## 3 Analysis of Sandstone Damage Characteristics Under Different Immersion Times and Temperatures  
During the fracture and failure process of sandstone, the development and propagation of internal cracks are often accompanied by the generation and transfer of energy. Acoustic emission (AE) can be used to monitor the internal damage of rocks [23,24]. The AE counts and cumulative counts reflect the extent of crack development and cumulative damage during rock rupture, while the AE energy indicates the amount of energy released during the failure of the sandstone. The acoustic emission signals obtained from the uniaxial compression tests of sandstone under different temperatures and soaking durations are illustrated in Figure 4.  
Figure 4: _Cont._  
Figure 3: Test flow chart. (**a**) Sandstone specimen. (**b**) Electric heating constant temperature drying oven. (**c**) Warm water tank. (**d**) Nano 30 sensor arrangement. (**e**) WAW-1000B electro-hydraulic servo universal testing machine. (**f**) AE win Express8 acoustic emission system. (**g**) Donghua DH3820 stress-strain acquisition system. (**h**) Nano sensor. (**i**) Insulation device. (**j**) Radial stress sensor. (**k**) Strain gauge. (**l**) Sandstone in the experiment. Figure 4: Acoustic emission ringing and energy of sandstone with different temperatures and immersion times. As illustrated in Figure 4, the acoustic emission signals of sandstone under different temperatures and soaking times exhibit similar patterns of change. The acoustic emission ringing and energy variations with time go through three distinct phases: 1 the quiet period, 2 the rapid increase period, and 3 the decay period. The cumulative ringing count of the acoustic emission increases gradually with loading time and reaches its maximum value after the peak stage. In the quiet period, due to the gradual compression of the primary pores and cracks within the sandstone, the acoustic emission signals (ringing counts, energy) of the sandstone are relatively low at all temperatures. In the rapid increase period, as the internal damage of the sandstone continues to intensify under axial force, cracks begin to develop and propagate. The acoustic emission signals (ringing count and energy) gradually increase, and near the peak stress, the acoustic emission activity suddenly surges, with the acoustic emission signals (ringing count and energy) reaching their maximum. In the decay period, as the stress-strain curve declines, macroscopic through-cracks develop both internally and on the surface of the rock, leading to complete failure of the rock. At this point, the acoustic emission signals (ringing count and energy) tend to stabilize and eventually disappear, while the cumulative acoustic emission ringing count reaches its maximum. To present the influence of different temperatures and soaking times on the acoustic emission (AE) damage characteristics of sandstone in a more intuitive and clear manner, we conducted a more detailed quantitative analysis of the maximum AE ring count, energy, and cumulative AE ring count of sandstone under various temperatures and soaking times. The results of this analysis are shown in Figure 5.  
As shown in Figure 5, at the same temperature, as the soaking time increases, the maximum AE ring count, maximum AE energy, and cumulative AE ring count of sandstone gradually decrease. When soaked for 3 h, the acoustic emission signals (ring count, energy) and cumulative ring count of sandstone at all temperature conditions reach their minimum values. At the same soaking time, as the temperature rises, for soaking durations of 0.5 h and 1 h, the maximum AE ring count and maximum AE energy of sandstone exhibit a trend of first increasing, then decreasing, and finally increasing again, while the cumulative AE ring count shows a gradual upward trend. For soaking durations of 3 h at different temperatures, the maximum AE ring count, maximum energy, and cumulative AE ring count of sandstone all gradually increase with rising temperature.