Shn Yin,Zhonghui Li,Dzho Song,Xueqiu He,Liming Qiu,Qun Lou,He Tin

a School of Civil and Resources Engineering,University of Science and Technology Beijing,Beijing 100083,China

b School of Safety Engineering,China University of Mining and Technology,Xuzhou 221116,China

c School of Municipal and Environmental Engineering,Henan University of Urban Construction,Pingdingshan 467036,China

Keywords:Gas–bearing coal Gas pressure Infrared temperature Infrared thermal image Infrared precursory law

ABSTRACT The stress and gas pressure in deep coal seams are very high,and instability and failure rapidly and intensely occur.It is important to study the infrared precursor characteristics of gas-bearing coal instability and failure.In this paper,a self-developed stress-gas coupling failure infrared experimental system was used to analyse the infrared radiation temperature(IRT)and infrared thermal image precursor characteristics of gas-free coal and gas-bearing coal.The changes in the areas of the infrared temperature anomalous precursor regions and the effect of the gas on the infrared precursors were examined.The results show that high-temperature anomalous precursors arise mainly when the gas-free coal fails under loading,whereas the gas-bearing coal has high-temperature and low-temperature anomalous precursors.The area of the high-temperature anomalous precursor is approximately 30%–40% under gasbearing coal unstable failure,which is lower than the 60%–70% of the gas-free coal.The area of the low-temperature abnormal precursor is approximately 3%–6%,which is higher than the 1%–2% of the gas-free coal.With increasing gas pressure,the area of the high-temperature anomalous precursor gradually decreases,and the area of the low-temperature anomalous precursor gradually increases.The highand low-temperature anomalous precursors of gas-bearing coal are mainly caused by gas desorption,volume expansion,and thermal friction.The presence of gas inhibits the increase in IRT on the coal surface and increases the difficulty of infrared radiation (IR) monitoring and early warning for gas-bearing coal.

1.Introduction

During deep coal mining,the crustal stress and gas pressure gradually increase,and instability damage occurs more rapidly and intensely than before.Coal and gas outbursts,rock bursts,and other coal and rock dynamic disasters are increasingly frequent,which seriously restricts the safe production of coal mines[1–6].Under the combined effect of stress and gas,coal and rock masses experience rapid instability damage.The energy in coal and rock masses is released as elastic,acoustic,thermal,and electromagnetic energy [7–12],and heat transfer and abnormal infrared precursory phenomena occur before the damage.Therefore,the acquisition and research of infrared precursor information on the gas-bearing coal-rock system instability is important for its monitoring and prevention.

To effectively capture the infrared precursor information of coal and rock fractures,predecessors have performed many experiments using infrared thermal imaging technology.Wu et al.and Liu et al.[13–16] systematically reviewed and analysed the manifestations and types of thermal infrared precursors,precursor time,spatial characteristics,and precursor mechanism of rock fracture.They believed that the infrared radiation temperature (IRT) effect was the strongest when the coal sample strength was approximately 70%under loading.Zhao et al.[17–19]analysed the acoustic and thermal effects of coal and coal-rock combinations in the failure process and the characteristics of anomaly information before failure by applying unidirectional and cyclic loading failure tests to impact-prone coal and coal-rock combinations.Ma et al.[20–22] found that the infrared variance difference in a coal sample would suddenly change upon failure.The change was synchronous with a sudden drop in the load curve,and an anomalous infrared thermal image sequence was observed.In addition,using original and continuous-difference infrared images,the spatial distribution characteristics and evolution process of coal failure were revealed,and the temporal and spatial precursory laws of coal failure were obtained.Li et al.[23,24]tested and studied the change rules of infrared radiation (IR),acoustic emission(AE),and surface potential during the rock sample deformation and failure.An anomalous IR signal appeared at the maximum or minimum point of the temperature curve,which was related to the anomalous acoustic electricity signal.The load ratio ranges of the sample damage and critical failure IR precursor points were 36.9%–49.2% and 73.6%–95.8%,respectively.Wu et al.[25] examined the rock stress catastrophe precursors from the perspective of infrared temperature mutation and conducted uniaxial compression infrared-AE experiments on siltstone.Their work shows that infrared temperature mutation precursors occur immediately before siltstone fracturing,including a sudden rise and drop,extreme value and mutation quantity precursors.Zhang et al.[26] analysed the inherent relationship among three factors and rock burst using far-infrared and visible light images and AE,and they found at least two large-scale temperature jumps before the rock burst occurred,i.e.the initial anomalous temperature point and the rock burst precursor point.Wang et al.[27] examined and detected IRT anomalies before rock failure,including the average temperature curve and thermal map anomalies.The thermal image anomaly recorded at the latter loading stage is consistent with the change in average temperature curve,which can be used as precursor information to predict rock failure.Lou and He [28]studied the infrared temperature distribution of concrete under loading.The results showed that the infrared temperature distribution obeyed a Gaussian distribution,and the rapid decline in theR2of the Gaussian distribution could be used as a precursor of concrete instability.

Scholars have also studied the infrared precursor characteristics of coal and rock samples as a function of the water saturation,fracture angle,and impact tendency.Liu et al.[29] tested and studied the IRT,infrared thermal image change characteristics,and fracture precursor information of a water-containing concrete sample under uniaxial compression loading.Zhang and Liu [30] reported that the nature of rock fractures was closely related to the change in IRT when circular pores were present.The radiation temperature of a shear fracture increases,but that of a tensile fracture does not significantly change.The banded heating of the final shear fracture area of the sample is an important infrared precursor of the rock fracture instability.Li et al.[31] determined the IR change rule of gas-bearing coal in the load failure process.The change in infrared temperature during gas-bearing coal failure was correlated with the coal body fracture area.Using infrared thermal images,the magnitude and location of deformation and failure of the coal body can be predicted.Cheng et al.[32] evaluated the sandstone IR under uniaxial compression at different fracture angles.The maximum and average IRT curves exhibited anomalous precursors during sample loading,and the maximum IRT precursor was related to the load ratio.Xu et al.[33] performed a comparative analysis of the infrared information of three types of coal samples with no,weak,and strong impact tendencies.They reported that there was no distinct high-temperature region during loading of the coal samples with no impact tendency.Before the coal samples with an impact tendency failed,the surface fracture area of the coal samples exhibited a temperature increase,while that of the samples without an impact tendency exhibited a temperature decrease.

Previous researchers have performed extensive studies on infrared failure precursors in gas-free coal and rock under loading,and the coal-rock failure process and mechanism are more complex due to gas erosion [34–36].The infrared failure precursor characteristics of gas-bearing coal and rock are even less pronounced.Infrared failure precursor monitoring in gas-bearing coal and rock is the premise of evaluating the risk of dynamic disasters.Therefore,in this paper,an infrared experimental system of stress-gas coupling damage is used to study the change rules of IRT and infrared thermal image precursors in gas-free coal and gas-bearing coal under the action of stress-gas coupling.The change in area of the precursor infrared temperature anomalous region in gas-bearing coal is analysed,and the effect of gas on the infrared precursors is examine.Then,comprehensive discriminative precursors including the infrared temperature and infrared thermal images of gasbearing coal failure are obtained,which is important for monitoring the coal-rock gas dynamic evolution process and evaluating coal stability.

2.Experimental system and methods

2.1.Experimental system

The infrared test system consists of eight sub-systems,i.e.a stress-gas coupling visual cylinder,a loading system,an infrared camera,an industrial camera,a high-pressure cylinder and pipeline,a digital pressure acquisition instrument,a vacuum pump,and a data acquisition system.The experimental system is shown in Fig.1.

The stress-gas coupling visual cylinder is equipped with an infrared window and visible-light window,allowing the infrared camera and industrial camera to observe and record the damage process of loading and the destruction of coal that contains gas.The infrared radiation data acquisition system uses an Optris PI450 high-resolution infrared thermal imager with the following parameters:an optical resolution of 382× 288 pixels,a temperature range of-20 to 100°C,a spectral range of 7.5–13.0 μm,a thermal sensitivity of 0.04 K,and an imaging rate of 10 Hz.In the stress-gas coupling visual cylinder,the infrared camera can capture and evaluate the effective range of 200 mm× 120 mm(width× height).The loading control system adopts an electrohydraulic servo testing machine that is controlled by the YAW microcomputer.In the gas charging and discharging subsystem,the gas in the chamber could be evacuated by the vacuum pump and input to the chamber by the gas transmission line.

2.2.Sample preparation

Coal samples were collected from the Huaibei mining area of Anhui province,China.According to the standards of the International Society for Rock Mechanics (ISRM),the coal samples were processed into a cuboid shape with dimensions of 50 mm× 50 mm× 100 mm (±0.5 mm).The samples were selected with a surface parallelism value within 0.05 mm and a surface flatness within 0.02 mm.Before the experiment,the coal samples were placed in the experimental room for 24 h to ensure that the temperature of the samples was consistent with the room temperature.To ensure safety,the gas used in the test was highpurity carbon dioxide.The data on sample number,gas pressure,and loading mode of the tested coal samples are presented in Table 1.

Table 1Statistical parameters of samples.

2.3.Experimental process

The experimental procedures are as follows.

(1) Lay out the experimental system,turn on the instrument in advance,bring the instrument to the best performance state,and test the gas tightness of the cylinders.

(2) Place the sample into the test cylinder and adjust the position of the infrared camera and industrial camera.

(3) Use the vacuum pump to exhaust the air from the test cylinder until the vacuum reaches -0.1 MPa;then,continue to vacuum for 1 h.

(4) Fill the test cylinder with gas and maintain the pressure stable for 4 h after it reaches the set value.

Fig.1.Schematic diagram of the experimental system.

(5) Run the experiment at a loading rate of 100 N/s through the load control mode and synchronously record the infrared and load data.

(6) Stop the experiment after the sample fails and release the gas inside the test cylinder.At the end of the experiment,record and process the experimental data.

3.Experimental results

3.1.Infrared temperature characteristics of gas-free coal and gasbearing coal

To reflect the radiation energy of the entire coal sample,the coal sample surface IRT is selected as an analysis index.The IRT includes the average infrared radiation temperature (AIRT) and maximum infrared radiation temperature(MIRT).The AIRT reflects the overall IR intensity on the coal surface,which is a general index and the average value of all points in the temperature field of the coal sample observation area.The MIRT reflects the hightemperature anomaly caused by fracture and friction during the loading process of the coal sample.It is the maximum value in the temperature field of the coal sample surface and reflects the anomalous change in IRT of the coal sample.

The IRTs of representative gas-free coal sample M1 and coal samples M3,M5,and M7 with different gas pressures are selected to analyse the compaction,elastic and plastic deformation,and fracture development stages in deformation and failure.

3.1.1.Infrared temperature characteristics of gas-free coal

In fracture of gas-free coal,the curves of the AIRT,MIRT,and stress–strain relationship are obtained,as shown in Fig.2.

The change in IRT in the gas-free coal failure process under loading is consistent with previous research results [20,21,37].In the compaction stage (OA),the IRT fluctuates and decreases.With the increase in load,the coal enters the elastic deformation stage(AB),and the IRT fluctuation gradually increases.In the damage and failure stage (BC),the IRT rapidly increases.When the main failure occurs,the AIRT and MIRT sharply increase by 0.298 and 0.830 °C,respectively.The IRT instantaneously decreases after the coal failure.

3.1.2.Infrared temperature characteristics of gas-bearing coal

Under the condition of containing gas,the curves of the AIRT,MIRT,and stress–strain relationship are obtained,as shown in Fig.3.The four typical stages of deformation and failure of the coal samples are analysed as follows.

(1) In the compaction stage (OA),the stress–strain curves are concave,which indicates that the initial microcracks in the coal sample gradually close under the external load,and the IRT on the coal sample surface decreases as a whole.Fig.3b shows that the AIRT of coal sample M5 decreases by 0.075 °C,and the MIRT decreases by 0.198 °C.

Fig.2.Relationship between the stress–strain curve and IRT of gas-free coal.

Fig.3.Relationship between the stress–strain curve and the IRT of gas-bearing coal.

(2) In the elastic deformation stage(AB),with increasing load on the coal sample,the stress–strain curve linearly increases.The IRT of coal sample M1 without gas greatly fluctuates.Meanwhile,the IRT of the gas-bearing coal samples fluctuates within the range of 0.1 °C,and the fluctuation degree is small.The presence of gas suppresses the fluctuation and increase in IRT at this stage.

(3) In the plastic deformation stage (BC),when the coal sample enters the plastic stage and the load increases to the yield strength,new cracks form,expand,connect,and develop into a fracture network,which forms macrofractures until the coal sample is destroyed.At this time,the IRT greatly fluctuates.Fig.3a–c reveals that the AIRT and MIRT correspond well with stress at this stage.After pointB,as shown in Fig.3a,the stress decreases,and the AIRT breaks its original calm and increases by 0.197 °C.

(4) In the fracture development stage (after pointC),at the moment of the main damage,at pointC,the IRT of coal rapidly increases.With the increase in gas pressure,the increase amplitude of the IRT of gas-bearing coal gradually decreases.After the coal sample becomes unstable,due to the residual stress,the coal sample retains a bearing capacity.In addition to the main crack,other cracks widely extend,and numerous macrocracks communicate,while the IRT begins to decrease but remains at a relatively high level.

The change in IRT in gas-free coal and gas-bearing coal mainly occurs at the plastic deformation and fracture development stages.Therefore,for the study of IRT precursors in gas-bearing coal,the middle and late stages of damage in the coal sample should be analysed.

3.2.Characteristics of infrared thermal images of gas-free coal and gas-bearing coal

To clearly characterize the evolution characteristics of the surface temperature field of coal in the loading process under gasfree and gas-bearing conditions,one should reduce the interference of the external environment on the infrared radiation of the sample surface and the difference in emissivity of each part of the sample [38,39].In this paper,the infrared temperature field data of the sample surface are extracted.As shown in Eq.(1),the infrared temperature field data of the initial period are selected for difference processing to obtain infrared thermal images of the sample surface infrared temperature difference.

whereDx×y,t=tis the infrared temperature field data over a period oftseconds;Mx×y,t=tthe infrared temperature field data attseconds;andMx×y,t=1the infrared temperature field data of the initial few seconds.

Simultaneously,the surface crack evolution of the sample was recorded with an industrial camera and compared with the IR isotherm cloud image.Due to space limitations,this paper takes gasfree coal sample M1 and the 0.6-MPa gas pressure coal M7 as examples for analysis.The infrared thermal image change results are shown in Fig.4,and σ is the current stress level of sample and σmaxis the maximum stress of sample.

3.2.1.Characteristics of infrared thermal images of gas-free coal

In fracture of gas-free coal,as shown in Fig.4a,before 74.2% of σmax,there is almost no change in the crack evolution image of the sample,but differential infrared thermal image cloud image shows that the surface temperature of the sample gradually increases.At 95.4%of σmax,the high-temperature region in the upper part of the sample further expands,and the crack in the middle and upper parts of the sample tends to expand.At 99.2% of σmax,instability failure occurs in the sample,and there is a large range of hightemperature regions in the differential infrared thermal image cloud image.Red high-temperature spots appear at the local failure region,which corresponds to the failure position at the red curve point in Fig.4a.

The change in infrared temperature field of gas-free coal sample M1 shows that the abnormal precursors of the infrared temperature field appear in the middle and late stages of sample failure,and they are mainly high-temperature abnormal precursors.

3.2.2.Characteristics of infrared thermal images of gas-bearing coal

The infrared temperature of sample M7 fluctuates and decreases during the loading process (Fig.3c).Fig.4b shows that at 12.3% of σmax,the sample is at the initial loading phase.The differential infrared thermal image cloud image shows that the infrared temperature of the sample surface is uniformly distributed at approximately 0 °C,and the crack evolution image exhibits no notable changes.At 58.9% of σmax,microcracks appear on the left side of the sample(see Ⅰin Fig.4b).Simultaneously,the surface temperature field of the sample changes,the temperature is unevenly distributed,and the low-temperature range (blue) increases.The left-side temperature is higher than the right-side temperature.At 61.6%of σmax,the microcracks on the left side of the sample gradually increase and span the entire sample from top to bottom(see Ⅱin Fig.4b).The differential infrared thermal image cloud image shows that the overall sample temperature decreases,and a hightemperature yellow band appears near the cracks on the left side of the sample,which corresponds to the crack formation.

At 94.5% of σmax,the sample enters the fracture development stage,the crack range continuously expands,and the cracks become deeper (see III in Fig.4b).The infrared thermal difference image exhibits high-temperature spots,which gradually expand into high-temperature bands from the left side to the middle of the sample,and the colour gradually intensifies.At 96.1% of σmax,the middle part of the sample fractures,and failure occurs.The differential infrared thermal image cloud image shows a wide range of high-temperature bands,which spread across the whole sample,and the overall sample surface temperature increases.

The differential infrared thermal image cloud image of gasbearing coal during loading reveals the temperature distribution state of the sample surface,which corresponds to the crack evolution process in the sample during loading.The change in colour of the differential infrared thermal image cloud image contains certain precursor information and reflects the coal failure form and location.Therefore,the differential infrared thermal image cloud image suitably shows the temperature distribution state of the sample surface,which corresponds well to the crack evolution process in the sample during loading.The change in the infrared thermal image can be used as a precursor characteristic of coal failure.

4.Infrared precursor characteristics of gas-free coal and gasbearing coal failure under loading

By analysing the IRT and infrared thermal image of gas-free coal and gas-bearing coal in the process of loading and failure,the authors find that the infrared precursor of coal mainly occurs in the middle and late stages of loading.Infrared thermal image precursors generally show nonuniform temperature field changes on the surface of coal,i.e.the infrared radiation of high-and lowtemperature precursor regions.

4.1.IRT field distribution precursor

Fig.4.Infrared thermal image and crack evolution diagram of gas-free coal and gas-bearing coal during loading.

To characterize the precursor characteristics of the infrared temperature field distribution on the surface of coal,different gas pressures are selected.Then,the authors generate the differential infrared thermal image cloud image of gas-bearing coal and a histogram of the IRT frequency distribution at representative times on the stress–strain curve.Gaussian fitting is conducted according to Eqs.(2) and (3) [28].The results are shown in Fig.5.

wherefis the Gaussian fitting function;xthe infrared temperature data;aa fitting parameter;μ the Gaussian distribution mean;σ the Gaussian distribution standard deviation;R2the goodness of fit between the differential infrared thermal image cloud image and the Gaussian distribution;yithe measured value;the fitted value;the average measured value;Nthe number of measured values;anditheith measured value.

Fig.5 reveals that the infrared thermal images and IRT frequency distribution histograms of gas-bearing coal at different stress levels exhibit large differences.Under the 0.2-MPa gas pressure,as shown in Fig.5b,at 65.5%of σmax,the infrared temperature distribution on the sample surface is uniform,and a high-IRT anomaly appears in the upper right corner.At this time,the IRT frequency distribution histogram is bilaterally symmetric and approximately Gaussian,with a single narrow peak at a frequency of 30% near -0.1 °C,which results in a lowR2.When the stress increases,the frequency distribution histogram gradually shows a rightward deflection.At 86.7%of σmax,the frequency distribution histogram shows a peak above 15% near 0.3 °C,and theR2increases.The precursor area of the high-temperature anomaly increases.At 98.4% of σmax,the precursor area of the hightemperature anomaly in the infrared thermal image contour map decreases and transfers to the middle of the sample,and the precursor area of the low-temperature anomaly increases.At 98.8%of σmax,a high-temperature anomalous precursor appears in the middle of the left side of the sample,and a low-temperature anomalous precursor appears in the lower part of the sample.The rightward deviation of the frequency distribution histogram becomes more pronounced and slowly decreases,and the Gaussian distribution is approximated.As a result,theR2increases to 94.12%.At this time,the anomalous high-and low-temperature ranges increase,the sample reaches a stress peak,and instability failure soon follows.At 100% of σmax,the frequency distribution histogram has multiple peaks exceeding 5%,and theR2increases to 97.60%.The high-temperature anomalous precursor area greatly expands,and the gas-bearing coal becomes unstable.

Due to space limitations,Fig.5a,c,and d are not described in detail.Gas-free coal sample M1 is more concentrated in the high-temperature regions.Similar to sample M3,the stress levels in samples M5 and M7 increase at gas pressures of 0.4 and 0.6 MPa.Both infrared thermal image and frequency distribution histogram significantly change.In other words,before the sample is destabilized,the high-and low-temperature regions become concentrated,and the high-and low-temperature anomaly distribution intervals further expand.Moreover,the temperature at the centre increases,the frequency distribution histogram gradually shows a rightward deviation,the peak degree becomes slow,and multiple peaks appear.In summary,both infrared thermal difference image contour map and IRT frequency distribution histogram contain clear infrared precursor information and have significant infrared precursor characteristics,which indicates that the coal sample will soon experience instability and failure.

Fig.5.Differential infrared thermal image cloud image and IRT frequency distribution histogram of gas-free coal and gas-bearing coal under loading.

Fig.5 (continued)

4.2.Changes in the areas of the infrared temperature anomalous precursor regions

The anomalous IRT field of coal near failure may be concentrated in a relatively small area.Examining the local anomalous changes in the IRT field in coal has a certain guiding significance for the monitoring and prediction of coal sample instability and failure [40].Gas-bearing coal will exhibit local areas of high-and low-temperature anomalies during loading (see Figs.5 and 6,respectively).Therefore,it is necessary to study the change in the area of the anomalous infrared temperature region.

As shown in Fig.6,the infrared thermal images of gas-bearing coal are matrices which compose of 382×288 temperature points,mrepresents each temperature point.The IRT field approximately conforms to the Gaussian distribution(see Fig.6,Gaussian fitting)[28,41].Based on the properties of the Gaussian distribution curve,the probability of attaining a value outside(μ-3σ,μ+3σ)is lower than 0.3%,which is a low-probability event that almost never occurs.The infrared temperature on the surface of the coal sample is uniformly distributed at approximately 0 °C (Fig.4b,12.3% of σmax) during the initial period of gas-bearing coal failure.At this time,there is almost no high-or low-temperature anomaly precursor,and the area of the high-and low-temperature anomaly precursor is less than 0.3%.With the loading progress,the surface temperature field of gas–bearing coal continuously evolves,the area of high-and low-temperature anomaly regions continuously increases,and the number of temperature points higher than μ0+3σ0and lower than μ0-3σ0increases.Based on this,to quantitatively express the change in the areas of the high-and lowtemperature anomalous IR precursor regions,this paper selects the mean value μ0and standard deviation σ0of the IRT field during the initial period as statistical indices.The temperature point higher than μ0+3σ0in the IRT field is called the hightemperature anomalous precursor temperature pointSh,and the temperature point lower than the μ0-3σ0temperature point is called the low-temperature anomalous precursor temperature pointSl.The area percentages of the high-and low-temperature anomalous precursor regions can be defined as wherePis probability distribution;andNhandNlthe area percentage of high temperature and low temperature anomaly precursor regions,respectively.

Fig.6.Anomalous precursor regions of the IRT field.

Eqs.(4)and(5)are used to calculate the temperature data of the IRT field.The ratio of the number of temperature points in the high-and low-temperature regions to the number of all temperature points in the statistical region is calculated to obtain the area percentage of the high-and low-temperature anomaly precursor regions.A diagram of the precursor region division is shown in Fig.7.

4.3.Infrared radiation thermal image precursor

In this paper,gas-free coal samples and gas-bearing coal samples under different gas pressures are selected,and the area percentages of the high-and low-temperature precursor regions at different stress stages are calculated,as shown in Figs.8 and 9,respectively.

Fig.7.Area percentage division of the high-and low-temperature precursor regions.

Fig.8 shows that before 40% of σmaxis reached,the precursor area of the high-temperature anomaly on the coal sample surface without gas and under different gas pressures is almost zero,which hardly changes with the IRT field at the time of initial loading.At this time,the coal sample is at the initial stage of compaction and elasticity.Under the effect of the external load,the coal sample only has elastic deformation,and the surface temperature field is relatively uniform,while there is no clear hightemperature anomalous precursor.At 50%of σmax,the elastic stage ends.Under the action of the external load,the IRT field begins to fluctuate.At this time,the high-temperature anomalous precursor area on the coal sample surface begins to increase,and a hightemperature anomalous IR precursor gradually appears.At 60% of σmax,the coal sample enters the plastic deformation stage (pointBin Fig.3),and the IRT field no longer remains stable and fluctuates greatly.At this time,the precursor area of the hightemperature anomaly on the coal sample surface increases.After 70% of σmax,the precursor region area of the high-temperature anomaly on the coal sample surface significantly increases.At 80% of σmax,the precursor area of the high-temperature anomaly on the coal sample surface increases to 5%–10%.At this time,the coal sample enters the stage of fracture development.A new fracture forms,expands,and connects with the original fracture,which forms a macrofracture.The large amount of friction between the fractures results in a high-temperature precursor region.At 90%of σmax,the precursor area of the high-temperature anomaly on the coal sample surface sharply increases and greatly fluctuates.At this time,the cracks widely and greatly expand,the macrocracks communicate with each other to a high degree,and the coal sample becomes unstable.At 100% of σmax,the coal sample fails,and the precursor area of the high-temperature anomaly on the coal sample surface is the largest.

Fig.9 shows that with increasing stress level,the lowtemperature anomalous precursor area on the surface of the gasfree coal slowly increases,while the low-temperature anomalous precursor area on the surface of the gas-bearing coal considerably fluctuates and exhibits an increasing trend.Before 50%of σmax,the precursor region of the low-temperature anomaly on the coal sample surface is small under different gas pressures.At this time,the thermal effect generated by the coal sample compression is exchanged with the convective heat of gas in the precursor region of the low-temperature anomaly on the coal sample surface at lower levels.At 60%–70% of σmax,cracks gradually form,and the gas inside the coal sample begins to desorb and expand,while the precursor region of the low-temperature anomaly increases.When the stress reaches 80% of σmax,many fractures form,which promotes the gas desorption and expansion in the coal matrix,and a certain amount of heat is absorbed,which further expands the area of the low-temperature anomalous precursor.After 90%of σmaxis reached,under the joint action of coal sample fracture friction and gas,the high heat production leads to alternating downward and upward trends of the low-temperature anomalous precursor area,which can be considered as a precursor of the critical gas-bearing coal failure.

Fig.8.Change in the area of the high-temperature precursor region.

Fig.9.Change in the area of the low-temperature precursor region.

Figs.8 and 9 show that the area of the high-temperature anomalous precursor is 30%–40% when the gas-bearing coal is unstable and fails,while the area of the low-temperature anomalous precursor is only approximately 3%–6%.The area of the high-temperature anomalous precursor of gas-free coal is 60%–70%,which is much higher than that of gas-bearing coal.The area of the low-temperature anomalous precursor is only approximately 1%–2%,which is much lower than that of gas-bearing coal.The analysis shows that more heat is generated by the staggered friction between cracks in the gas-free coal;thus,the hightemperature anomalous precursor areas are large.Gas-bearing coal has a larger low-temperature anomalous precursor area due to the presence of gas.With increasing gas pressure,the area of the hightemperature anomalous precursor gradually decreases,and the area of the low-temperature anomalous precursor gradually increases.The analysis shows that at a greater gas pressure,more heat is absorbed due to the gas desorption,expansion,and convection across the coal surface,the area of high-temperature precursor regions decreases,and the area of low-temperature precursor regions increases.The heat generated by the staggered friction between cracks is significantly greater than the heat absorbed by the gas desorption,expansion and convection across the coal surface when the gas-bearing coal becomes unstable and fails.The area of the high-temperature anomalous precursor is significantly larger than that of the low-temperature anomalous precursor when the coal is unstable and fails,which also explains the rapid increase in IRT of gas-bearing coal.

Through the changes in the areas of the high-and lowtemperature precursor regions at different stress stages,there are precursor damage and critical failure points in gas-bearing coal.According to Figs.8 and 9,the precursor points of coal sample damage appear at 60% of σmax,and the precursor points of critical failure appear at 90% of σmax.Consistent with previous research results [21,23,24],coal sample damage precursor points appear at 40%–60%of the load,and critical failure precursor points appear at 80%–95% of the load.Therefore,the change in anomalous highand low-temperature areas can be used as a precursor index for the instability and failure of gas-bearing coal.

5.Discussion

5.1.Thermodynamic analysis of infrared precursors of coal body failure

In the loading process of gas-bearing coal,the IRT always changes.Although the transformation of IR energy in the loading process of gas-bearing coal is complex,it still follows the law of energy conservation.According to the first law of thermodynamics,Eq.(6) can be obtained.

whereWis the work performed by the outward gas expansion;Q1the initial gas pressure;Q2the gas pressure after expansion;V1the initial gas volume;V2the gas volume after expansion;andKthe ratio of the constant pressure and constant volume heat capacity of gas.

When gas expands and flows outwards,the energy absorbed by the gas reduces the IRT of the coal sample;hence,the reduced IRT can be expressed as [42]

where ΔT1is the temperature at which gas volume expansion decreases;T1the initial temperature before gas expansion;T2the temperature after gas expansion;andcthe specific heat of coal.

Gas desorption is an endothermic process.When a coal volume of ΔVis desorbed,the decrease in IRT ΔT2can be expressed as[43]

where ΔVis the gas desorption volume;ΔT2the endothermic micro-decomposition of gas,and the negative sign indicates a decrease in the gas desorption process;andqdthe endothermic micro-decomposition of gas.

By combining Eqs.(7)and(8),the effect of gas on the IRT of the coal body can be expressed as

where ΔTis the IRT affected by gas.

In summary,gas desorption and volume expansion reduce the IRT of gas-bearing coal,and the presence of gas suppresses the increase in IRT on the coal body surface with the result that the coal body exhibits an anomalous precursor of a decrease in IRT.

5.2.Effect of gas on the infrared precursors of coal failure

Without gas,the change in IRT of coal failure under loading is mainly caused by two mechanisms,i.e.the thermoelastic effect and friction-thermal effect,which has been widely confirmed[13,44–46].Generally,with the increase in load,the IRT of coal gradually increases,which is related to the thermoelastic effect.In the damage and failure stages,a large amount of sliding friction within cracks produces a substantial amount of heat,which makes the IRT rapidly increase.This stage is mainly related to the frictionthermal effect.When the main failure occurs,the heat generated by crack friction is transferred to the sample surface,which is a significant high-temperature anomalous precursor(pointC,Fig.2a).Coal bodies are rock-like materials with poor homogeneity that contain primary pores and fissures.In the loading and failure process of the coal sample,a small amount of heat exchange occurs with the outside environment[27].Therefore,in the gas-free case,there is also a small amount of low-temperature anomalous precursor.

Gas plays an important role in the infrared precursors of gasbearing coal failure.Due to the gas pressure and continuous movement of gas molecules,the attraction among the molecules inside the coal body is affected,which changes the structural strength of the coal body[47].At the initial loading stage,gas hinders the internal gas discharge of the coal body.As loading progresses,the increase in stress makes the coal body temperature continuously increase,the gas desorption and volume expansion make the coal body temperature continuously decrease,and the IRT basically remains unchanged (stageAB,Fig.2).Before the gas-bearing coal becomes unstable and fails,due to fracture expansion,the staggered friction temperature of the coal body increases,and the gas desorption and volume expansion absorb heat inside the coal body.Simultaneously,the heat exchange between gas and sample surface removes some of the heat,and the IRT of the coal body surface fluctuates to varying degrees;that is,there are anomalous high-and low-IRT precursors(stageBC,Fig.2).The presence of gas suppresses the appearance of high-temperature infrared radiation precursors in the coal body,which reduces the IRT in the process of coal body destruction.In other words,there is a low-temperature anomalous precursor region in the coal body during the process of coal body destruction.However,due to the main heat conduction effect of friction heat on the fracture surface,the high-temperature anomalous precursor region is larger than the low-temperature anomalous precursor region when the gas-bearing coal becomes unstable(100%σmax,Fig.4),and the overall coal body temperature increases,which results in a temperature rise precursor.

5.3.Interpretation of the infrared precursor phenomenon of gasbearing coal for temperature change in coal mining

Many applications in the field show that before coal and gas outbursts occur,the IRT of the working face decreases,the coal wall cools,and cold gas escapes through drilling cracks.When coal and gas outbursts occur,the working space,such as the working face,shaft,and chamber,will be subjected to a strong and cold gas flow [41,42].However,it has also been reported that the coal body temperature increases when the gas in the coal body is compressed or escapes [48].This finding is consistent with the results of this study,where anomalous infrared high-and lowtemperature precursors were observed before the gas-bearing coal became unstable.The staggered friction between fractures increases the coal body temperature,which makes the coal body exhibit a high-IRT anomalous precursor.The presence of gas inhibits the increase in IRT on the coal body surface,and the coal body exhibits a low-IRT anomalous precursor.This phenomenon also reveals that because of the particularity of the geophysical environment and complexity of the stress field,the crustal stress and gas pressure of deep coal greatly increase.Gas makes the coal failure process and mechanism more complex,and the infrared precursor characteristics become less notable.Therefore,the study of infrared precursor information of the instability and failure of gas-bearing coal has a certain monitoring and early warning significance for the prediction and prevention of coal-rock dynamic disasters such as rock burst and coal and gas outbursts.

6.Conclusions

In this paper,a self-developed stress-gas coupling damage infrared experimental system is used to study the IRT and infrared thermal image precursor characteristics of gas-free coal and gas-bearing coal under loading failure,analyse the change rule of the IRT anomalous precursor area,and reveal the effect of gas on the infrared precursors of coal failure.The following conclusions are obtained.

(1) The IRT of gas-free coal gradually increases under loading and reaches the maximum value at the main failure,which shows the high-temperature anomalous precursor.Due to the presence of gas,the IRT of gas-bearing coal shows a large fluctuation under loading,which mainly occurs in the plastic deformation and fracture development stages,and demonstrates high-and low-temperature anomalous precursors.With increasing gas pressure,the high-temperature anomalous precursor gradually weakens,and the low-temperature anomalous precursor gradually increases.

(2) The changes in the infrared thermal image and IRT frequency distribution histogram of gas-bearing coal during loading and failure processes have infrared precursor information.The high-and low-temperature regions of the infrared thermal image become concentrated,and the high-and low-temperature anomaly distribution intervals expand.The IRT frequency distribution histogram gradually shows a rightward deviation,the peak degree becomes slow,and multiple peaks appear,which indicate that the coal sample is becoming unstable.

(3) The precursor index for the change in area of the high-and low-temperature abnormal precursor regions of gasbearing coal is established.The area of the hightemperature anomalous precursor is approximately 30%–40% when the gas-bearing coal is unstable,which is lower than the result of 60%–70% corresponding to the gas-free coal.The area of the low-temperature abnormal precursor is approximately 3%–6%,which is higher than the 1%–2%value of gas-free coal.With increasing gas pressure,the area of the high-temperature anomalous precursor gradually decreases,and the area of the low-temperature anomalous precursor gradually increases.

(4) The high-temperature anomalous precursor of gas-free coal is mainly caused by the thermoelastic effect and frictionthermal effect.The high-and low-temperature anomalous precursors of gas-bearing coal are mainly caused by gas desorption,volume expansion,and thermal friction.The staggered friction between fractures increases the coal body temperature,causing the coal body to exhibit a hightemperature anomalous precursor.The presence of gas suppresses the increase in IRT on the coal body surface,which causes the coal body to exhibit a low-temperature anomalous precursor.

Acknowledgements

This work is supported by the National Natural Science Foundation of China(No.52074280),the National Natural Science Foundation of China (No.52004016),and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.