# Statistical Analysis of Acoustic Emission in Uniaxial Compression of Tectonic and Non-Tectonic Coal
## 3 Results  
The AE original amplitude spectrum for non-tectonic coal, and tectonic coal are shown in Figure 3a,b. The most common AE parameter for statistical analysis is absolute energy E [17], which can be calculated by fast numerical integration of the squared voltage of signals \(E=1/R\int_{i}^{t}U^{2}(t)dt\), where R = 10 k\(\Omega\) is inner resistance, U is the voltage signal. The amplitude dB follows this relation with voltage, dB = [20 lg (U\({}_{\text{peak}}\)/1\(\upmu\)V)].  
The compressive stress curve and AE absolute energy of coal samples during the entire test process are shown in Figure 3c,d for non-tectonic coal, and tectonic coal. The entire compression process can be roughly divided into three stages. The first stage is the microstructure compaction stage (1). In this stage, the stress curve is concave, which reflects that the micro-porous fractures in the coal underwent a compaction process. In addition, the tectonic coal was more active than the non-tectonic coal. This result occurred because the extensive pores experience damage under the external stress and emit acoustic events. In this process, AE cumulative energy rarely occurs. When there is obvious AE cumulative energy, the stress curve begins to transform to a linear and elastic straight line. Therefore, the appearance of the cumulative AE energy may be considered as the end of this stage.  
Figure 2: The sketch map of experimental system. (**a**) loading equipment, (SHIMADZU AGI-250 high-precision electric-servo digital experiment machine, Japan), and insert shows the coal sample and acoustic emission (AE) sensor arrangements, (**b**) real-time AE detection device. (**c**) the original AE signal feature extraction diagram, A proper setting of the peak definition time (PDT) ensures correct identification of the signal peak for rise time and peak amplitude measurements. Proper setting of the hit definition time (HDT) ensures that each AE signal from the structure is reported as one and only one hit. The AE system needs hit lockout time (HLT) to get ready for the next signal detection.  
The second stage is the approximate linear elastic stage (2). At this stage, the stress curve of tectonic coal is smoother, and there are fewer AE events. In addition, two stress drop phenomena occur near 0.5 MPa for the non-tectonic coal. These stresses' drop phenomena are accompanied by multiple AE signals. This result occurred because the micro-cavities combine to form a macroscopic weak surface. However, the coal body does not completely lose the loading capacity, and the entire coal structure shares the load. At this stage, the rate of AE events for non-tectonic coal is 5 to 10 times greater than that for tectonic coal.  
The third stage is the flow failure stage (3). After a relatively large stress drop, the coal sample enters into a stage where the stress oscillates in a limited range but the strain increases. At this stage, the stress curves of the two coal samples generally enter a platform. However, the tectonic coal generally increases and the non-tectonic coal generally decreases, which is accompanied by a dramatic enhancement in AE cumulative energy. The number of AE events roughly maintains the level of the previous stage. At failure, both coal samples dramatically lose the loading capacity and release the AE signal.