## 4 Experimental Results
### AE Characteristics
#### 4.3.2 Distribution of AE Events  
AE energy reflects the global acoustic information about different kinds of sandwiched coal-rock specimens. However, it cannot be applied to describe the local acoustic characteristics of coal elements and rock elements. To provide deeper insight into the progressive failure characteristics of sandwiched coal-rock specimens, the three-dimensional (3D) distribution of AE events was also investigated in detail. This reflects the spatial evolution of micro-cracks and internal damages inside the coal elements and rock elements at different heights [10]. Figure 7 shows the 3D distribution of accumulated AE events inside the C-R specimen during the loading process. Under the compaction effects at stage I (see Figure 7a,c), earlier AE events appeared randomly on both the coal element (50-100 mm) and rock element (0-50 mm). The number of AE events inside the C-R specimen increased slightly during the elastic deformation process at stage II, which were mainly caused by the initiation and propagation of micro-cracks at the height of 50-100 mm. As shown Figure 7d-g, AE events were mostly inside the coal element. The distribution space of AE events in this area also expanded progressively from the top section to the bottom section. Furthermore, the number and density of AE events inside the coal element (50-100 mm) were apparently more than those in the rock element (0-50 mm). That is, the coal element in the C-R specimen primarily experienced the micro-damage at stage II due to its lower strength. In terms of the rock element in the C-R specimen, little internal damage appeared and AE events were random at this stage. It is worth noting that several AE events were spread across the interface of the coal element and rock element at the later period of stage II (see Figure 7g). However, most of them were located at the left-top corner of the rock element. This indicates that local damage of the rock element was induced by the damage to the coal element. At stage III, the number of AE events in Figure 7h,i grew significantly inside the C-R specimen. Corresponding to the intensive release of accumulated energy in Figure 6a, the AE events were almost distributed throughout the coal element. This indicates that the internal damage inside the coal element was exacerbated during stage III. Furthermore, the AE events scattered at the top corner of the rock element also increased in number. Both the distribution area and density were enlarged. When it turned into stage IV, the number of AE events inside the C-R sandwiched coal-rock specimen continued to grow (see Figure 7i,j). This was attributed to the connection of macro-cracks, the formation of the sliding plane, and the dislocation of material particles. AE events inside the coal element were greater than those in the rock element. The distribution space of AE events in the rock element was still centralized near the top corner, which demonstrated the induced damage was not spread across the entire rock element at stage IV. The 3D distribution of the accumulated AE events inside the R-C specimen at different loading times is presented in Figure 8. As a whole, AE events in Figure 8a-d started to generate both on the coal element (0-50 mm) and the rock element (50-100 mm) at stage I. This was attributed to the closure of natural pores and cracks inside the specimen. With the increase of the loading time, a more rapid increase of AE events was monitored inside the coal element. The distribution space of AE events also expanded progressively inside the coal element. As shown in Figure 8e-h, primary AE events were located at the height of 0-50 mm (coal element) during the elastic deformation stage. The total number of AE events inside the rock element also increased. However, the growth rate was obviously less than that in the coal element. At stage III, a drastic increase of AE events was observed inside the coal element, which quickly distributed throughout the whole coal element (see Figure 8h,i).