During the loading process of the specimen, a large number of microcracks will appear. At the same time, it will be accompanied by the release of elastic wave and energy. Monitoring and analyzing the AE signal during the loading process, is helpful to understand the micro-fracturing process in the rock specimen.. Acoustic emission can capture the elastic wave released by crack propagation in the specimen. Conversely, crack propagation location and the damage degree of the rock mass can be expressed by the acoustic emission characteristic parameters such as the event counts and so on. According to these characteristic parameters, the acoustic law of fracturing and damagein the sandstone under different shear angles can be revealed.the results will provide theoretical basis for the analysis of rock failure precursors.
4.1 Analysis of acoustic emission counting characteristics of sandstone under different shear angles
Acoustic emission counts refers to the times that the ringing pulse crosses the threshold in a unit time. This parameter is a very important and basic for AE signal, and its change law can reflect the real-time state of rock in the whole failure process (Ji et al., 2012). The AE count during the failure process of the specimen was recorded, and the distribution curve of AE count and cumulative count are drawn according to the recorded results, as shown in Figure 5.
It can be obtained from Figure 5 that the evolution of shear force, AE ringing count and cumulative count with time in the process of variable angle shear. The evolution law of characteristic parameters of AE ringing count can well reflect the failure process of sandstone specimens. The loading process of specimens can be divided into four stages: compaction stage, elastic stage, plastic stage and failure stage. Compaction stage: at this stage, some micro-cracks inside the specimen will be compacted to produce lots of intense AE signals.Elastic stage: at this stage, the AE activity gets into the "quiet period", and the stress begins to increase linearly with the loading time, which lasts for a long time. Plastic stage: at this stage, the AE activity gets into the "active period", and the stress reaches about 85% of the peak load. At this time, some tiny cracks can be observed on the surface of the specimen, which appear and continue to propagate. Failure stage: at this stage, the AE activity is extremely intense, and the stress reaches the peak value. The specimen is instantly farcturing along the shear plane, and then the AE ringing count and the stress decreases rapaidly until the end of the experiment.
With the increase of the angle, the required loading time decreases in the compaction stage, elastic stage, plastic stage and fracturing stage. Under the same experimental conditions, the normal stress on the shear plane changes with the change of loading angles, and the friction of rock particals on the shear plane also changes, which makes the peak load, shear stress and normal stress at different shear angles inconsistent.
The AE parameters of failure process have different evolution characteristics under different shear angles:
1) 50° shear angle
The peak shear force of two groups specimen are about 50 kN, and the loading time is about 330 s. At the compaction stage, the intense activity time of AE lasts about 10 s, and the shear force is about 10% of the peak load. The active time of AE in plastic stage lasts for 15-20 s. When the shear force reaches about 85% of the peak load, the AE activity of the specimen gets into the "active period" and continues until failure completely.
2) 55° shear angle
Compared with 50° specimens, the peak shear stress and loading time of this group of specimens are reduced by about 20% and 18% respectively. The duration of acoustic emission "active period" began to increase.
3) 60° shear angle
The acoustic emission "active period" of 60° specimen lasts for the longest time, reaching about 20s. It is also found that this group of specimens will have strong acoustic emission activity for 3-5 seconds in the elastic stage of loading process.
The comparison shows that there are differences between deformation characteristics and AE characteristic parameters evolution with the increase of shear angle. At the same stage, the AE activity of different angle specimen is basically the same. With the increasing of the shear angle, the intense activity time of AE in the compression stage and the "active period" of AE activity in the plastic stage gradually increase. This shows that the development of micro-cracks in the shear plane appear moreearlier with the decrease of normal stress. In addition, it is observed that the rock failure is more sudden and more violent with the increase of shear angle. This phenomenon indicates that the rock gradually transforms from plasticity to brittleness. Therefore, the AE ringing count characteristics can better predict the instability of sandstone engineering.
4.2 Analysis of acoustic emission amplitude characteristics of sandstone under different shear angles
Amplitude in the AE characteristic parameter is the maximum amplitude of AE waveform., which is one of the most important characteristic parameters for measuring signal size. . Its unit is dB, which has a direct relationship with the signal strength. It is usually used to identify the type of wave source, the measurement of strength and attenuation. During the test, the threshold value of AE instrument was set to 40 dB, and the amplitude of AE only changes in the range of 40-100 dB. The relationship between AE signal amplitude and loading time of sandstone failure process under different shear angels are shown in Figure 6.
It can be seen from Figure 6 that with the increase of normal stress on the shear plane of the specimen, the variation of AE amplitude presents a law of "increase-decrease-increase" during the loading process. Under different shear angles, the AE amplitude signal points present concave distribution, and the amplitude signals are high at both ends and low in the middle. The AE amplitude signals are relatively strong during the compaction stage and the failure stage during the loading process, and most of the amplitude signals are in the range of 70 dB-90 dB. However, the amplitude signal in the elastic stage is relatively weak, and most of the amplitude signals are in the range of 40 dB-70 dB.
The high decibel AE signals of 50° group specimens are mainly distributed in the compression stage and failure stage, the duration of high decibel signals in the compression stage are about 30 s, and about 20 s at failure stage, the rest signals are about 50 dB for 50°-1 specimen and about 70 dB for 50°-2 specimen, and the overall signal changes smoothly. The high decibel AE signals of 55° group specimens are mainly distributed in the compression stage and failure stage, the duration of high decibel signals in the compression stage is about 40 s, and about 25 s at failure stage, and the overall amplitude signals are distributed smoothly during the loading process. The high decibel AE signals of 60° group specimens are mainly distributed in the compression stage and failure stage, the duration of high decibel signals in the compression stage is about 50 s, and about 35 s at failure stage.
It can be seen that with the increase of shear angle, the duration of high decibel signal in the compression stage and failure stage increases continuously during the loading process, which indicates that the micro-crack propagation in the specimen becomes more seriously with the normal stress on the shear plane of the specimen decreasing. As shown in Figure 6, the high decibel amplitude signal generation interval is the internal micro-crack compression stage and the plastic-failure stage, while the low decibel amplitude signal generation interval is the elastic stage. At the compression stage and plastic-failure stage, the crack closure and crack growth will be more intense.. However, at the elastic stage, the crack closure and crack growth are stable and slight. Therefore, the generating the high decibel amplitude signals has obvious difference between these two stages. It can be concluded from the relationship between the amplitude and the fracturing law of the specimen that the magnitude of the signal can predict the fracturing of the specimen to a certain extent.
4.3 Analysis of peak frequency characteristics of sandstone under different shear angles
Waves with different frequencies correspond to different types of AE sources and microscopic crack pattern of rocks. With the failure process of rock, the frequency of AE signal will changes with the loading level and deformation degree. Therefore, the development process of AE peak frequency can reflect the fracturing information and the loading state of rock (Ji et al., 2012). Under different shear angles, the relationship between peak frequency of AE signal and loading time are shown in Figure 7 .
It can be seen from Figure 7 that the peak-frequency signals of AE under different shear angles are basically concentrated in the intermediate frequency signal region and the ultra-high frequency signal region, while the signal distribution in section I is generally less.
The acoustic emission peak frequencies of the three groups of specimens are mainly distributed in the intermediate frequency signal area of about 100 kHz and the ultra-high frequency signal area of about 300 kHz during loading.Under different shear angles, there are obvious differences of signal distribution in section I. Among them, the signal distribution in section I of 50° group specimens is more and evenly distributed. the signals in section I of 55° group specimens are less and concentrated the range of the intermediate frequency and ultra-high frequency. The number of signals in section I of 60°-1 specimen are the least. With increase of the shear angle, the signal distribution in section I of the high-frequency signal area shows a decreasing trend.
Based on the above analysis and the fracturing process of the spencimen, it can be seen that during the test loading process, the AE peak frequency signal is mainly the intermediate frequency signal. The peak frequency signal of AE in elastic stage is also dominated by IF frequency signal. As an important AE parameter, the peak frequency signal may be of great significance to determine the fracturing type of rock failure. The different peak frequency of sound wave may be related to the failure mechanism of the fracturing location. With the increasing of shear angle, the normal stress of shear plane and the confining pressure effect decreases gradually, which leads to the weakening of friction shear effect. Compared with the peak frequency signal distribution, it is found that high frequency signal decreases significantly with the increase of shear angle, which may have a significant corresponding relationship with the rock friction and shear effect. Therefore, further investigation on the relationship between the peak frequency signal of acoustic emission and the fracturing type of rock failure has important guiding value for judging and predicting the instability mechanism of rock engineering.