Experimental Study on the Acoustic Emission Characteristics of Fractured Granite after Repeated High Temperature-Water Cooling

: Using the MTS816 rock mechanics servo tester, an acoustic emission monitoring system and high-speed digital photographic equipment, uniaxial compression tests were conducted on granite specimens containing single fracture slabs after repeated treatment (treatment times 1, 5, 10, 15 and 20) with three types of high temperature (250, 350 and 450 ◦ C) water cooling, respectively, to analyze the basic mechanical parameters, acoustic emission change characteristics and fracture evolution of the specimens during the uniaxial compression process. It is shown that the heating temperature and the number of treatments not only have a deteriorating effect on the basic mechanical parameters of the specimens but also have an important effect on the changes in the basic parameters of acoustic emission at different compression stages. At 250 ◦ C, the acoustic emission characteristics of the specimens at the initial tightening stage tended to decrease ( N = 1 and 5 times) then, increase ( N = 10 and 15 times) and then decrease ( N = 20 times) as the number of treatments increased. At the same set temperature, the percentage of the bottom amplitude value of the acoustic emission of the specimen gradually decreases, and the percentage of the high amplitude value gradually increases as the number of treatments increases. After the specimen undergoes one and ﬁve treatments at 250 ◦ C, the maximum acoustic emission energy value changes less, the maximum acoustic emission energy value decreases with the increase of treatment times in an approximately exponential function, the specimen is transformed from the brittle damage mode to the plastic damage mode and the effect of the prefabricated fracture on the damage of the specimen gradually disappears.


Introduction
At present, countries all over the world are facing the trend of increasing depletion of traditional energy sources. On the one hand, fossil energy represented by coal and oil is a nonrenewable resource, and after a long period of mining and utilization, the problems of gradually increasing difficulty in mining, increasing mining costs and energy depletion arises. On the other hand, the burning of fossil fuels brings many ecological and environmental problems, such as water pollution, climate warming and frequent haze, which have a greater impact on people's health and quality of life [1,2]. The development and utilization of new energy sources is an urgent problem for mankind, and geothermal energy, as a clean and renewable energy source, is receiving more and more attention from governments because of its wide distribution, stable source, huge reserves and green and recyclable characteristics [3,4].
Geothermal energy is mainly stored in dry heat rock resources, which are dominated by dense metamorphic rocks and granites. For the development of dry heat rock resources, the development of enhanced geothermal systems ( Figure 1) is currently the main focus [5,6]. EGS is mainly developed by first drilling into the reservoir and then using hydraulic fracturing technology to construct a well-connected fracture network in the low-porosity, low-permeability dry-thermal rock reservoir to form an artificially hyperpermeable reservoir with good permeability; thus, the dry-thermal rock reservoir contains a large number of penetrating fractures [7][8][9][10][11]. In order to sustain the extraction of underground energy, make the mining work safer and more stable, as well as prevent the disasters caused by the weakening of rock mechanical properties after mining. An in-depth study on the fundamental mechanical response of fracture-bearing granites after high-temperature water cooling cycles is of scientific value for the scheme design and exploitation of deep geothermal resources [12,13].
from governments because of its wide distribution, stable source, huge reserves and green and recyclable characteristics [3,4].
Geothermal energy is mainly stored in dry heat rock resources, which are dominated by dense metamorphic rocks and granites. For the development of dry heat rock resources, the development of enhanced geothermal systems ( Figure 1) is currently the main focus [5,6]. EGS is mainly developed by first drilling into the reservoir and then using hydraulic fracturing technology to construct a well-connected fracture network in the low-porosity, low-permeability dry-thermal rock reservoir to form an artificially hyperpermeable reservoir with good permeability; thus, the dry-thermal rock reservoir contains a large number of penetrating fractures [7][8][9][10][11]. In order to sustain the extraction of underground energy, make the mining work safer and more stable, as well as prevent the disasters caused by the weakening of rock mechanical properties after mining. An in-depth study on the fundamental mechanical response of fracture-bearing granites after hightemperature water cooling cycles is of scientific value for the scheme design and exploitation of deep geothermal resources [12,13]. The repeated effects of rock conduction heating and water cooling caused by EGS development can lead to changes in the internal pore and microfracture structure of dryheated rocks, which in turn can lead to changes in the mechanical properties of dry-heated rocks, and a lot of research work has been carried out to address this issue [14][15][16][17]. Zhou C et al. [18] found that the cooling effect of high-temperature borehole fracturing fluid led to a thermal shock phenomenon in hydraulic fracturing tests. Zhu D. et al. [19] studied the mechanical properties of standard granite specimens after 1-20 cycles of heated water cooling in the range of 250 to 650 °C and analyzed the degradation process of the specimens based on SEM test results, in which temperature plays a controlling role on their internal damage. Xu C. et al. [20] investigated the effect of the quenching number on the tensile properties of granite by Brazilian splitting tests and found that the tensile strength of granite was negatively correlated with the temperature and quenching number. For the same number of quenches, the tensile strength and wave velocity decreased with increasing temperature. When the number of quenches is greater than five, the effect of the  The repeated effects of rock conduction heating and water cooling caused by EGS development can lead to changes in the internal pore and microfracture structure of dryheated rocks, which in turn can lead to changes in the mechanical properties of dryheated rocks, and a lot of research work has been carried out to address this issue [14][15][16][17]. Zhou C et al. [18] found that the cooling effect of high-temperature borehole fracturing fluid led to a thermal shock phenomenon in hydraulic fracturing tests. Zhu D. et al. [19] studied the mechanical properties of standard granite specimens after 1-20 cycles of heated water cooling in the range of 250 to 650 • C and analyzed the degradation process of the specimens based on SEM test results, in which temperature plays a controlling role on their internal damage. Xu C. et al. [20] investigated the effect of the quenching number on the tensile properties of granite by Brazilian splitting tests and found that the tensile strength of granite was negatively correlated with the temperature and quenching number. For the same number of quenches, the tensile strength and wave velocity decreased with increasing temperature. When the number of quenches is greater than five, the effect of the quenching number on the tensile strength and wave velocity is relatively small. Yin T. et al. [21] investigated the effects of cyclic heating and cooling treatments on the fracture mode of granite through bending fracture tests and found that cyclic treatments could lead to severe degradation of bending resistance, a decrease in fracture toughness, a smoother post-peak curve of the load-displacement curve and a prolonged active period of acoustic emission, which indicates the enhanced ductility of granite. The internal particles of the rock fracture slip under load to form cracks, and the energy generated during the fracture releases the acoustic emission signal. Therefore, the use of acoustic emission signals to characterize the damage state of rock materials can well describe the processes of fracture development, extension and penetration within the rock [22][23][24][25]. Ganne P. et al. [26] studied the connection between the acoustic emission and microfracture using the acoustic emission technique, classified four stages in the rock-bearing process by the accumulated energy of acoustic emission and defined the transition threshold for each stage. Several scholars [27,28] demonstrated the change in the damage mode of granite from a brittle fracture to a quasi-brittle fracture with the increase of temperature through the study of the acoustic emission technique. Ge Z. [29] studied the acoustic emission response characteristics and b-values of circulating heated and water-cooled granites. The uniaxial compression test confirmed 450 • C as the temperature threshold of granite and found that the sensitivity of the granite b-value to the temperature and number of cycles before 450 • C was low and grew slowly. After the heating temperature of 450 • C and 5 cycles, its sensitivity is stronger and increases sharply with the increase in the temperature and number of cycles. Zhu D. et al. [30,31] conducted compressive and tensile tests on granite after hot and cold cycles using the MTS816 rock testing machine, studied the acoustic emission response characteristics of the specimens during loading and proposed the concepts of mechanical damage and thermal impact damage based on the basic mechanical and acoustic emission parameters, based on which a specimen damage model was constructed.
However, none of the above research results have addressed the mechanical properties of fracture-bearing granites associated with repeated hot and cold impacts. In view of this, this paper contains a single fissure slab granite as the object of study. The specimens were treated with multiple hot and cold impacts in the range of 250 • C~450 • C, using the MTS816 test and the uniaxial compression test on the treated specimens, while using the acoustic emission monitoring system and high-speed digital photography technology for real-time data acquisition of the test process. Based on the experimental results, we focus on analyzing the influence of temperature and the number of treatments on the variation pattern of the acoustic emission characteristic parameters of the specimen, and analyze and discuss the fracture evolution mechanics mechanism during uniaxial compression of granite containing a single fracture by comparing typical acoustic emission ringing events and digital photographic results.

Specimen Processing and Handling
This paper selected granite specimens from Linyi City, Shandong Province, China. The color of the specimen is dark gray and through XRD test analysis it can be seen that the main minerals of the specimen consist of quartz, potassium feldspar, plagioclase, secondary mineral composition by including black mica, hornblende, etc., where the quartz and potassium feldspar content reached 95.00%. The average density of the specimens was 2.95 kg/m 3 , and the average ultrasonic velocity reached 2.87 km/s. The test results showed that the specimens were relatively dense and hard.
According to the fabrication method in the literature [32][33][34][35], in this paper, large granite blocks were cut and polished to produce rectangular slab specimens with dimensions of (160 ± 2) mm × (80 ± 2) mm × (30 ± 1) mm, then a penetration crack with a length of 20 ± 1 mm was cut in the center of the specimen by a high-pressure waterjet, and the completed specimen was processed as shown in Figure 2.
Using the MXQ1700 high-temperature atmosphere furnace ( Figure 3) for heating granite specimens containing a single fissure, the equipment has a temperature control accuracy of ±1 • C and the power of 4 KW, and the equipment has the characteristics of uniform heating and small thermal shock. The specimens were heated to 250, 350 and 450 • C at a heating rate of 10 • C/min according to the test design. Ten specimens were heated at each set temperature, and the heating test was completed in three batches. After heating to the set temperature and continuing to maintain a constant temperature for two hours, crucible tongs were used to remove the specimen and quickly put it into a container of cold water with 20 • C, and the specimen is dried briefly after the cooling is completed, so the first heating-water cooling treatment is completed. Following this treatment, the specimens were subjected to 1, 5, 10, 15 and 20 heating-water cooling iterations in sequence. of 20 ± 1 mm was cut in the center of the specimen by a high-pressure waterjet, and the completed specimen was processed as shown in Figure 2. Using the MXQ1700 high-temperature atmosphere furnace ( Figure 3) for heating granite specimens containing a single fissure, the equipment has a temperature control accuracy of ±1 °C and the power of 4 KW, and the equipment has the characteristics of uniform heating and small thermal shock. The specimens were heated to 250, 350 and 450 °C at a heating rate of 10 °C/min according to the test design. Ten specimens were heated at each set temperature, and the heating test was completed in three batches. After heating to the set temperature and continuing to maintain a constant temperature for two hours, crucible tongs were used to remove the specimen and quickly put it into a container of cold water with 20 °C, and the specimen is dried briefly after the cooling is completed, so the first heating-water cooling treatment is completed. Following this treatment, the specimens were subjected to 1, 5, 10, 15 and 20 heating-water cooling iterations in sequence.  Table 1 shows the test results for the basic physical parameters, such as mass, volume and wave velocity, of the treated granite specimens. It can be seen that at the heating temperatures of 350 °C and 450 °C, with the increase in the number of treatments, the overall trend of the specimen density and wave velocity decreases, especially at a temperature of   Using the MXQ1700 high-temperature atmosphere furnace ( Figure 3) for heating granite specimens containing a single fissure, the equipment has a temperature control accuracy of ±1 °C and the power of 4 KW, and the equipment has the characteristics of uniform heating and small thermal shock. The specimens were heated to 250, 350 and 450 °C at a heating rate of 10 °C/min according to the test design. Ten specimens were heated at each set temperature, and the heating test was completed in three batches. After heating to the set temperature and continuing to maintain a constant temperature for two hours, crucible tongs were used to remove the specimen and quickly put it into a container of cold water with 20 °C, and the specimen is dried briefly after the cooling is completed, so the first heating-water cooling treatment is completed. Following this treatment, the specimens were subjected to 1, 5, 10, 15 and 20 heating-water cooling iterations in sequence.  Table 1 shows the test results for the basic physical parameters, such as mass, volume and wave velocity, of the treated granite specimens. It can be seen that at the heating temperatures of 350 °C and 450 °C, with the increase in the number of treatments, the overall trend of the specimen density and wave velocity decreases, especially at a temperature of  Table 1 shows the test results for the basic physical parameters, such as mass, volume and wave velocity, of the treated granite specimens. It can be seen that at the heating temperatures of 350 • C and 450 • C, with the increase in the number of treatments, the overall trend of the specimen density and wave velocity decreases, especially at a temperature of 450 • C after 20 times of hot and cold shock treatment. The specimen internal fissures have been developed very rich and the propagation speed of the acoustic wave in the interior of the rock is significantly reduced.

Loading and Data Acquisition Systems
In this study, the uniaxial compression of the specimen was carried out using the MTS816 loading system manufactured by MTS Corporation, USA. The MTS816 vertical loading load range was 0~1459 KN, and the actuator stroke range was 0~100 mm. The displacement control mode is used for loading, the actuator descent speed is 0.05 mm/min and the data acquisition system automatically collects the load-displacement curve. The acoustic emission data acquisition system is used to collect the characteristic parameters of the specimen during uniaxial compression in real-time. The acoustic emission acquisition sets the transducer with a 200 kHz resonance frequency and the preamplifier and postamplifier are set to 40 dB. In order to minimize the influence of acoustic impedance, etc., the coupling process between the acoustic emission probe and the specimen is lifted using petroleum jelly. During the loading process, a high-speed digital image acquisition system is used to acquire the specimen loading rupture evolution process in real-time, as shown in Figure 4. using petroleum jelly. During the loading process, a high-speed digital image acquisition system is used to acquire the specimen loading rupture evolution process in real-time, as shown in Figure 4.

Test Results
Firstly, two complete granite specimens were tested in uniaxial compression and their stress-strain curves are shown in Figure 5. The average values of the peak strength, peak strain, average elastic modulus and secant modulus of the complete specimen obtained from its stress-strain curve ( Figure 5) were 184.44 MPa, 9.63 × 10 −3 , 27.33 GPa and 15.20 GPa, respectively, with dispersion coefficients of 0.53%, 1.62%, 4.24% and 2.12%, respectively, which indicate that the selected granite has better homogeneity and less dispersion of mechanical parameters.

Characteristics of Basic Mechanical Parameters, Number of Acoustic Emission Events and Maximum Amplitude Variation
For reasons of space, only the uniaxial compressive strength, average modulus of elasticity, number of acoustic emission events, cumulative number of acoustic emission events and maximum amplitude distribution of the specimens after 1~20 treatments at a high temperature of 250 °C are analyzed in this paper with an increasing number of treatments, as can be seen from Figure 6.

Test Results
Firstly, two complete granite specimens were tested in uniaxial compression and their stress-strain curves are shown in Figure 5. The average values of the peak strength, peak strain, average elastic modulus and secant modulus of the complete specimen obtained from its stress-strain curve ( Figure 5) were 184.44 MPa, 9.63 × 10 −3 , 27.33 GPa and 15.20 GPa, respectively, with dispersion coefficients of 0.53%, 1.62%, 4.24% and 2.12%, respectively, which indicate that the selected granite has better homogeneity and less dispersion of mechanical parameters.
Processes 2023, 11, x FOR PEER REVIEW 6 of 19 using petroleum jelly. During the loading process, a high-speed digital image acquisition system is used to acquire the specimen loading rupture evolution process in real-time, as shown in Figure 4.

Test Results
Firstly, two complete granite specimens were tested in uniaxial compression and their stress-strain curves are shown in Figure 5. The average values of the peak strength, peak strain, average elastic modulus and secant modulus of the complete specimen obtained from its stress-strain curve ( Figure 5) were 184.44 MPa, 9.63 × 10 −3 , 27.33 GPa and 15.20 GPa, respectively, with dispersion coefficients of 0.53%, 1.62%, 4.24% and 2.12%, respectively, which indicate that the selected granite has better homogeneity and less dispersion of mechanical parameters.

Characteristics of Basic Mechanical Parameters, Number of Acoustic Emission Events and Maximum Amplitude Variation
For reasons of space, only the uniaxial compressive strength, average modulus of elasticity, number of acoustic emission events, cumulative number of acoustic emission events and maximum amplitude distribution of the specimens after 1~20 treatments at a high temperature of 250 °C are analyzed in this paper with an increasing number of treatments, as can be seen from Figure 6.

Characteristics of Basic Mechanical Parameters, Number of Acoustic Emission Events and Maximum Amplitude Variation
For reasons of space, only the uniaxial compressive strength, average modulus of elasticity, number of acoustic emission events, cumulative number of acoustic emission events and maximum amplitude distribution of the specimens after 1~20 treatments at a high temperature of 250 • C are analyzed in this paper with an increasing number of treatments, as can be seen from Figure 6. under this condition gradually increases, the opening gradually increases and the initial stage of uniaxial compression produces a higher number of acoustic emission events and a maximum amplitude. When N ≥ 15 times, the number of microcracks inside the specimen basically tends to stabilize, and the comparison of the results under the two conditions of N = 15 and N = 20 can be seen (Figure 6e,f), and the distribution patterns of the number of acoustic emission events and maximum amplitude of the two specimens in the initial stage of uniaxial compression are nearly the same. Under the same temperature condition, the number of treatments has a certain influence on the number of accumulated acoustic emission events during uniaxial compression of the specimen, but temperature often plays a decisive role. At 250, 350 and 450 °C ( Figure  7), the cumulative number of acoustic emission events of the specimens during uniaxial compression tended to increase and then decrease, with the highest level of cumulative acoustic emission events of the specimens at 250 °C. Compared with the intact slab granite specimens at 25 °C, the cumulative acoustic emission events of the specimens were almost at the same level for both conditions at a high temperature of 250 °C when N = 1 and 5 times, with the values of 136.73 × 10 3 and 118.32 × 10 3 , respectively. However, when N = 10 times, the cumulative acoustic emission events of the specimens showed a significant increase, and compared with N = 5, the cumulative acoustic emission events increased from 118.32 × 10 3 to 292.03 × 10 3 , an increase of 146.81%. When N = 15 and 20 times, the cumulative acoustic emission events of the specimens were 312.36 × 103 and 306.28 × 103, respectively, with an increase of 6.97% and 6.88% compared with N = 10 times. A significant decrease in the cumulative number of acoustic emission events was observed at 350 °C and 450 °C (Figure 7c,d), and at 450 °C, when N = 5, 10, 15 and 20 times, the cumulative (1) The number of treatments has a significant effect on the deterioration of the basic mechanical parameters of slab granite containing a single fissure, and the time required for specimen damage during uniaxial compression shows an overall increasing trend. After five cold and hot treatments, the granite slab specimens containing a single fissure initially showed plasticity characteristics, and the compressive strength decreased from 185.12 MPa to 171.47 MPa by 7.37%, and the average modulus decreased from 27.33 GPa to 24.89 GPa by 8.93% compared with the specimens at 25 • C (Figure 6a,c). After 20 cold and hot treatments, their compressive strength and average modulus decreased to 155.99 MPa and 23.67 Gpa, 15.74% and 13.39%, respectively (Figure 6a,f). This shows that the peak strength and average modulus decrease of granite containing a single fissure fluctuate in the range of 2.21% to 15.74% and 7.77% to 13.39%, respectively, after 20 cold and hot treatments at 250 • C.
(2) The number of treatments has an important effect on the number of acoustic emission meter events and the maximum amplitude value of the specimen in the initial stage of uniaxial compression. When N = 1 and 5, the specimens under both working conditions have only a few AE events in the initial stage of uniaxial compression, accounting for 3.92% and 8.47% of the accumulated acoustic emission events, respectively, and the maximum amplitude is mainly distributed between 40 and60 dB, and the acoustic emission events are concentrated in the stages before and after the specimen rupture (Figure 6a-c). This phenomenon indicates that when N = 1 and 5, only very minor cold and thermal shock damage is generated inside the specimen. In contrast, when N = 10, 15 and 20, a large number of AE events appear in the initial stage of uniaxial compression for the specimens in the three working conditions, accounting for 17.12%, 19.87% and 21.43% of the cumulative number of acoustic emission events, respectively, and the range of maximum amplitude values has been extended to 40-90 dB (Figure 6d-f). This means that at a high temperature of 250 • C, when 10 ≤ N < 15, the number of microcracks inside the specimen under this condition gradually increases, the opening gradually increases and the initial stage of uniaxial compression produces a higher number of acoustic emission events and a maximum amplitude. When N ≥ 15 times, the number of microcracks inside the specimen basically tends to stabilize, and the comparison of the results under the two conditions of N = 15 and N = 20 can be seen (Figure 6e,f), and the distribution patterns of the number of acoustic emission events and maximum amplitude of the two specimens in the initial stage of uniaxial compression are nearly the same.
Under the same temperature condition, the number of treatments has a certain influence on the number of accumulated acoustic emission events during uniaxial compression of the specimen, but temperature often plays a decisive role. At 250, 350 and 450 • C (Figure 7), the cumulative number of acoustic emission events of the specimens during uniaxial compression tended to increase and then decrease, with the highest level of cumulative acoustic emission events of the specimens at 250 • C. Compared with the intact slab granite specimens at 25 • C, the cumulative acoustic emission events of the specimens were almost at the same level for both conditions at a high temperature of 250 • C when N = 1 and 5 times, with the values of 136.73 × 10 3 and 118.32 × 10 3 , respectively. However, when N = 10 times, the cumulative acoustic emission events of the specimens showed a significant increase, and compared with N = 5, the cumulative acoustic emission events increased from 118.32 × 10 3 to 292.03 × 10 3 , an increase of 146.81%. When N = 15 and 20 times, the cumulative acoustic emission events of the specimens were 312.36 × 103 and 306.28 × 103, respectively, with an increase of 6.97% and 6.88% compared with N = 10 times. A significant decrease in the cumulative number of acoustic emission events was observed at 350 • C and 450 • C (Figure 7c,d), and at 450 • C, when N = 5, 10, 15 and 20 times, the cumulative acoustic emission event numbers of the specimens were 64.23 × 10 3 , 68.96 × 10 3 , 73.50 × 10 3 and 70.50 × 10 3 , respectively. The change of cumulative acoustic emission event numbers with small magnitude, which indicates that after five treatments at this temperature, ductile damage occurs in the mineral composition of the specimen, and the number of acoustic emission events decreases significantly during the loading process, and the damage to the internal structure of the specimen reaches its extreme value at this moment, and the influence of the number of treatments on the damage of the specimen gradually weakens.

Maximum Amplitude Frequency Distribution Characteristics
The maximum amplitude is the maximum amplitude of a single acoustic emission event during the loading rupture of the specimen. For a single acoustic emission event, it is not significant to analyze its maximum amplitude, but it is important to analyze the distribution law of the maximum amplitude of all acoustic emission events to reveal the damage evolution law and differences of the specimen. According to relevant research results, in general, small-scale crack sprouting and expansion during specimen rupture tend to correspond to smaller amplitude values, while the sprouting and expansion of large-scale cracks correspond to larger amplitude values [36].

Maximum Amplitude Frequency Distribution Characteristics
The maximum amplitude is the maximum amplitude of a single acoustic emission event during the loading rupture of the specimen. For a single acoustic emission event, it is not significant to analyze its maximum amplitude, but it is important to analyze the  Figure 8 shows the frequency distribution of the maximum amplitude values of acoustic emission during uniaxial compression of the treated granite specimens containing single fracture slabs. As can be seen from Figure 8, more than 50% of the maximum acoustic emission values of the specimens at the temperature levels of 25 • C, 250 • C and 350 • C are distributed in the 40-50 dB range, indicating that when T ≤ 350 • C, multiple cold and heat treatments only lead to small-scale cracks inside the specimens. However, when T = 450 • C, the maximum amplitude of acoustic emission during uniaxial compression of the specimens tends to increase. Figure 8a shows that the average percentage of the frequency of the maximum amplitude of acoustic emission in the 40-50 dB interval for the two-room temperature specimens during uniaxial compression is 65.66%, distributed in the 50-60 dB, 60-70 dB, 70-80 dB and 80-90 dB intervals with the average percentages of 25.42%, 7.82%, 1.10% and 0, respectively. When T = 250 • C, 350 • C and 450 • C, the frequency distribution of the maximum amplitude of acoustic emission of the specimens was 61.44%, 52.24% and 45.90% in the 40-50 dB range, with a decreasing trend. The frequency distribution was 25.06% in the 50-60 dB range, with a slight increase at 450 • C. The average percentages of the frequency distribution in the 50-60 dB range were 25.06%, 20.66% and 28.55%, which increased slightly under the high temperature of 450 • C. The average percentages of frequency distribution in the 60-70 dB and 70-80 dB ranges increased significantly, and the average percentages in the 80-90 dB range showed an increasing trend, as shown in Figure 8b-d. The frequency distribution pattern of the maximum amplitude of acoustic emission from the specimen can be seen that the specimen has been subjected to hot and cold shock at 450 • C, and large-scale fracture damage has occurred inside. 70-80 dB and 80-90 dB intervals with the average percentages of 25.42%, 7.82%, 1.10% and 0, respectively. When T = 250 °C, 350 °C and 450 °C, the frequency distribution of the maximum amplitude of acoustic emission of the specimens was 61.44%, 52.24% and 45.90% in the 40-50 dB range, with a decreasing trend. The frequency distribution was 25.06% in the 50-60 dB range, with a slight increase at 450 °C. The average percentages of the frequency distribution in the 50-60 dB range were 25.06%, 20.66% and 28.55%, which increased slightly under the high temperature of 450 °C. The average percentages of frequency distribution in the 60-70 dB and 70-80 dB ranges increased significantly, and the average percentages in the 80-90 dB range showed an increasing trend, as shown in Figure  8b-d. The frequency distribution pattern of the maximum amplitude of acoustic emission from the specimen can be seen that the specimen has been subjected to hot and cold shock at 450 °C, and large-scale fracture damage has occurred inside.

Acoustic Emission Maximum Energy Analysis
Acoustic emission energy signals can reflect the information of the rock deformation damage process timely and accurately, so they are widely used in the health monitoring of geotechnical structures. Acoustic emission energy values are used by most scholars for

Acoustic Emission Maximum Energy Analysis
Acoustic emission energy signals can reflect the information of the rock deformation damage process timely and accurately, so they are widely used in the health monitoring of geotechnical structures. Acoustic emission energy values are used by most scholars for quantitative evaluation studies of rock damage because they are less dependent on the voltage threshold and operating frequency during the test period and can better reflect the rock rupture damage process [37][38][39]. In this paper, the effects of T and N on the degree of damage to the specimens were analyzed separately by the regular variation of the maximum energy value of the acoustic emission near the peak stress point, and at all three setting temperatures, the maximum energy value of acoustic emission during specimen loading tended to decrease with increasing N.
When T = 250 • C, the maximum energy value of acoustic emission during specimen loading decreased from 7.38 × 10 4 mv·µs to 6.96 × 10 4 mv·µs when the specimen experienced N = 5 cold and hot shocks compared with N = 1, with a reduction of only 5.67% as shown in Figure 9a. The maximum energy value decreased to 4.36 × 10 4 mv·s when undergoing 10 treatments, a decrease of 37.36%. When N = 15 and 20 times, the maximum energy value during the loading of the specimen decreased substantially and consistently to 4.32 × 104 mv·µs and 4.26 × 104 mv·µs, with a decrease of 41.46% and 42.28%, respectively. It can be seen that when the specimen undergoes N = 10~20 treatments at 250 • C, the degree of damage to the specimen changes less, and its damage value basically tends to a constant value. When T = 350 • C and 450 • C, the changes in the maximum energy values of acoustic emission were basically the same as those at 250 • C, and the changes tended to decrease as an approximately exponential function (Figure 9b,c). of damage to the specimens were analyzed separately by the regular variation of the maximum energy value of the acoustic emission near the peak stress point, and at all three setting temperatures, the maximum energy value of acoustic emission during specimen loading tended to decrease with increasing N.
When T = 250 °C, the maximum energy value of acoustic emission during specimen loading decreased from 7.38 × 10 4 mv·μs to 6.96 × 10 4 mv·μs when the specimen experienced N = 5 cold and hot shocks compared with N = 1, with a reduction of only 5.67% as shown in Figure 9a. The maximum energy value decreased to 4.36 × 10 4 mv·s when undergoing 10 treatments, a decrease of 37.36%. When N = 15 and 20 times, the maximum energy value during the loading of the specimen decreased substantially and consistently to 4.32 × 104 mv·μs and 4.26 × 104 mv·μs, with a decrease of 41.46% and 42.28%, respectively. It can be seen that when the specimen undergoes N = 10~20 treatments at 250 °C, the degree of damage to the specimen changes less, and its damage value basically tends to a constant value. When T = 350 °C and 450 °C, the changes in the maximum energy values of acoustic emission were basically the same as those at 250 °C, and the changes tended to decrease as an approximately exponential function (Figure 9b,c). Among the existing research results, no unified, representative or widely accepted theory has been obtained on the correspondence between the basic mechanical parameters and physical signal response during loading damage to brittle materials, which largely restricts the scope of application of acoustic emission technology. At present, the study on the quantitative relationship between acoustic emission characteristic parameters and basic mechanical parameters is still a hot and difficult research area. Therefore, based on the existing studies, this paper further explores the variation law of basic macromechanical parameters with the maximum energy value of acoustic emission to provide a theoretical reference for the safe development of enhanced geothermal energy using geophysical methods. Figure 10 shows that when the maximum energy value of acoustic emission is within a certain range, the basic macroscopic mechanical parameters of the specimen show an increasing trend with the increase in its value. At T = 250 • C, when the energy value of the acoustic emission of the specimen was in the range of 4.75 × 10 4 mv·µs~7.00 × 10 4 mv·µs, the peak intensity and peak strain of the specimen were positively correlated with the maximum energy, and the average elastic modulus was basically a certain value (E = 25.05 GPa in this study), (Figure 10a). When T = 350 • C, the energy value of the acoustic emission of the specimen was in the range of 4.00 × 10 4 mv·µs~6.50 × 10 4 mv·µs, the peak intensity, peak strain and average elastic modulus of the specimen were positively correlated with the maximum energy, where the average elastic modulus increased more (Figure 10b). At T = 450 • C, when its value was in the range of 2.36 × 10 4 mv·µs~6.83 × 10 4 mv·µs, the peak intensity and peak strain of the specimen were positively correlated with the maximum energy, and the average modulus of elasticity increased and then decreased with the increase of the maximum energy value (Figure 10c). restricts the scope of application of acoustic emission technology. At present, the study on the quantitative relationship between acoustic emission characteristic parameters and basic mechanical parameters is still a hot and difficult research area. Therefore, based on the existing studies, this paper further explores the variation law of basic macromechanical parameters with the maximum energy value of acoustic emission to provide a theoretical reference for the safe development of enhanced geothermal energy using geophysical methods. Figure 10 shows that when the maximum energy value of acoustic emission is within a certain range, the basic macroscopic mechanical parameters of the specimen show an increasing trend with the increase in its value. At T = 250 °C, when the energy value of the acoustic emission of the specimen was in the range of 4.75 × 10 4 mv·μs ~ 7.00 × 10 4 mv·μs, the peak intensity and peak strain of the specimen were positively correlated with the maximum energy, and the average elastic modulus was basically a certain value (E = 25.05 GPa in this study), (Figure 10a). When T = 350 °C, the energy value of the acoustic emission of the specimen was in the range of 4.00 × 10 4 mv·μs ~6.50 × 10 4 mv·μs, the peak intensity, peak strain and average elastic modulus of the specimen were positively correlated with the maximum energy, where the average elastic modulus increased more (Figure 10b). At T = 450 °C, when its value was in the range of 2.36 × 10 4 mv·μs ~ 6.83 × 10 4 mv·μs, the peak intensity and peak strain of the specimen were positively correlated with the maximum energy, and the average modulus of elasticity increased and then decreased with the increase of the maximum energy value (Figure 10c).

Discussion
Acoustic emission events are mainly energy releases caused by internal crack sprouting and expansion of brittle materials as well as the closing friction of the original cracks [40][41][42], and typical acoustic emission events can reflect the damage characteristics of materials in real-time. Therefore, in order to study the fracture evolution characteristics of granite slab specimens containing fractures during uniaxial compression after repeated hightemperature water cooling, acoustic emission monitoring data and high-speed digital photographic techniques can be synthesized to determine the mechanical boundary conditions during crack sprouting, expansion and penetration damage of the specimens. For the reason of space, only the characteristics of acoustic emission events during typical rupture of specimens undergoing 1, 5, 10, 15 and 20 cold and hot treatments at 250 • C were analyzed, and the effects of the number of cold and hot treatments and fractures on the rupture evolution of slab granite were investigated by combining high-speed digital photographic images, as shown in Figure 11.
The stress-strain conditions at initial crack initiation (A), crack extension (B), penetration (C), and complete damage (D) of the specimen can be determined by acoustic emission event characteristics in combination with digital photography. As shown in Figure 10a Table 2) showed that the stresses at the moments of crack initiation, crack extension, penetration and damage to the specimens all tended to decrease with the increase of N.
The number of cold and hot shock treatments N increases at 250 • C, and the specimen transforms from the brittle damage to the plastic damage. After one and five cold and hot treatments, the specimens have obvious brittle cleavage damage characteristics, and a large amount of elastic energy gathered inside the specimen is suddenly released at complete damage, leading to an instantaneous closure of the intermediate cracks, as shown in Figure 11a,b. When experiencing 10, 15 and 20 cold and hot treatments, the specimens gradually transformed from the splitting damage to the plastic damage, and the elastic energy released when the specimens were completely damaged gradually decreased, and the presence of cracks had no effect on the damage characteristics of the specimens, as shown in Figure 11c-e. [40][41][42], and typical acoustic emission events can reflect the damage characteristics of materials in real-time. Therefore, in order to study the fracture evolution characteristics of granite slab specimens containing fractures during uniaxial compression after repeated high-temperature water cooling, acoustic emission monitoring data and high-speed digital photographic techniques can be synthesized to determine the mechanical boundary conditions during crack sprouting, expansion and penetration damage of the specimens. For the reason of space, only the characteristics of acoustic emission events during typical rupture of specimens undergoing 1, 5, 10, 15 and 20 cold and hot treatments at 250 °C were analyzed, and the effects of the number of cold and hot treatments and fractures on the rupture evolution of slab granite were investigated by combining high-speed digital photographic images, as shown in Figure 11.
The stress-strain conditions at initial crack initiation (A), crack extension (B), penetration (C), and complete damage (D) of the specimen can be determined by acoustic emission event characteristics in combination with digital photography. As shown in Figure  10a, the stresses corresponding to A, B, C and D during uniaxial compression of the specimens after experiencing one cold and hot treatment are 174.43 MPa, 177.17 MPa, 180.65 MPa and 177.52 MPa; the strains at the corresponding moments are 9.32 × 10 −3 , 9.48 × 10 −3 , 9.65 × 10 −3 and 9.74 × 10 −3 . The stress-strain statistics of the four rupture characteristic points A, B, C and D during uniaxial compression of the specimens after 5, 10, 15 and 20 cold and hot treatments ( Table 2) showed that the stresses at the moments of crack initiation, crack extension, penetration and damage to the specimens all tended to decrease with the increase of N.
The number of cold and hot shock treatments N increases at 250 °C, and the specimen transforms from the brittle damage to the plastic damage. After one and five cold and hot treatments, the specimens have obvious brittle cleavage damage characteristics, and a large amount of elastic energy gathered inside the specimen is suddenly released at complete damage, leading to an instantaneous closure of the intermediate cracks, as shown in Figure 11a,b. When experiencing 10, 15 and 20 cold and hot treatments, the specimens gradually transformed from the splitting damage to the plastic damage , and the elastic energy released when the specimens were completely damaged gradually decreased, and the presence of cracks had no effect on the damage characteristics of the specimens, as shown in Figure 11c

Conclusions
In this paper, we analyzed the changes in acoustic emission characteristics during uniaxial compression of slab granite specimens containing single fissures after repeated cold and hot treatments at different temperatures, and explained the effect law of cold and hot impacts on such specimens, and came to the following conclusions: (1) Compared with the specimens at room temperature, after 1, 5, 10, 15 and 20 treatments at 250 • C, the decreases in peak strength were 2.21%, 7.37%, 8.43%, 9.67% and 15.74%, respectively; the decreases in average elastic modulus were 2.86%, 8.93%, 7.76%, 9.92% and 13.39%, respectively. When treated one and five times, respectively, there was no significant change in the degree of damage inside the specimens. The overall acoustic emission characteristics of the specimen in the initial stage of uniaxial compression tend to decrease (at N = 1 and 5 times), then increase (at N = 10 and 15 times), and then decrease (at N = 20 times) as the number of treatments increases.
(2) At the three high temperatures, the frequency distribution of the maximum amplitude values of the specimens showed significant changes compared with room temperature, and the average frequency in the 40-50 dB interval of the distribution tended to decrease as the number of treatments increased. Under the two high temperatures of 250 • C and 350 • C, the average frequency of distribution in the interval of 50-60 dB does not change significantly, but increases significantly at 450 • C; the average frequency of distribution in the intervals of 60-70 dB and 70-80 dB increases significantly; the average frequency of distribution in the interval of 80-90 dB shows an increasing trend.
(3) After the specimen undergoes one and five treatments at a high temperature of 250 • C, the maximum energy value of acoustic emission changes less, and its value decreases from 7.38 × 10 4 mv·µs to 6.96 × 10 4 mv·µs, with a decrease of only 5.67%; when N = 10, the energy value of acoustic emission decreases significantly to 4.36 × 10 4 mv·µs, with a decrease of 37.36%. However, when N = 15 and 20 times, the maximum acoustic emission energy values did not show significant changes compared with N = 10-timestreated specimens. The changes in the maximum acoustic emission energy values of the specimens at 350 • C and 450 • C were basically the same as those at 250 • C, and the changes tended to decrease as an approximately exponential function.
(4) The overall stress values at the moment of crack initiation, extension, penetration and damage to the specimen during uniaxial compression tend to decrease with the increase in the number of treatments. The specimens were converted from brittle to plastic damage mode with the increase of the number of treatments at 250 • C. The instantaneous closure of the prefabricated single fissures in the middle of the specimens was observed at N = 1 and 5. The presence of fissures had no significant effect on the damage characteristics of the specimens at N = 10, 15 and 20.