al. [29] introduced a theory of damage for intact rocks under uniaxial compression, rooted in the linear pattern of energy dissipation and its dissipation coefficient. [PERSON] et al. [30] built upon the mechanical characteristics and energy development in rocks to suggest a novel brittleness index based on energy (BIE). Further exploring uniaxial compression, [PERSON] et al. [31] crafted a methodology for characterizing rock damage that incorporates the energy dissipation coefficient. [PERSON] et al. [32] delved into the mechanisms of energy loss, the progression of damage, and the acoustic emission (AE) event distribution in rocks, using AE testing under uniaxial compression. [PERSON] et al. [33] identified a link between energy loss and the deformation behavior of rocks under cyclic stress, leading to the creation of a model for predicting damage in fractured rock masses. Continuing with cyclic stress studies, [PERSON] et al. (2018) analyzed the changes in energy and the fatigue properties of sandstone, resulting in a model to forecast fatigue damage. [PERSON] et al. (2018), focusing on rock brittleness, proposed a new index and calculated rock damage via an energy-centric approach. Lastly, [PERSON] et al. (2018), through uniaxial compression tests on sandstone with varying moisture levels, established a damage variable for sandstone grounded in the theories of AE energy and energy dissipation. The research highlighted above serves as a valuable foundation for further exploration into how energy evolution impacts the failure properties of rocks. However, these studies predominantly address rock failure through tests under simple uniaxial or standard triaxial stress conditions, overlooking the complexities of failure and energy dynamics under triaxial CLU scenarios. Additionally, the common practice of integrating stress-strain curves to estimate rock energy during loading tends to ignore the distinct contributions of elastic and plastic strains, leading to potential inaccuracies in the calculations. To bridge this gap, this study leverages an MTS-815 electro-hydraulic servo-controlled testing system to perform both standard triaxial tests and CLU experiments on sandstone samples, subject to six varying confining pressures and constant axial displacement rates. This approach allows for a detailed analysis of energy evolution and distribution within sandstone throughout the failure process under different confining pressures, with particular attention paid to sudden shifts in energy as indicators of failure criteria.