# Mechanical and Energy Evolution Characteristics of Sandstone under True Triaxial Cyclic Loading
## 1 Introduction  
In the process of underground engineering construction, due to the interference of artificial repeated drilling, blasting, roadway excavation, support, and natural geological tectonic movement, the rock mass is often subjected to cyclic loading, and rock failure under cyclic loading is a progressive fracturing process [1; 2; 3; 4]. In addition, in engineering activities such as mining adjacent rock strata or roadway excavation, rock mass stress increases and decreases periodically to form cyclic disturbance, which will also make the rock mass bear cyclic load or even graded cyclic load. The mechanical properties and energy evolution of rock mass under cyclic loading influence the long-term stability of underground engineering. Many domestic and foreign scholars have investigated the mechanical characteristics of rocks under cyclic loading. [PERSON] et al. [5] studied salt rock's deformation and damage characteristics under uniaxial cyclic loading. [PERSON] et al. [6; 7] conducted a uniaxial cyclic loading test and reported that rock deformation under axial cyclic loading undergoes initial, stable, and accelerated stages. [PERSON] et al. [8] found thatthe number of cyclic loads in the process of fatigue failure of coal rocks can reflect the entire rock process from compaction and strain-hardening stages to the strain-softening stage; it can also reflect the process of damage evolution. [PERSON] et al. [9] analyzed sandstone deformation, strength, and damage under different cyclic loading frequencies. [PERSON] et al. [10] examined salt rock strength, residual deformation, and elastic modulus under cyclic loading. [PERSON] et al. [11] found that the plastic deformation of coal samples under cyclic loading is the largest in the first cycle and then gradually decreases, and the axial stress and strain are positively correlated. To further understand the mechanical and damage behavior of rock under true three-dimensional disturbance, it is necessary to study the deformation, energy dissipation, and damage evolution characteristics of rock under true triaxial cyclic loading. This will further reveal the damage mechanism of rock gradual deterioration caused by true three-dimensional cyclic disturbance, which has important guiding significance for prediction and prevention of rock dynamic disasters. The elastic deformation, residual deformation, microcracks, and pores of rocks under load directly reflect the transfer process of energy. Therefore, studying the energy evolution of rocks under cyclic loading is essential for dynamic disaster prediction and safety evaluation of rock masses in underground engineering. [PERSON] et al. [12] studied the relationship among the fatigue energy, fatigue deformation, and damage evolution of rocks under cyclic loading and unloading through a uniaxial cyclic loading and unloading test. [PERSON] [13] investigated the evolution of sandstone's total energy, elastic energy, and dissipated energy under cyclic loading and the relationship between them. [PERSON] et al. [14] conducted uniaxial cyclic loading and unloading tests on rocks, studied the energy accumulation and dissipation characteristics of rocks under different loading and unloading paths, and revealed the evolution and distribution of energy during rock failure. [PERSON] et al. [15] discussed the relationship between the dissipated energy and electromagnetic radiation of coal rocks under cyclic loading. [PERSON] et al. [16] established a damage criterion based on the energy dissipation obtained from the cyclic impact load test of granite, which could well characterize the relationship between the damage and the number of impacts. Most of the above experimental studies were carried out under uniaxial or conventional triaxial conditions. However, the majority of the rock masses in underground engineering practice are in the true triaxial state (\(\sigma_{1}>\sigma_{2}>\sigma_{3}\)) with unequal 3D stress.