## 4 Discussion
### IR and AE Combined Damage Evolution Analysis of Different Coal Impact Types
The weak-impact coal has a lower degree of development in its primary fissure structure, weaker connectivity, and a denser structure. The skeleton has a certain load-bearing capacity and exhibits overall brittle characteristics [59; 60]. In the early stages of coal loading, the external input energy mainly accumulates in the form of elastic energy, and the released dissipated energy and generated IRT and AE signals are relatively lower. Due to the brittleness of coal, when the stress drop occurs, cracks in the coal body rapidly develop and expand, and the dissipated energy increases rapidly, corresponding to a sudden increase in IRT and AE signals (Figure 9b). The pore and fracture development degree of strong impact coal is very low, with a dense and hard structure and strong load-bearing capacity of the skeleton [61]. In the early stages of coal loading, the externally input energy is almost entirely stored in the form of elastic properties inside the coal body, and the dissipated energy hardly increases. At the moment of coal failure, the accumulated elastic energy is rapidly released and converted into dissipated energy, resulting in a rapid increase in IRT and AE signals (Figure 9c). Combining the IR and AE combined instability precursor index (Figure 7) and the coal energy evolution law (Figure 9), it can be seen that before the IRT and AE combined instability precursor point, the energy inside the coal is mainly accumulated in the form of elastic energy, with less dissipated energy being released. After the IRT and AE combined instability precursor point, the dissipated energy increases rapidly. At this time, the internal damage of the coal body intensifies, and instability failure is about to occur. As the impact tendency increases, the development of primary pores in the coal decreases and the structure becomes denser. During the early and middle stages of coal loading, the damage degree is minimal, and internal energy accumulates. Near the moment of failure, the coal body gradually transitions from tensile failure to shear failure (Figure 6). The stronger the impact tendency, the faster and more severe the shear failure, the more dissipated the energy released by the coal body failure, the stronger the IR and AE signals generated, and the closer the precursor point of instability is to the moment of coal body instability failure. At present, there are methods for monitoring rock bursts, such as microseismic, electromagnetic, IR and AE approaches. The microseismic monitoring method is usually used for dynamic monitoring in large areas, and it is not sensitive enough to fractures with low intensity. The electromagnetic radiation monitoring method has obvious non-contact advantages, but electromagnetic radiation is easily interfered with by electrical equipment and cable circuits. Coal damage and failure generate IR while also releasing elastic waves, which generate AE signals. As a non-contact method, IR does not require direct contact with the coal body, but the IR mainly monitors the changes in the IRT on the surface of the coal body [14; 62]. AE, as a contact monitoring method, can monitor the internal damage changes of the coal and rock mass, but it requires a large number of sensors to be deployed on the coal and rock mass, which often has problems such as heavy workload, easy interference from mechanical vibration, and the complex propagation path of elastic waves in the rock mass [63; 64]. This paper combines IR and AE monitoring, integrates the IR data of the coal surface temperature field with the AE data of internal coal damage, and establishes the IR and AE sensitive combined index \(K\) and combined damage model. It fully utilizes the advantages of IR and AE signals, which can significantly reflect the damage and failure information of coal bodies and improve the accuracy of monitoring and early warning for different impact types coal.