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Article

Experimental Study on Failure Characteristics and Energy Release Evolution of Coal Under Microwave Irradiation

1
School of Mechanics and Engineering, Liaoning Technical University, Fuxin 123000, China
2
Shanghai Datun Energy Co., Ltd., Xuzhou 221611, China
3
School of Geology and Mining Engineering, Xinjiang University, Urumqi 830000, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(17), 9522; https://doi.org/10.3390/app15179522 (registering DOI)
Submission received: 11 July 2025 / Revised: 15 August 2025 / Accepted: 22 August 2025 / Published: 29 August 2025
(This article belongs to the Section Applied Physics General)

Abstract

In order to reveal the failure characteristics and burst tendency of coal after microwave irradiation, the microstructure damage effect of microwave irradiation on coal was explored. The microstructure damage, burst tendency and acoustic emission energy characteristics of coal samples before and after microwave irradiation were quantitatively evaluated, and the mechanisms behind porosity growth and the weakening effect of microwave irradiation were revealed. The results show that the damage amount DE of coal samples after microwave irradiation is 0.018, the crack damage amount DP is 0.015, and the crack damage amount accounts for 83.3% of the damage of coal samples. It is determined that the damage-weakening effect of microwave-irradiated coal is affected by the increase in cracks. The monitoring data show that the acoustic emission signals generated by its development are concentrated in the crack compaction stage and elastic stage, the energy dissipated by crack failure is proportional to the number of cracks, and the ability of coal samples to accumulate energy is inversely proportional to the number of cracks. After microwave irradiation, the mechanical properties of coal samples are weakened, the uniaxial compressive strength index is reduced by 67.66%, and the burst energy index is reduced by 66.67%, indicating that microwave irradiation can effectively reduce the bursting tendency of coal.

1. Introduction

With the increasing mining depths and production of coal mines in China, the frequency and intensity of rock burst as a dynamic disaster phenomenon are also increasing, which restricts the safe and efficient mining of coal mines [1,2,3,4]. Based on the main influencing factors and disaster mechanisms of rock burst, some scholars have put forward the engineering routes of rock burst prevention and control of energy reduction, energy release and resistance [5,6,7]. Hydraulic56 fracturing, hydraulic slotting, blasting stress relief, coal seam water injection, large-diameter borehole stress relief and other coal and rock mass stress relief methods have been proposed [8,9,10,11,12]. The above stress relief methods have achieved good effects and can effectively control the rock burst risks of the mine.
However, these methods have limitations or face environmental protection challenges in terms of stress relief effectiveness, timeliness and the waste of water resources. For example, drilling stress relief and hydraulic slotting create passive changes in the mechanical properties of coal. When the rock burst coal seam is hard (f ≥ 3), coal seam water injection and coal body blasting are applied to weaken the strength of the coal body. However, coal seam water injection takes a long time, and, while coal body blasting can relieve stress in time, its process is complex, there is the possibility of dud blasting, and blasting itself may cause rock burst.
Since the 1990s, numerous studies have been conducted on the weakening effects of rocks under microwave irradiation. Kingman et al. [13,14] found that, after irradiating copper ore with 5 kW of microwaves for 1 s, the bond work index decreased by 33%; meanwhile, after irradiating it with 10 kW of microwaves for 0.5 s, the bond work index decreased by 74%. Kingman et al. irradiated different types of ores with 2.6 kW of microwaves for 4 min and found that the bond work index of the ores significantly decreased. Nejati et al. [15] irradiated basalt samples with 1 kW and 2 kW of microwaves for 30 s. No significant changes were observed in the appearance of the samples after 1 kW microwave irradiation, while three macroscopic cracks appeared on the surfaces of the samples after 2 kW microwave irradiation.
Therefore, it is very important to find a coal seam stress relief technology with better applicability. In order to solve the above problems, microwave rock-breaking technology has been gradually applied to the coal mine. It has the properties of cleanliness, high efficiency and strong penetration, and the minerals contained in the rock have good compatibility with microwave technology and a high absorption rate [16,17,18]. For example, Hu et al. [19,20] conducted microwave heating on strongly impact-prone coal samples under various combinations of power (0.45 kW~1.8 kW) and time (30 s~480 s) and found that high-power, short-duration microwave combinations were more effective in reducing the impact tendencies of coal bodies. Wen et al. [21] discovered that microwave heating primarily promoted the generation and expansion of microcracks in coal samples under the influence of microwave power of 1 kW and a heating time of 120 s. Hassani et al. [22] found that microwave irradiation technology can promote the formation of new cracks in coal and effectively reduce its strength, so this technology has been gradually applied in coal mining.
Some scholars have carried out corresponding research on microwave weakening in coal fracture development. The research results show that microwave thermal stress acting on the coal and rock mass will lead to the expansion or formation of new cracks on its surface and internal primary cracks, promote pore development to form a fracture network and increase the number of microcracks in the coal and rock mass [23,24,25,26]. The evolution law of mineral pores under microwave irradiation was analyzed by means of experiments and numerical simulations. It was found that the pyrolysis of some minerals in the coal and rock after microwave irradiation led to the development of pores, which is the fundamental reason that microwaves could be used to assist ore crushing and grinding [27,28]. Scholars have also applied microwave irradiation to rocks such as granite, sandstone and basalt and found that the development degree of microcracks is proportional to the damage and deterioration characteristics of rocks. The influence on the mechanical properties of rocks is mainly caused by intergranular pore expansion and the internal fracture of particles [29,30,31].
In addition, some scholars have studied the energy evolution characteristics of the coal and rock mass failure process after microwave weakening and fracture development. Qi et al. [32] explored the changes in the physical and mechanical properties of coal and the evolution characteristics of the failure deformation energy by controlling the microwave power. It was found that the physical and mechanical properties of coal samples treated by microwave irradiation generally showed a downward trend, and the accumulation of elastic energy was weakened. By analyzing the physical and mechanical properties of coal samples before and after the microwave irradiation of oil-rich coal with different water content, it was found that the wave velocity of microwave-treated coal decreased, and the energy accumulation and dissipation capacity were further weakened. Tang et al. [33] demonstrated the possibility of microwave action regarding the prevention and control of rock burst disasters by controlling the microwave irradiation time, and they found that the proportion of dissipated energy increased after microwave treatment, and the energy-based brittleness index decreased. Meanwhile, in [34,35], the authors compared and analyzed the physical and mechanical properties of basalt before and after microwave irradiation and found that microwaves can weaken the rock strength and effectively reduce the intensity of rock burst.
At present, research on microwave weakening in coal and rock mass technology mostly focuses on the evolution characteristics of the pore structure, the formation mechanisms of cracks and the energy evolution characteristics of the coal and rock mass under different conditions. However, the energy release characteristics of the whole process of coal failure have not been clarified, and the corresponding relationship between pore development and the energy evolution of coal samples after microwave is unclear. Therefore, in this paper, coal samples before and after microwave irradiation are taken as the research objects. Based on the porosity measured by nuclear magnetic resonance, the damage index for the evaluation of the porosity development degree is established. The influence of the porosity development degree on the mechanical index of the coal burst tendency and the characteristics of the acoustic emission energy is analyzed. The relationship between the damage amount, mechanical properties and energy is also explored. The purpose is to clarify the evolution mechanism of energy accumulation and the dispersion of coal compression failure after microwave irradiation and to provide a basis for the application of microwave irradiation coal technology and the selection of reasonable parameters for the prevention and control of rock burst disasters.

2. Test Methods

2.1. Coal Sample Preparation

The coal samples used in this work consisted of X5 coal from Kuqa, Xinjiang, China. The lump coal was sent to the laboratory within 24 h after the packaging of the underground preservative film, and the coal sample size was 50 mm × 50 mm × 100 mm. Regarding the cube specimen, the standard process of sample preparation follows the ‘Methods for determining the physical and mechanical properties of coal and rock’ [36].

2.2. Test Scheme

2.2.1. Experimental Equipment

The experimental equipment described in this paper mainly includes microwave equipment, nuclear magnetic resonance test equipment and pressure test equipment.
Microwave equipment: A Xinhang commercial microwave oven (Henan Xinhang Microwave Technology Co., Ltd, Henan, China)was used as the microwave generator (Figure 1b), with a frequency of 2.45 GHz (the most commonly used frequencies for microwave heating equipment are 915 MHz and 2.45 GHz) [25] and power of 0 kW–2 kW, which could continuously and stably irradiate the specimen.
Nuclear magnetic resonance test equipment: The MesoMR23-060H nuclear magnetic resonance analysis imaging system (Shanghai Neway Electronic Technology Co., Ltd, Shanghai, China) was used (Figure 1c).
Pressure test equipment: This included three parts—a pressure loading system, an acoustic emission monitoring system and a high-speed camera. The mechanical loading system adopted the E45.305 static electro-hydraulic servo testing machine (Metes Industrial Systems Co., Ltd, Shenzhen, China) produced by MTS (Material Testing System) in the United States (Figure 1d).
Acoustic emission monitoring system: The DS5-8A (8-channel) acoustic emission instrument host (Beijing Soft Island Era Technology Co., Ltd, Beijing, China) (Figure 1e) produced by Beijing Soft Island Times Technology Co., Ltd. was used to collect acoustic emission (AE) transient waveforms in real time. It has full-waveform acquisition and processing and real-time AE positioning functions. In the experiment, eight R15 sensors were used to collect AE signals. The operating frequency was 100~400 kHz, and the sensor layout is shown in Figure 2.
High-speed camera: The Japanese NAC high-speed camera GX-3 (NAC Corporation of Japan, Tokyo, Japan) was used. It has ultra-high photosensitivity, 21.7 μm pixels and an excellent dynamic range. The shooting speed can reach 198,000 frames per second. The number of frames in this experiment was 4000, and the resolution was 1024 × 768.

2.2.2. Experimental Procedure

In order to explore the mechanisms and characteristics of coal energy accumulation and dispersion, mechanical loading tests were carried out on coal samples before and after microwave irradiation. The experimental procedure was as follows (see Figure 1).
(1) The coal sample was dried in a 110 °C vacuum drying oven for 24 h and then taken out. The coal sample was left to stand for 24 h to ensure the complete cooling of its internal temperature, and it was weighed in a dryer. After two consecutive dryings, the coal sample was considered to be dry when the mass reduction was not more than 2%.
(2) Then, the preparation of microwaved coal samples and non-microwaved coal samples was performed. The microwave irradiation procedure involved placing the standard specimen in the microwave generator and irradiating it for 120 s with 1 kW power. The selection of these microwave parameters was based on the research of Wen et al. [23]. When using these parameters to microwave coal samples, the coal samples will not produce obvious incandescence phenomena, and the samples themselves will not spontaneously ignite. In order to ensure that the microwave electromagnetic field distribution of each sample was the same under the same microwave irradiation conditions, the coal sample was placed in the same position in the microwave generator. The non-microwaved coal sample was used as the natural coal sample control group without microwave treatment. The coal samples without microwave treatment and after microwave treatment were marked as CGN and CGY, respectively. Three samples were selected from each group of five for detailed analysis, with the selection intended to exclude samples with incomplete acoustic emission signals or abnormal stress values to ensure the representativeness and reliability of the data.
(3) Nuclear magnetic resonance (NMR) was used to assess the coal sample before and after microwave irradiation, and the porosity of the coal samples before and after microwave irradiation was measured. The two groups were labeled HC1 and HC2, respectively.
(4) Then, the uniaxial compression tests of coal samples after microwave treatment and coal samples without microwave treatment were performed. The loading process of the coal samples was synchronized with acoustic emission monitoring and a high-speed camera.

2.3. Nuclear Magnetic Resonance Principles

The nuclear magnetic resonance (NMR) test is based on the magnetic field of the object, the energy released by the external radio frequency field, the echo attenuation signals in different structures inside the material and the electromagnetic waves generated by the external gradient in magnetic field detection. The structure of the object can be drawn [37], and this can be used to analyze the defects or damage inside the material and its pore size distribution.
When the nucleus is superimposed with the radio frequency field, the energy level transition of the absorbed energy is known as nuclear magnetic resonance; after the action of the radio frequency field, the process of restoring the equilibrium state of the nucleus occurs, which is called relaxation [37]. In a porous medium, the pore structure is characterized by the inversion of the transverse relaxation time (T2) of the confined fluid. The pore size is different, and the relaxation time of the pore water in the transverse direction is also different. Therefore, the pore size distribution of the rock can be obtained according to the inverted T2 spectrum [38]. The relationship between T2 and the pore geometry parameters is expressed according to Equation (1):
1 T 2 = ρ 2 S V p o r e
In the formula, T2 is the transverse relaxation time, ρ2 is the surface relaxation rate and (S/V) pore is the specific surface area of the pore. In a porous medium, the larger the pores, the longer the relaxation time of water in the pores. The smaller the pores, the greater the binding degree of water in the pores and the shorter the relaxation time. In other words, the position of the peak in the relaxation time curve is related to the pore size, and the area of the peak is related to the pore proportion of the corresponding pore size.

2.4. Microwave Irradiation Principles

A microwave is a type of high-frequency electromagnetic wave whose electromagnetic spectrum is between those of a red wave and radio wave. The frequency range is 300 MHz–300 GHz, and the wavelength range is 1 mm–100 cm [39]. Microwave irradiation has the characteristics of selectivity, integrity and high efficiency [40]. At present, the application of microwave irradiation technology in coal and rock masses mostly relies on its selectivity. Rock materials with different dielectric properties have different absorption degrees of microwave energy [41]. Thus, microwave irradiation is selected to selectively crack and weaken the coal and rock mass.

2.5. Numerical Simulation of Microwave Irradiation

In order to further investigate the thermal stress distribution of coal samples under microwave action, this study used the COMSOL 6.0 software to perform electromagnetic thermal coupling calculations on the coal samples. We obtained the overall temperature distribution, temperature gradient and electric field mode distribution of the coal samples under microwave irradiation.

2.5.1. Geometric Model Establishment and Simulation Condition Setting

Firstly, a three-dimensional geometric model of the microwave generator was established, with the following specific dimensions: width 315 mm, depth 345 mm and height 230 mm. The dimensions of the coal body were based on the dimensions of the coal specimen, with a diameter of 50 mm and a height of 100 mm. The specific geometric model is shown in Figure 3.
In order to avoid the influence of moisture and explore the impact of coal pores on thermal stress and energy, this study set the internal pores of the coal as air in the numerical simulations. The simulation process adopted the electric thermal force multi-field unidirectional coupling method: first, the electric field is solved in the frequency domain; then, the transient temperature field is solved based on the distribution of the electric field strength; finally, the thermal stress and ideal gas pressure are calculated to obtain the stress field distribution and pore deformation characteristics. The pressure in the pore area is calculated using the ideal gas equation and acts as a pressure load on the pore wall. In the simulation, the electric field of microwave irradiation was set as a 2.45 GHz waveguide input with power of 1 kW, using a copper material cavity and waveguide, and the bottom limit of the sample was set to allow only upward deformation.

2.5.2. Characteristics of Temperature Gradient Distribution

In Figure 4, it can be seen that the overall temperature gradient inside the coal body after microwave irradiation shows a relatively uniform distribution. As a whole heating method, wave irradiation can easily lead to temperature gradients inside coal samples, resulting in an increase in the extreme values of the thermal stress distribution. Due to the difference in internal thermal stress, the porosity of the coal sample will increase, forming new pores. When there are primary pores in the coal sample, this will further intensify the temperature gradient, causing more drastic temperature changes around the pores and facilitating the generation of thermal stress differences, resulting in a weaker coal matrix near the pores. This renders these areas more susceptible to damage and allows the pores to further develop and extend. In addition, when the distance between pores is relatively close, whether during the heating process of microwave irradiation or under the load compression conditions after microwave treatment, these pores are more easily interconnected and further develop and extend, thereby forming larger-scale cracks.

3. Experimental Results and Analysis

3.1. Porosity Variation Characteristics Before and After Microwave Irradiation

3.1.1. T2 Spectrum Analysis

The change in the nuclear magnetic resonance T2 spectrum curve reflects the change in the pore structure in the coal before and after microwave treatment. According to the different spectral peaks presented in the nuclear magnetic resonance T2 spectrum curve, three types of pore cracks, namely micropores, mesopores and macropores, can be identified [42]. Therefore, the peak height, peak position and smoothness between the peaks of the T2 curve represent the number, scale and connectivity of coal pores before and after microwave irradiation, respectively. The NMR T2 curves of the coal samples before and after microwave irradiation are shown in Figure 5.
According to the relaxation time, ranging from small to large, the distribution of micropores, mesopores and macropores in the coal samples in this experiment can be identified in turn: the peak value of micropores is distributed within T2 = 0.01~1 ms; the peak value of mesopores is distributed within T2 = 1~100 ms; and the peak value of macropores is distributed within T2 = 100~10,000 ms. The trends of the T2 spectrum curves of the same groups of coal samples are roughly the same, among which the peak values of mesopores and macropores are similar, and they are lower than the peak values of micropores. However, the peak values of the T2 curves of different groups of coal samples (coal samples before and after microwave treatment) have obvious changes. Compared with non-microwaved coal samples, the porosity of the coal samples increased from 1.63% to 3.00% after microwave irradiation, indicating that the internal pore structures of the coal samples changed under microwave irradiation. Moreover, pores of various sizes were developed to varying degrees, resulting in an increase in number. The stationarity of the inter-peak curve of the T2 spectrum can reflect the connectivity between pores [19]. The corresponding peaks of micropores, mesopores and macropores in the diagram are relatively independent, indicating that the connectivity between coal pores before and after microwave is poor.

3.1.2. Pore Damage Analysis

The change in pore structure will inevitably cause different degrees of damage and deterioration in the strength of the sample. Considering that the change in pore volume is entirely caused by microwave irradiation damage, ignoring the mutual damage between the internal pores of coal samples during microwave irradiation [43], this paper proposes the pore damage index D p of coal samples, which reflects the ratio of the pore volume change to the unit volume before and after microwave irradiation, so as to quantitatively evaluate the degree of pore development, as shown in Equation (2):
D p = V W V T V 0
In the formula, D p is the amount of pore damage of coal samples caused by microwave irradiation; V T is the pore volume of a non-microwaved coal sample; V W is the pore volume of a coal sample after microwave treatment; and V 0 is the sample volume.
According to the nuclear magnetic resonance test, the changes in the pore volume of the coal samples before and after microwave are as shown in Table 1. The pore structure parameters measured in the test are substituted into Equation (2), and the pore damage D p is 0.013 and 0.017, respectively. The average pore damage value is 0.015, indicating that microwave irradiation has a damage-weakening effect on coal samples.

3.2. Analysis of Damage Quantity of Coal Samples

In order to characterize the weakening effect of porosity changes on coal strength, according to the commonly used damage variable definition method in damage mechanics [44], combined with the literature [45], the elastic energy change can be used to define the initial damage amounts of coal samples. The calculation method is shown in Equation (3):
D = U d / U
In the formula, D is the initial damage amount of the coal sample; U d is the dissipated energy consumed by pore compaction and friction between pores in the uniaxial compression test; and U is the total input energy of the uniaxial compression test.
The total stress–strain curve of uniaxial compression can be divided into five stages according to the development process of cracks [46]: the crack compaction stage, the elastic stage, the stable development stage of microcracks, the unstable development stage and the post-peak stage. When the coal sample enters the stable development stage of a microfracture, the development of the microfracture causes the coal sample to be further damaged and energy is released. In order to avoid errors in the initial damage calculation of the coal sample after microwave irradiation caused by energy release in this stage, this study selects the stage before the stable development of microcracks to calculate the accumulated elastic energy. Therefore, it is necessary to determine the boundary point between the elastic stage and the stable development stage of the microfracture. In this work, the different fracture development stages of the full stress–strain curve are further divided by incorporating the characteristics of the acoustic emission ringing number curve.
Taking the boundary points of different fracture development stages as the characteristic threshold point [47], the process is divided into the stress point of crack closure λ c c , stress point of crack development λ c i , stress point of unstable development λ c d and peak stress point λ c f . Among them, the stress point of crack closure λ c c corresponds to the starting point of the smooth section of the cumulative ringing number curve of acoustic emission; the stress point of crack development λ c i corresponds to the end point of the stationary phase of the cumulative ringing number curve of acoustic emission; the stress point of unstable development λ c d corresponds to the obvious rise point of the number of acoustic emission rings after the end of the linear elastic stage; and the peak stress λ c f usually reflects the peak stress point of the stress–strain curve.
Taking the non-microwaved coal sample CGN2 as an example, combined with the full stress–strain curve and the number of acoustic emission rings, the characteristic threshold points are determined, as shown in Figure 6.
The pressure test process of coal and rock masses is an isolated closed system. In this process, there is no heat exchange between the coal sample and the outside. According to the first law of thermodynamics [48,49], the total input energy of the pressure test equipment to the coal sample can be expressed according to Equation (4):
U = U d + U e
In the formula, U is the total input energy for the uniaxial compression test; U d is the dissipated energy consumed in the coal sample by compaction and friction between pores in the uniaxial compression test; and U e is the elastic energy accumulated in the coal sample in the uniaxial compression test.
According to the definitions of each stage of the full stress–strain curve [46], the crack compaction stage and the elastic stage before the stress point of crack development λ c i are taken when calculating the initial damage amount of the coal sample. The energy diagram of the coal sample regarding λ c i is shown in Figure 7.
During the uniaxial compression test, only the vertical force inputs energy to the coal sample. The total input energy U , elastic energy U e and dissipated energy U d of the uniaxial compression test before the stress point of crack development λ c i can be expressed according to Equations (5)–(7), respectively:
U = 0 ε 1 λ 1 d ε
U e = λ 1 2 2 E 0
U d = U U e
In the formulas, we consider the strain value corresponding to the coal sample regarding the stress point of crack development λ c i ; λ 1 is the value of stress point λ c i ; and E 0 is the elastic modulus of the coal sample in the elastic stage.
From Equations (5) to (7), the total input energy, elastic energy and dissipation energy before the stress point of crack development λ c i under the uniaxial compression test can be obtained. We substitute them into Equation (3) to calculate the initial damage amount of the coal sample before and after microwave irradiation. The results are shown in Table 2, indicating the damage of pores in coal samples before and after microwave irradiation. The initial damage increment in coal samples before and after microwave irradiation is the damage caused by the increase in porosity after microwave irradiation. Therefore, the damage amount of a coal sample is defined according to Equation (8):
D E = D 2 D 1
In the formula, D E is the damage amount of the coal sample; D 1 is the initial damage amount of a non-microwaved coal sample; and D 2 is the initial damage amount of a coal sample after microwave treatment.
From Table 2, it can be seen that the initial damage of coal samples increases after microwave irradiation. The initial damage of non-microwaved coal samples amounts to 0.078~0.099, and the average initial damage amounts to 0.089; the initial damage amount of coal samples after microwave treatment is 0.095~0.117, and the average initial damage amounts to 0.106. Substituting these into Equation (8), the damage amount of the coal sample after microwave irradiation under a change in elastic energy is 0.017.
According to Section 3.1, the average pore damage value after microwave irradiation is 0.015, which is compared with the average coal sample damage, as shown in Figure 8.
Comparing the amounts of pore damage and coal sample damage before and after microwave irradiation, the amount of pore damage and the amount of coal sample damage are in the same order of magnitude, and the difference is small. Therefore, the proportion of pore damage can be defined as η , as shown in Equation (9):
η = D P D E × 100 %
The damage amount DE of the coal sample after microwave irradiation is 0.018, and the crack damage amount DP is 0.015. Therefore, η is 88.24%, which means that the average pore damage accounts for 88.24% of the coal sample damage. The analysis shows that the damage-weakening effect of microwave irradiation on the coal body is mainly affected by the increase in porosity.

3.3. Effects of Damage on Mechanical Properties

Microwave irradiation leads to an increase in internal pores in coal samples, resulting in damage to their microstructures, which in turn affects the physical and mechanical properties of coal. In order to further explore the damaging effects of pore growth on the mechanical properties of coal samples, the influence of porosity on the rock burst tendencies of coal samples was quantitatively characterized based on the damage amount.
The rock burst tendency index can directly reflect the physical and mechanical properties of coal. The full stress–strain curves of coal samples obtained by the uniaxial compression test using a static electro-hydraulic servo testing machine are shown in Figure 9, and the trends of the full stress–strain curves of each group of coal samples are similar. The peak variance before and after microwave irradiation is only 1.90 and 0.27. Thus, they can be used for data analysis. The peak and bursting energy indices before and after microwave irradiation are shown in Table 3.
Considering the differences in the sensitivity of coal samples to microwave irradiation, it is difficult to accurately control the failure load within 75%~85% during cyclic loading, resulting in a large calculation error for the elastic deformation energy (WET). In this experiment, the bursting energy index (KE) and uniaxial compressive strength (Rc), which are more reliable, were selected for analysis.
The paired t-test results regarding the uniaxial compressive strength and impact energy indices of coal samples before and after microwave irradiation are as follows: the p-value for CGY group coal samples and CGN group coal samples is 0.0106 (p < 0.05), and the Cohen’s d value is 4.87; the p-value for the impact energy index is 0.0237 (p < 0.05), and the Cohen’s d value is 3.14. This indicates that microwave treatment significantly weakens the uniaxial compressive strength and impact energy of coal samples, and it can effectively reduce the burst tendency of coal. The changes in the rock burst tendency index before and after microwave irradiation under different initial damage amounts are shown in Figure 10. The values of the bursting energy index and uniaxial compressive strength for coal samples after microwave irradiation decrease with an increase in the damage amount.
The initial damage amounts of coal samples without microwave irradiation were 0.078, 0.089, and 0.099, and the initial damage amounts of coal samples after microwave irradiation were 0.095, 0.107, and 0.117. The average initial damage amount of coal samples after microwave irradiation increased from 0.089 to 0.106, and the overall damage amount of coal samples was 0.017. The bursting energy index of the coal samples decreased from 3.51~6.25 to 1.01~2.05 after microwave irradiation, with an average decrease of 66.67%. After microwave irradiation, the uniaxial compressive strength of the coal samples decreased from 7.59~10.95 MPa to 2.26~3.49 MPa, with an average decrease of 67.66%.
In summary, the rock burst tendency of coal samples after microwave irradiation is greatly reduced—that is, with an increase in the damage amount of a coal sample after microwave irradiation, its physical and mechanical properties are greatly weakened.

3.4. Effects of Damage on Energy Characteristics

3.4.1. Variations in Acoustic Emission Parameters in Different Stages

In order to better analyze the energy change characteristics of the coal sample failure process by incorporating acoustic emission event information, the stress–time curve and acoustic emission ringing count are normalized. The stress–time curves and the characteristic threshold points of the coal samples before and after microwave irradiation under uniaxial compression are shown in Figure 11.
Compared with non-microwaved coal samples, the duration of the fracture compaction stage in coal samples after microwave irradiation increased, the duration of the elastic stage and stable fracture development stage of microfractures decreased significantly, the duration of the unstable fracture development stage decreased, and the acoustic emission ringing count for each stage increased. The time spent by the coal samples in each stage before and after microwave irradiation and the ringing counts of acoustic emission are shown in Table 4. The duration of the fracture compaction stage for coal samples after microwave irradiation increased by 15.51% on average. The duration of the elastic stage was reduced by 23.16% on average; the average duration of the stable fracture development stage decreased by 69.69%. The duration of the unstable fracture development stage decreased by 12.26% on average.
After microwave irradiation, the coal sample is affected by pore damage, and the pre-peak ringing count per unit time increases from 0.48 times/s to 0.58 times/s. The cumulative emission ringing count increased by 105.77% on average; in particular, the number of acoustic emission rings in the crack compaction stage increased by 146%, that in the elastic stage increased by 133%, that in the stable fracture development stage of microcracks increased by 109%, and that in the unstable fracture development stage increased by 84%. Microwave irradiation has a great influence on the acoustic emission signal in the crack compaction stage in coal samples. The cumulative ringing count curve changed from showing multiple sudden rises before microwave irradiation to a steady rise after microwave irradiation, indicating that the microfractures in the coal body after microwave irradiation were uniformly broken and penetrated in turn.
Combined with the loading process of the coal samples, the acoustic emission signals are analyzed at every stage. The changes in the acoustic emission energy characteristics of the coal samples before and after microwave irradiation at different stages are shown in Table 4. After microwave irradiation, the average acoustic emission energy of the coal samples increased by 19.68%. Except for the downward trend of the acoustic emission energy in the stable fracture development stage of microfractures, after microwave irradiation, the acoustic emission energy in all stages increased. The average acoustic emission energy in the fracture compaction stage increased by 60.29%, the average acoustic emission energy in the elastic stage increased by 24.53%, the average acoustic emission energy in the stable fracture development stage of microfractures decreased by 38.17%, and the average acoustic emission energy in the unstable fracture development stage increased by 33.79%.

3.4.2. The Evolution Law of Strain Energy in Different Stages

The changes in strain energy (the product of displacement and stress) corresponding to the fracture compaction stage, elastic stage, stable fracture development stage and unstable fracture development stage of coal samples before and after microwave irradiation are shown in Figure 12.
After microwave irradiation, the strain energy of coal samples in the fracture compaction stage showed an upward trend under the action of microwave irradiation, and the strain energy of coal samples in the other stages decreased significantly. In the stage of fracture compaction, the average strain energy of the coal samples before and after microwave irradiation was 1.68 J and 1.98 J, respectively, with an increase of about 17.9%. In the elastic stage, the average strain energy of the coal samples before and after microwave irradiation was 11.73 J and 6.12 J, respectively, being reduced by 47.8%. In the stage of the stable fracture development of microcracks, the average strain energy of the coal samples before and after microwave irradiation was 15.78 J and 2.03 J, respectively, being reduced by 87.1%. In the stable fracture development stage, the average strain energy of the coal samples before and after microwave irradiation was 11.69 J and 3.10 J, respectively, being reduced by 73.5%. In the pre-peak stage, with an increase in coal sample damage, the accumulated strain energy before and after microwave irradiation decreased from 40.9 J to 13.2 J, representing a decrease of 67.7%, and the overall ability to accumulate energy decreased with the increase in coal sample damage.
Through the above analysis, it is found that the damage amount of coal samples after microwave irradiation is proportional to the acoustic emission ringing count and acoustic emission energy—that is, the energy dissipated by pore development increases with the increase in the damage amount of the coal sample, resulting in a decrease in the strain energy accumulated by the sample. The ability to accumulate strain energy in the elastic stage, the stable fracture development stage and the unstable fracture development stage decreases to varying degrees, while that in the fracture compaction stage increases.

3.5. The Influence of Damage on the Macroscopic Fracture Characteristics of Coal Samples

The development of pores and energy release during the fracture process of coal samples under a load can be quantitatively described using fractal theory, enabling us to assess the degree of macroscopic fracture of coal samples under a load. In the experiment, coal sample fragments were screened using sieves with particle sizes of 2, 5, 10, and 20 mm, and the masses of fragments in the corresponding particle size range were weighed. The mass fractions of fragments in different particle size ranges were calculated, as shown in Table 5.
According to reference [13], the relationship between the frequency of fragment mass and the number of fragments can be obtained using the mass Mmax of the largest block, the cumulative number N of fragments with characteristic sizes less than x and the corresponding cumulative mass M(x) of fragments. When the coal body is damaged and the number of small-sized fragments produced increases, the number of large-sized fragments will correspondingly decrease. Meanwhile, coal sample fragments exhibit a fractal distribution, and their fractal characteristics can be obtained using screening statistical methods to measure the fractal dimension. The fractal dimension of coal sample fragments can be calculated based on their mass and diameter according to Equation (10):
ln M ( x ) / M t = ( 3 D ) ln ( x / x m )
In the formula, x is the diameter of the fragment; M(x) is the cumulative mass of fragments smaller than x in size; Mt is the total mass; xm is the maximum fragment diameter; and D is the fractal dimension. The final calculation results are shown in Table 5. Compared to that before microwave irradiation, the degree of fragmentation of the coal after microwave irradiation is higher, and the fractal dimension D is also larger.

4. Discussion

4.1. Microwave-Induced Damage Mechanisms: Porosity Development

In this study, the nuclear magnetic resonance results showed an increase in the porosity of coal after microwave treatment, confirming the pore development characteristics observed by Lin et al. [50] and Rong et al. [25] in coal after microwave treatment. Li et al. [51,52,53,54] investigated the pore evolution characteristics of coal under different microwave times using nuclear magnetic resonance technology and CT scanning technology. They found that the opening effect and pore thinning effect played a major role in the process of microwave-promoted pore development. This study further examined the pore structure evolution of coal after microwave treatment, with a decrease in the proportion of micropores and an increase in the proportion of mesopores and macropores. The increase in proportion was quantified, providing new ideas and data support for subsequent research on the influence of microwaves on the pore structure evolution of coal.

4.2. Coal Damage Index Caused by Changes in Porosity After Microwave Irradiation

Hu et al. [20,21] and Wen et al. [21] found that the impact tendency of coal decreased after microwave treatment. This study first proposed the pore damage DP and coal sample damage increment DE and further quantified the influence of pore damage on coal’s uniaxial compressive strength and impact energy index using the ratio of the average pore damage to the coal sample damage increment.

4.3. Evolution of Coal Cracks and Energy Dissipation Characteristics Under Microwave Irradiation

According to previous research [55,56,57] on the effects of microwave radiation on rock crack propagation, the microwave radiation of coal leads to the formation of new cracks or promotes the development of existing cracks. The difference in energy accumulation before and after microwave irradiation is due to the different forms and spatial distributions of cracks in the coal body. After internal cracks are developed, major cracks affecting the strength of the coal body will be formed [58], which is manifested in the process of uniaxial compression of the rock. A staged rupture [59] indicates that the coal body preferentially penetrates along the direction of defects, such as microcracks or holes, forming a fracture network and causing damage.
On the basis of previous research, this study further found that energy dissipation during coal loading is mainly based on compression (inelastic collision) and friction between pores. After microwave irradiation, the internal squeeze space in the crack compaction stage increases, the plastic deformation increases, and the dissipated energy increases under loading. In the elastic stage, the internal cracks of the coal body are compacted by the load to form an entity, and the influence of the microstructure on its physical and mechanical properties is weakened. Therefore, the degree of energy accumulation in the elastic stage after microwave treatment is reduced, but the difference is small. As the loading process progresses to the fracture stage, cracks appear on the surface of the coal sample, and the accumulated energy inside the coal sample is sharply released, resulting in acoustic emission energy events. There are a large number of low-energy releases caused by microcrack expansion and fracture, the dissipation energy increases, the accumulated energy decreases, and the rock burst tendency weakens.

5. Conclusions

This work establishes a novel quantitative framework. Through the characteristics of changes in the mechanical properties and acoustic emission energy in the uniaxial compression tests of coal samples before and after microwave irradiation, the weakening effect of coal damage on coal with rock burst tendencies was analyzed. The test results and conclusions are as follows:
(1) The crack damage quantity (DP) was determined based on the changes in porosity measured by nuclear magnetic resonance. The results showed that the damage amount DE of the coal sample after microwave irradiation was 0.018, the crack damage amount DP was 0.015, and the crack damage accounted for 83.3% of the damage of the coal sample. The weakening effect of microwave irradiation on coal damage is mainly affected by the increase in porosity.
(2) After microwave irradiation, the physical and mechanical properties of the coal sample weakened, and the uniaxial compressive strength index decreased by 67.66%. The instantaneous energy release decreased, and the burst energy index decreased by 66.67%. The rock burst tendencies of the coal samples decreased.
(3) After microwave irradiation, the average acoustic emission energy of coal samples increased by 19.68%, and the increase is proportional to the damage amount of the coal sample. The strain energy accumulated by coal samples after microwave irradiation is reduced; this is inversely proportional to the damage amount of the coal sample.
Based on the experimental findings, this study recommends specific microwave treatment parameters (e.g., 1 kW power, 120 s irradiation time) for the pretreatment of coal seams in rock burst-prone areas. This approach can significantly reduce the coal’s strength and energy accumulation, providing a practical solution for the mitigation of rock burst risks in deep mining operations. The methodology and data derived from this study can serve as a reference for engineers and practitioners seeking to apply microwave irradiation for rock burst control, with potential applications in both laboratory and field-scale settings. However, due to the sample size of n = 3 in both groups before and after microwave treatment, the applicability of the results may be limited. Future research should consider increasing the sample size to improve the generalizability and reliability of the conclusions. Future research will focus on optimizing the microwave processing technology for coal, including adjusting the microwave power and exposure time, as well as increasing the sample size, to evaluate the long-term impact on coal seam stability. Additionally, we plan to explore the effects of microwave treatment on the mechanical properties, failure mechanisms and energy evolution patterns of coal–rock composite materials.

Author Contributions

C.D. undertook the main research work, including experimental scheme design, experimental data collection and processing, chart drawing, and the initial writing of the paper, as well as coordinating the content framework and the logical organization of the entire text. A.C. provided coal sample sources and coordinated the acquisition of on-site coal mine samples and supported the setting of the microwave test conditions and the organization of on-site coal mine data. Y.W. and W.G. proposed research ideas and technical routes, guided the experimental design and data analysis, coordinated research progress and paper revisions and were responsible for the final draft. H.L. and Y.S. participated in the microwave irradiation experiments and acoustic emission data processing, as well as nuclear magnetic resonance T2 spectrum analysis, and contributed to the writing and revision of some sections of the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This work was sponsored by the Xinjiang Uygur Autonomous Region Key Research and Development Task Special Project (2024B01005-1, 2022B01034).

Acknowledgments

We acknowledge the X5 coal from the EH coal mine. Special thanks are given to the anonymous reviewers for their constructive and valuable comments. We appreciate their guidance despite their busy schedules. We would like to express our gratitude to K.Z., Y.Z. and S.W. for their guidance and assistance in the numerical simulation and statistical analysis of important data in this experiment. Through these works, they validated and supplemented the research results.

Conflicts of Interest

Chuanhong Ding was employed by the company Shanghai Datun Energy Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Experimental steps and corresponding devices. (a) Drying treatment of natural coal samples; (b) microwave generator; (c) low-field nuclear magnetic resonance instrument; (d) MTS (Material Testing System) pressure testing machine; (e) acoustic emission monitoring system.
Figure 1. Experimental steps and corresponding devices. (a) Drying treatment of natural coal samples; (b) microwave generator; (c) low-field nuclear magnetic resonance instrument; (d) MTS (Material Testing System) pressure testing machine; (e) acoustic emission monitoring system.
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Figure 2. Acoustic emission (AE) sensor layout.
Figure 2. Acoustic emission (AE) sensor layout.
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Figure 3. Microwave generator and three-dimensional geometric model of coal sample.
Figure 3. Microwave generator and three-dimensional geometric model of coal sample.
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Figure 4. Distribution characteristics of temperature gradient in coal samples under microwave for 120 s.(a) Electric field mode distribution of coal samples microwaved for 120 s; (b) temperature field distribution of coal samples microwaved for 120 s; (c) temperature gradient distribution of coal samples microwaved for 120 s.
Figure 4. Distribution characteristics of temperature gradient in coal samples under microwave for 120 s.(a) Electric field mode distribution of coal samples microwaved for 120 s; (b) temperature field distribution of coal samples microwaved for 120 s; (c) temperature gradient distribution of coal samples microwaved for 120 s.
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Figure 5. NMR relaxation time T2 spectra of coal samples before and after microwave irradiation.
Figure 5. NMR relaxation time T2 spectra of coal samples before and after microwave irradiation.
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Figure 6. Stress–strain curve and acoustic emission characteristics of coal sample CGN2 under uniaxial compression.
Figure 6. Stress–strain curve and acoustic emission characteristics of coal sample CGN2 under uniaxial compression.
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Figure 7. Energy diagram of stress point of crack development λci.
Figure 7. Energy diagram of stress point of crack development λci.
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Figure 8. Comparison of coal sample damage amounts.
Figure 8. Comparison of coal sample damage amounts.
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Figure 9. Total stress–strain curves of two sets of coal samples (CGN and CHY) before and after microwave irradiation.
Figure 9. Total stress–strain curves of two sets of coal samples (CGN and CHY) before and after microwave irradiation.
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Figure 10. Changes in uniaxial compressive strength (left) and impact energy index (right) with initial damage amount. For samples treated with microwaves, their mechanical properties significantly decrease as the internal damage increases.
Figure 10. Changes in uniaxial compressive strength (left) and impact energy index (right) with initial damage amount. For samples treated with microwaves, their mechanical properties significantly decrease as the internal damage increases.
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Figure 11. Acoustic emission ringing counts of coal samples before and after microwave irradiation. (a) Stress–strain curve and acoustic emission ringing count of CGN1 sample; (b) stress–strain curve and acoustic emission ringing count of CGY1 sample; (c) stress–strain curve and acoustic emission ringing count of CGN2 sample; (d) stress–strain curve and acoustic emission ringing count of CGY2 sample; (e) stress–strain curve and acoustic emission ringing count of CGN3 sample; (f) stress–strain curve and acoustic emission ringing count of CGY3 sample.
Figure 11. Acoustic emission ringing counts of coal samples before and after microwave irradiation. (a) Stress–strain curve and acoustic emission ringing count of CGN1 sample; (b) stress–strain curve and acoustic emission ringing count of CGY1 sample; (c) stress–strain curve and acoustic emission ringing count of CGN2 sample; (d) stress–strain curve and acoustic emission ringing count of CGY2 sample; (e) stress–strain curve and acoustic emission ringing count of CGN3 sample; (f) stress–strain curve and acoustic emission ringing count of CGY3 sample.
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Figure 12. Curves depicting variations in strain energy of coal samples before and after microwave irradiation at different stages.
Figure 12. Curves depicting variations in strain energy of coal samples before and after microwave irradiation at different stages.
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Table 1. Nuclear magnetic pore volumes of coal samples before and after microwave treatment.
Table 1. Nuclear magnetic pore volumes of coal samples before and after microwave treatment.
Type of SampleDebase Saturated SemaphorePore Volume of Coal SampleSample Volume/cm3Porosity /% Pore   Damage   Amount   D p
HC1Before microwave irradiation2227.1280.4029.241.410.017
After microwave irradiation5131.2060.903.03
HC2Before microwave irradiation3022.0120.5431.261.840.013
After microwave irradiation5218.6640.932.97
Table 2. The initial damage amounts of coal samples before and after microwave irradiation.
Table 2. The initial damage amounts of coal samples before and after microwave irradiation.
Microwave ProcessingSample NumberInitial Damage Quantity DAverage Initial Damage D Damage   Amount   D E of Coal Sample
Non-microwaved coal sampleCGN10.078 D 1 : 0.0890.017
CGN20.089
CGN30.099
Coal sample after microwave treatmentCGY10.095 D 2 : 0.106
CGY20.107
CGY30.117
Table 3. Physical and mechanical properties of coal samples before and after microwave irradiation.
Table 3. Physical and mechanical properties of coal samples before and after microwave irradiation.
IndexCGN1CGN2CGN3Average ValueStandard DeviationCGY1CGY2CGY3Average ValueStandard Deviation
Stress peak value/MPa10.958.987.599.171.693.493.152.262.970.64
Bursting energy index6.255.273.515.011.391.952.051.011.670.57
Note: CGN and CGY each have a sample size of n = 3.
Table 4. Coal samples before and after microwave irradiation at different stages, acoustic emission ringing counts and energy characteristics.
Table 4. Coal samples before and after microwave irradiation at different stages, acoustic emission ringing counts and energy characteristics.
Sample NumberFissure Compaction StageElastic StageStable Development Stage of MicrofracturesUnstable Rupture Development Stage
Duration/sCGN141.4979.059.5325.64
CGN240.0643.5234.9315.50
CGN343.1924.6237.039.71
Average value41.5849.0627.1616.95
CGY145.7845.198.704.77
CGY254.4940.175.2515.74
CGY343.8227.7410.7516.27
Average value48.0337.708.2312.26
Acoustic emission ringing countCGN156,555236,400256,489318,857
CGN29548112,990207,745389,639
CGN362,72492,853119,251146,340
Average value42,942147,414194,495284,945
CGY1163,406466,608523,751615,707
CGY275,313359,247417,590566,189
CGY377,774202,745277,911388,395
Average value105,498342,867406,417523,430
Acoustic emission energy/aJCGN13.42 × 1063.91 × 1076.33 × 1063.71 × 107
CGN24.32 × 1051.28 × 1072.72 × 1076.34 × 107
CGN35.21 × 1062.22 × 1062.84 × 1064.17 × 106
Average value3.02 × 1061.80 × 1071.21 × 1073.49 × 107
CGY11.04 × 1073.08 × 1079.04 × 1063.85 × 107
CGY22.53 × 1063.01 × 1077.28 × 1061.43 × 107
CGY31.62 × 1066.47 × 1066.18 × 1068.72 × 107
Average value4.84 × 1062.25 × 1077.50 × 1064.67 × 107
Table 5. Mass percentages and fractal dimensions D of coal sample fragments before and after microwave irradiation.
Table 5. Mass percentages and fractal dimensions D of coal sample fragments before and after microwave irradiation.
SampleIndexFragment Size/mmFractal Dimension D
<22~55~1010~20>20
CGN1Quality/g0.440.950.480.73191.251.33
Mass fraction/%0.230.490.250.3898.66
CGN2Quality/g0.931.150.424.45186.371.47
Mass fraction/%0.480.590.222.3096.40
CGN3Quality/g1.353.235.6016.29170.711.50
Mass fraction/%0.681.642.848.2686.58
CGY1Quality/g2.324.182.002.64181.591.78
Mass fraction/%1.202.171.041.3794.22
CGY2Quality/g2.515.252.278.92161.351.78
Mass fraction/%1.392.911.264.9589.49
CGY3Quality/g2.983.873.287.81170.071.81
Mass fraction/%1.592.061.744.1590.46
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Ding, C.; Cao, A.; Liu, H.; Wen, Y.; Guo, W.; Shi, Y. Experimental Study on Failure Characteristics and Energy Release Evolution of Coal Under Microwave Irradiation. Appl. Sci. 2025, 15, 9522. https://doi.org/10.3390/app15179522

AMA Style

Ding C, Cao A, Liu H, Wen Y, Guo W, Shi Y. Experimental Study on Failure Characteristics and Energy Release Evolution of Coal Under Microwave Irradiation. Applied Sciences. 2025; 15(17):9522. https://doi.org/10.3390/app15179522

Chicago/Turabian Style

Ding, Chuanghong, Anye Cao, Haonan Liu, Yingyuan Wen, Wenhao Guo, and Yang Shi. 2025. "Experimental Study on Failure Characteristics and Energy Release Evolution of Coal Under Microwave Irradiation" Applied Sciences 15, no. 17: 9522. https://doi.org/10.3390/app15179522

APA Style

Ding, C., Cao, A., Liu, H., Wen, Y., Guo, W., & Shi, Y. (2025). Experimental Study on Failure Characteristics and Energy Release Evolution of Coal Under Microwave Irradiation. Applied Sciences, 15(17), 9522. https://doi.org/10.3390/app15179522

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