An open-end high-power microwave-induced fracturing system for hard rock

Xia-Ting Feng,Jiuyu Zhang,Feng Lin,Chengxiang Yang,Shiping Li,Tianyang Tong,Xiangxin Su

Key Laboratory of Ministry of Education on Safe Mining of Deep Metal Mines,Northeastern University,Shenyang,110819,China

Keywords:Hard rock engineering High-power microwave Microwave intelligent fracturing Dynamic feedback True triaxial stress

ABSTRACT Microwave pre-treatment is considered as a promising technique for alleviating cutter wear.This paper introduces a high-power microwave-induced fracturing system for hard rock.The test system consists of a high-power microwave subsystem (100 kW),a true triaxial testing machine,a dynamic monitoring subsystem,and an electromagnetic shielding subsystem.It can realize rapid microwave-induced fracturing,intelligent tuning of impedance,dynamic feedback under strong microwave fields,and active control of microwave parameters by addressing the following issues:the instability and insecurity of the system,the discharge breakdown between coaxial lines during high-power microwave output,and a lack of feedback of rock-microwave response.In this study,microwave-induced surface and borehole fracturing tests under true triaxial stress were carried out.Experimental comparisons imply that high-power microwave irradiation can reduce the fracturing time of hard rock and that the fracture range(160 mm)of a 915-MHz microwave source is about three times that of 2.45 GHz.After microwave-induced borehole fracturing,many tensile cracks occur on the rock surface and in the borehole:the maximum reduction of the P-wave velocity is 12.8%.The test results show that a high-power microwave source of 915 MHz is more conducive to assisting mechanical rock breaking and destressing.The system can promote the development of microwave-assisted rock breaking equipment.

1.Introduction

Mechanical rock breaking is considered the most effective among numerous methods and has been widely applied in civil and mining engineering(Hassani,2010;Hassani et al.,2016).However,in hard and extremely hard rocks,mechanical excavation often encounters problems of cutter wear,cutting failure,and reduced driving speed,which can greatly limit its wider use in hard rock engineering (Zheng et al.,2016).In addition,rockburst accidents often occur in deep hard rock excavation,which threaten the safety of construction personnel and equipment(Feng,2017;Zhang et al.,2020;Niu et al.,2022).Microwave heating can weaken the mechanical properties of rock and decrease the level of stress concentration,making it a promising technique (Motlagh,2009;Lu et al.,2020).

The microwave response characteristics of various rocks have been studied by the open-end type or multi-mode cavity (Koiwa et al.,1975;Lindroth et al.,1993;Makul et al.,2014;Feng et al.,2021;Ma et al.,2022).The former is considered more suitable for assisting mechanical rock breaking.During the high-power microwave output,the test system is faced with the problems of its discharge and an excess of reflected power(Takahashi et al.,1986).The large volume and weight prevent it from convenient in situ applications.At present,microwave-assisted mechanical rock breaking remains in the laboratory stage,for which the lack of appropriate equipment is a main reason.In field applications,the complex environment in the tunnel can affect the stability of the equipment,such as high temperature,humidity,and dust.In addition,when combined with a tunnel boring machine(TBM),it is necessary to consider power allocation,rapid fracturing,and impedance matching (an additional impedance is added to the microwave system so that the load impedance is equal to the microwave system impedance,reducing the reflected microwave power) (Zheng,2018).

In industry,medicine,and science,the frequencies available for microwave heating are 915 MHz,2.45 GHz,5.8 GHz,and 22.215 GHz,respectively (Gwarek and Celuch-Marcysiak,2004).Most researchers chose a microwave source with a frequency of 2.45 GHz for fracturing of hard rock (Wei et al.,2019;Nicco et al.,2020;Li et al.,2021).The penetration depth has a negative correlation with microwave frequency.The high-frequency microwave is mainly concentrated to within a small range in the rock,limiting the range of fracturing(Hartlieb et al.,2018).The penetration depth of microwave with a frequency of 915 MHz is about 2.68 times that of 2.45 GHz (Hassani et al.,2016),which is conducive to the formation of a larger fracture area.A high-power microwave can enhance the heating effect and increase the fracturing efficiency of rocks (Deyab et al.,2021;Lu et al.,2019a).In addition,when the microwave applicator is combined with the TBM,a higher-power microwave is required to fracture the rock mass in a short enough time.

In practice,due to the impedance mismatch (i.e.the load impedance is not equal to the microwave system impedance),some microwave energy can be reflected to the magnetron(the reflection characteristic can be indicated by the voltage standing wave ratio(VSWR)),causing damage thereto (Zheng et al.,2017).Before the test,the impedance of the microwave system is often adjusted by a manual tuner,which is matched with the load impedance to prevent excessive reflected power (Hassani et al.,2016;Lu et al.,2019b).However,during the test,the load impedance may change due to rock response(dynamic load),e.g.rock spalling,thus a single static adjustment cannot ensure that the microwave system is always in a state of low reflective power,especially when applying high-power microwave irradiation.In previous microwave apparatus,the output of microwave energy was controlled by fixing the microwave power and irradiation time(Wei et al.,2019),which cannot directly reflect the heating effect of rock,nor can it be adjusted according to the feedback.Therefore,it is necessary to develop a novel high-power microwave fracturing apparatus for hard rock,including rapid fracturing,intelligent impedance matching,dynamic feedback,and active parameter control.

A high-power microwave-induced fracturing apparatus for hard rock has been developed by Northeastern University of China.It can overcome the problems of the instability and insecurity of the system,the discharge breakdown between coaxial lines during high-power microwave output,and the lack of feedback in the rockmicrowave response.The test system and key technology are described herein with details of preliminary tests designed to assess its performance.

2.Test system composition

Feng et al.(2021) developed a novel true triaxial system for microwave-induced fracturing of hard rocks.This updated test system incorporates a new microwave source into the existing apparatus,consisting of a high-power microwave-induced fracturing subsystem and a high-power microwave shielding subsystem,etc.(Fig.1).One of its key features is real-time monitoring of the system’s operation and the heating effect of rocks.It can make real-time adjustments based on feedback,including reflected microwave power,water temperature,water flow rate,VSWR (proportional to the reflected microwave power),and rock temperature,realizing the dynamic control of microwave energy output.

Fig.1.An open-end high-power microwave-induced fracturing test system for hard rock under true triaxial stress.①-High-voltage power supply,②-Microwave source,③-Shielding cavity,④-Infrared camera,⑤-True triaxial testing machine,⑥-Lifting platform,⑦-Intelligent impedance tuner,⑧-Coaxial applicator.

2.1.A high-power microwave-induced fracturing subsystem

The microwave components serve distinct functions and can be divided into four parts: high-voltage power supply,microwave source,microwave transmission,and microwave protection(Fig.2).The high-voltage power supply adopts advanced series resonant switching circuit topology technology,including the anode and the filament power supply.This design exhibits strong anti-ignition ability and rapid fault protection,ensuring its reliable performance even in the complex tunnel environments.The anode power supply adopts a modular design,consisting of five identical and independent parallel-connected switching power supplies.This configuration simplifies operation and enhances flexibility.The power supply employs a sealed cold-water circulation system,characterized by its compact size and high cooling efficiency.

Fig.2.Schematic diagram of a high-power microwave fracturing test system for hard rock.1 -High-voltage power supply,2 -Lifting platform,3 -Cold-water circulation,4 -Magnetron,5-Water load,6-Circulator,7-Impedance intelligent tuner,8-CCD industrial camera,9-Shielding cavity,10-Infrared thermal imager,11-Control box,12-Surface applicator,13 -Infrared temperature sensor,14 -Distributed optical fiber,15 -Coaxial applicator,16 -Demodulator,17 -AE instrument,18 -AE sensor.

The microwave source is the key component used for converting electrical energy into microwave energy,including magnetron,excitation cavity,and cold-water circulation apparatus.It operates at a microwave frequency of 915 MHz and has a maximum output power of 100 kW.The cold-water circulation apparatus ensure a clean water supply to the magnetron,which solves the problem of water supply convenience and water quality in field conditions.Microwave transmission refers to the efficient conveyance of microwave energy to the rock surface with l minimal loss such as rectangular waveguide and coaxial line,etc.

The microwave protection includes a large-capacity circulator,water load,automatic alarm apparatus,and impedance intelligent tuner.The circulator,functioning as a three-port radio-frequency(RF)apparatus,enables the microwave to be transmitted clockwise or counter-clockwise in a single direction (preventing reflected microwave energy from reentering the magnetron).When connected to a water load,it has the capability to isolate reflected microwave energy,boasting a maximum power capacity of 100 kW and the ability to withstand a maximum reflected power of 40 kW.The automatic alarm apparatus plays a pivotal role in assessing the operational status of the equipment according to the monitored information.When the equipment fails during operation,the power supply will be cut off timeously,or if there is a failure,the power supply will not be able to start.The impedance intelligent tuner can rapidly match the dynamic load in real-time,ensuring that the microwave source and rock consistently remain in an optimal matching state.

2.2.Electromagnetic shielding subsystem under high-power microwave

In the test,high-power microwave is more likely to produce electromagnetic wave leakage,which can be harmful to both humans and electronic equipment.Therefore,the suppression of microwave leakage is crucial for safe operation.Electromagnetic shielding is employed to isolate the space with a plane composed of conductive or magnetic conductors,which can be divided into overall shielding and local shielding.Overall shielding involves the creation of a high-power microwave shielding room by sealing the laboratory’s walls,ceilings,doors,and windows with metal plates and metal mesh,effectively separating the microwave source from the experimental operation.Local shielding utilizes metal material to shield monitoring equipment,sensors,and computers,including a metal cavity,metal cabinet,metal box,cut-off waveguide,absorbing plate,and aluminum foil,etc.The joint part of microwave source and testing machine is shielded by a metal cavity,which limits most electromagnetic waves to within the cavity.Meanwhile,the microwave energy within the cavity is absorbed by the water in the barrel to prevent microwave resonance.Aluminum foils,metal meshes,and magnetic elements are used to seal the rock specimen,loading plates,and gaps,while monitoring equipment is shielded with a metal box,metal cabinet,metal mesh,and a cut-off waveguide.Following the shielding treatment,there is no microwave leakage outside the shielding room,and the leakage intensity at a distance of 1 m from the irradiation port falls below the safety threshold (leakage intensity ＜5 mW/cm2),and monitoring equipment and sensors work normally.

2.3.True triaxial testing machine

The true triaxial testing machine consists of loading frame,true triaxial stress loading,servo-motor control,and control software,etc.,which are described in detail in Feng et al.(2021).This testing machine is capable of replicating the stress state of “five-face compression and one-face irradiation” of a rock mass near the excavated face.It can provide a maximum load of 5000 kN in three principal stress directions and can be adapted to cubic rock specimens with a side-length of 400 mm.Piston displacement and rock load in three directions are monitored by a displacement sensor and an oil pressure sensor respectively.

2.4.Dynamic monitoring subsystem of thermal response and fracture evolution

The dynamic monitoring system is mainly composed of an infrared thermal imager,a demodulator,an optical fiber,a CCD industrial camera,an AE sensor,a cut-off waveguide,an infrared temperature sensor,etc.This system is designed to enable real-time monitoring of the heating effect and thermal fracture under extremely strong microwave irradiation.As stress is applied and microwave irradiation is initiated,the load,displacement,temperature,and image are automatically transmitted to the control software for display and storage,which is convenient for unattended use under high-power microwave irradiation.

3.Key problems solved in the development of the system

3.1.Instability and insecurity of the system during high-power microwave-induced fracturing

The high-voltage power supply and the magnetron play pivotal roles in ensuring the stable output of high-power microwave energy.Practice has shown that most of the magnetron damage is due to untimely ignition protection,leading to secondary or multiple ignitions.Secondly,during high-power microwave-induced fracturing,the sudden or excessive reflection of power can impair the stability of the magnetron.The manual tuner,with its inability to provide prompt and precise adjustments,poses challenges for the system in rapidly responding to sudden changes in dynamic load conditions.

The power supply utilizes the dual protective measures of a parallel resonant circuit and rapid power-cutting,which confers a strong anti-ignition ability.In case of ignition or a short-circuit,the resonant inductive circuit prevents a rapid increase in current,thus averting peak current surges and eliminating the effect of an ignition fault on the power supply.When an ignition fault is detected,the fast protection circuit cuts off the power quickly,with a response time of 10 μs.Fig.3 shows the anode voltage and current at a microwave power level of 100 kW.Once the rated power is reached,both voltage and current remain stable with little fluctuations,indicating that the power supply can stably provide power to the high-power magnetron.

Fig.3.Anode voltage and current at microwave power of 100 kW.

The impedance intelligent tuner and threshold protection serve a dual purpose: they swiftly adjust the system’s impedance of minimizing reflected power and provide real-time data feedback to the microprocessor.When the reflected power exceeds the safety threshold,the microwave output is proactively disconnected.The key is to establish the relationship between the three-pin position and the load impedance,which can complete impedance matching within seconds.The model of three-pin tuner is constructed using a high frequency structure simulator (HFSS),then a database of three-pin position and load impedance is generated,and a relationship model between them is developed using neural networks.When the microwave system does not match the rock load,the optimal position of the three pins is determined using an optimal algorithm,realizing the rapid matching of rock dynamic load under high-power microwave irradiation.Fig.4 shows the evolution of VSWR during microwave-induced surface fracturing.When the VSWR is greater than 1.6,it is judged that the system is in a mismatched state.By adjusting the insertion depth of the three pins,the VSWR can be quickly adjusted to below the threshold,with a response time of approximately 3 s.Employing the above method,the system can output high-power microwave energy in a safe and stable manner.

Fig.4.The VSWR evolution during microwave-induced surface fracturing.

3.2.Active control of microwave parameters during high-power microwave-induced fracturing

The extent of rock fracturing is affected by stress,water content,and mineral content to varying degrees.The experimental findings indicate that thermal fracture requires a relatively long incubation time and is mainly occurring within a certain period,while also exhibiting a threshold temperature effect (Zhang,et al.,2022).Fixed microwave power and irradiation time may result in insufficient fracturing or low fracturing efficiency.Therefore,it is necessary to achieve dynamic feedback of rock response and make timeous adjustment to microwave parameters.

The active control of microwave parameters involves dynamically adjustments to the microwave output according to feedback.The key is obtaining real-time feedback on the rock’s response under extremely strong microwave irradiation.The self-designed infrared temperature sensor is installed on the applicator to monitor the heating characteristics of rock specimens,which overcomes the interference of the strong electromagnetic field.The details are provided as follows:the sensor and its wire are shielded using metal tubes and metal mesh,respectively.An observation hole is set on the side surface of the applicator.A cut-off waveguide(microwaves cannot pass through the waveguide) is connected to the observation hole,and the sensor is then installed in the cut-off waveguide.The control software can process data from the sensor and adjust the microwave parameters according to the feedback.The temperatures monitored by the two methods are in agreement(Fig.5a),confirming the accuracy of the temperature monitored by the infrared temperature sensor.As shown in Fig.5b,when the rock is heated to 50°C at 5 kW,the microwave power is automatically adjusted to 30 kW,and the output is then stopped after heating to 225°C.This result demonstrates the feasibility of controlling microwave output by heating effect.This technique lays the foundation for the intelligent control in microwave-assisted mechanical rock breaking.

Fig.5.(a) The accuracy of temperature monitored by the infrared temperature sensor,and (b) Example of active control of microwave parameters.

3.3.Discharge breakdown of contact end surface between coaxial lines during high-power microwave-induced borehole fracturing

The coaxial line is the key component of microwave-induced borehole fracturing.Due to the limitations of machining conditions,transportation,and installation,the use of multi-segment connections is inevitable in the long-distance coaxial transmission of microwave energy.In general,the coaxial line is composed of the outer conductor,the inner conductor and the supporting medium,with electromagnetic wave confined to the air gap between them.The inner and outer conductors between the two sections are connected with thread or bayonet,and the quality of the contact end surface at these junctions significantly impacts the tolerable power of the coaxial line.From a microscopic perspective,any surface is uneven.In practical applications,it is difficult to ensure the precision of machining and assembling for the contact end surface.When numerous gaps or burrs exist,there’s a risk of generating a high field intensity under high-power microwave irradiation.This high field intensity can lead to tip discharges,resulting in an ignition fault in the microwave system and even combustion of the microwave device (Fig.6a).

Fig.6.(a) Contact end surface discharge fault of outer conductor,and (b) Heating characteristics under different applied microwave powers.

Ensuring the continuity of contact surface conduction is crucial to avoid the aforementioned situation.The following measures are taken: (1) To reduce the contact area,grooves are carved into the ends of both the inner and outer conductor ends;(2) Before assembly,the contact end surface is polished with fine sandpaper to eliminate visible burrs;(3) Conductive silver glue is applied to the contact end surface to ensure complete contact;(4) The outer conductors of two coaxial lines are connected with copper sleeves to ensure sufficient force on the contact surface.In addition,efforts have been made to suppress electric field intensity at the contact end surface: (1) The length of the coaxial line is optimized using HFSS to prevent the contact end surface from aligning with the peak of the electric field;(2)To avoid sudden changes in the electric field,the support block (formerly solid) is fabricated as a honeycomb shape;(3)The position of the inner conductor contact surface is adjusted to avoid as alignment with the outer conductor contact surface.Using these adjustment methods,the problem of the discharge on the contact end surface during is obviated during high-power microwave-induced borehole fracturing.When the microwave power is 60 kW,the heating rate is 6.57°C/s (Fig.6b),indicating the stable transmission of high-power microwave energy along the coaxial line.

4.Test

4.1.Test scheme

Basalt specimens from Chifeng City,Inner Mongolia,China,were typical hard brittle,microwave sensitive rocks with good homogeneity and no defects visible to the naked eye.Size,parallelism,and flatness of the specimens were carefully controlled to avoid any significant stress concentrations or geometric effects on strength.In addition,a rubber gasket with a thickness of 3 mm was placed between the specimens and the loading plate.X-ray diffraction(XRD)analysis indicated that the basalt was mainly composed of plagioclase(about 48.5%),pyroxene(about 30%),olivine(about 16.5%),iron(about 4%),and other compositions(about 1%).The basalt specimens with dimensions of 400 mm × 400 mm × 400 mm were used for both microwave-induced surface and borehole fracturing tests.The diameter and depth of the basalt specimens with boreholes were 90 mm and 250 mm respectively.These specimens were naturally dried in the laboratory for a minimum of seven days before testing.Before conducting the tests,the physico-mechanical properties of the basalt specimens were measured.These properties included an average uniaxial strength of 386 MPa,an average tensile strength of 18 MPa,an average density of 2920 kg/m3,an average P-wave velocity of 6180 m/s,and complex permittivity (ε) at 915 MHz and 2.45 GHz was (8.3 -9) -(1.7 -1.9)i and (6.2 -7) -(0.9 -1.33)i,respectively.The time mode was selected to control the microwave output,and the true triaxial stress remained constant during microwave heating.Under the same heating time and microwave energy (increased at a rate of 2 kW/s),microwave-induced surface fracturing tests were conducted under different microwave frequencies (Table 1).Another microwave source with a maximum power of 15 kW at a frequency of 2.45 GHz was used (Feng et al.,2021).The fracture characteristics of rock specimens under different microwave parameters were compared by acoustic emission(AE),P-wave velocity,and visual observation.

Table 1Test scheme.

4.2.Test results and analysis

4.2.1.Microwave-induced surface fracturing characteristics

4.2.1.1.Heating characteristics.The heating characteristics of the basalt specimens were monitored within a metal cavity in real time,maintaining a distance of 50 mm between the applicator and the rock surface.The highest temperature within the monitoring area served as an indicator of the heating characteristics.Fig.7 shows the heating and temperature distribution characteristics of the basalt specimens under different microwave power levels.Since the microwave power increased gradually,the initial temperature of the rock specimens differed when reaching the rated power.To compare the heating rates of different microwave powers,the initial temperature was set to 16.7°C.The relationship between temperature and irradiation time exhibited a nearly linear trend,with the heating rate gradually increased with increasing power.At a power level of 70 kW,the heating rate reached 4.83°C/s,indicating that the basalt is a highly-sensitive rock.This behavior remained consistent even after the specimens had been dried,as previously observed (Zhang et al.,2022).After the test,an assessment of the temperature distribution across the irradiated surface was determined.The temperature contour displayed a roughly elliptical shape and gradually decreased from the center to the surroundings,while the heated area was related to the size of the aperture surface of the applicator.

Fig.7.Heating and temperature distribution characteristics of the basalt specimens under different microwave powers:(a) Schematic diagram of surface temperature monitoring layout;(b)Heating rates of the basalt specimens under different powers;(c)The temperature distribution on the irradiation surface(30 kW,60 s);(d)The temperature distribution on the side (30 kW,150 s).

To measure the heating depth in the basalt specimens,the applicator was shifted from position 1 to position 2,and the lateral surface was covered with aluminum foil.An auxiliary specimen was placed next to the foil to shield against microwave heating from the side (Fig.7a).After the test,the specimen was promptly removed,and the temperature on the side of the rock specimen was measured using an infrared camera.The penetration depth (the depth is referred to as “skin depth” in certain applications,e.g.by geophysicists)is the depth at which the microwave decays from the surface to an initial power of 1/e (e denotes the base of natural logarithms)(Hassani et al.,2016).For dielectric materials like rocks,the loss factor is significantly smaller than the dielectric constant,allowing for the calculation of microwave penetration depth using the following formula(Hassani et al.,2016;Peng et al.,2010):z=λ0λ0is the wavelength of the microwave at the given frequency(m),ε′represents the dielectric constant of the material,and ε′′stands for the loss factor of the material.The calculated penetration depths at frequencies of 915 MHz and 2.45 GHz are 79.2-92.1 mm and 36.3-57 mm,respectively.As illustrated in Fig.7d,when the highest temperature at the boundary is 225°C,the temperature at the critical depth is 82.8°C and the penetration is 90.7 mm,but the heating depth (i.e.the depth at which the temperature is measurably higher than the initial temperature) is greater than the penetration depth,at about 150.8 mm.

4.2.1.2.Fracturing characteristic.Fig.8a and c shows the basalt specimens after microwave-induced surface fracturing (915 MHz,2.45 GHz).Despite differing microwave conditions,the thermal fracture characteristics remain similar,with varying degrees of fracturing.During testing,the rock specimens spalled several times in a short time,accompanied by loud crackling sounds.After the test,there were rock spalling pits left on the irradiation surface,and interconnected tension cracks formed on theZ-surface and side surface.Longitudinal cracks on theZ-surface are more numerous than the transverse cracks,with more cracks on theY-surface than that on theX-surface.This is related to the amount of stress applied to the surface,where areas with lower stress exhibited greater thermal deformation.P-wave velocity was measured in three directions before and after each test,and the degree of fracturing of a rock specimen fracturing is quantified by the reduction in P-wave velocity.Fig.8b and d demonstrates reductions in P-wave velocity in three directions after microwave-induced surface fracturing(915 MHz,2.45 GHz).The reduction and distribution of P-wave velocity exhibit anisotropic behavior,with the most substantial reduction observed in theZ-direction is the largest.This anisotropy may be related to the magnitude of stress in three directions.The wave velocity in theXandY-directions decreases only within a specific depth range,aligning with the fracture characteristics of the rock.

Fig.8.The basalt specimens and P-wave velocity reduction distribution after microwave-induced surface fracturing:(a,b)70 kW,80 s,915 MHz;and(c,d)15 kW,292 s,2.45 GHz.

Under the same microwave energy output,the degree of fracturing of Specimen II exhibits significantly greater fracturing compared to Specimen I.Many visible cracks occur in Specimen II,forming a complex network of cracks with main crack widths reaching 2-4 mm,but the main visible cracks on the side are concentrated in the range of 0-50 mm.The maximum reduction in wave velocity of the two specimens is 8.7%and 64.8%respectively,and their fracture depths are 160 mm and 70 mm respectively.The aforementioned situation of Specimen II involves excessive fracturing.Extending the heating time of 292 s increases the degree of rock fracturing but has little effect on the range of fracturing.This phenomenon is closely related to the penetration depth of microwave energy in the rock,which indicates that it is not feasible to enlarge the fracture depth by increasing the heating time.Thermal fracturing,induced by microwave heating,occurs when thermal stress is insufficient to overcome its strength,causing fractures to stop extending to greater depths.The unheated region remains elastic,while the process repeated in the heated region,with fracture preferentially propagating along the direction of the original crack width.This time inconsistency poses a challenge for simultaneous rock breaking using microwave pretreatment and TBM.Low-frequency microwaves can enhance the depth of rock breaking and higher power levels can reduce the fracturing time,which can alleviate the aforementioned problems.

The AE activity can reflect the development process of thermal fracture and can determine its critical points of action.During the true triaxial stress loading,a small amount of AE activity,indicates that the rock specimens are always in an elastic state.Due to the continuous spalling in the test,it is difficult to monitor the heating characteristics.Fig.9 shows the AE activity during microwaveinduced surface fracturing,and the fracture process is similar under different microwave parameters.In the early stage,the AE activity shows the characteristic of intermittent surging,and the macroscopic fracture mainly entails rock spalling.After that,continuous AE activity occurs,mainly upon new crack development and crack propagation.In addition,there is a calm period (little to no AE activity) before a violent fracture,similar to the sudden fracture of brittle rock with small deformation.The fracture process of Specimen I is described below.There is almost no AE activity in the first 58 s,indicating that rock fracturing requires a long incubation time.At 58 s,the corresponding temperature is 231.2°C,and the AE activity suddenly increases for the first time,implying that the rock specimens suffer a fracture.Between 60 and 80 s,several intermittent sudden increases in the AE activity,indicate that many thermal cracks are generated.Before 74 s,the cumulative AE event count was 10,107,accounting for only 20.8% of the total events.

Fig.9.The AE activity during microwave-induced surface fracturing: (a) 70 kW,80 s,915 MHz;and (b) 15 kW,292 s,2.45 GHz.

4.2.2.Microwave-induced borehole fracturing characteristics

Microwave-induced borehole fracturing can release elastic strain energy stored in the rock mass and forms a low-stress zone,which can effectively mitigate the risk of rockbursts in deep hard rock excavation.Fig.10a illustrates the basalt specimens after microwave-induced borehole fracturing.After the test,a network of tensile cracks forms on the surface and within the borehole,which is mainly distributed in the depth range of the borehole.The cracks within the borehole connect to the surface cracks,indicating the formation of a fracture plane traversing through the specimens.P-wave velocities were measured at various points of the rock specimens before and after the test.Fig.10b illustrates the distribution of P-wave velocity reduction under the microwave-induced borehole fracturing.The P-wave velocity reduction shows anisotropy,with the most significant reduction occurring in theZ-direction,exceeding that in the other two directions,with a maximum decrease of 12.8%.The P-wave velocity reduction in theX-andYdirections is mainly distributed near the borehole,while in theZdirection,it decreases to varying degrees.This variation in P-wave velocity reduction is closely related to the direction of crack propagation.Cracks on theX-surface andY-surface run quasi-parallel to theZ-surface,significantly affecting the P-wave velocity in theZdirection.The degradation of the rock matrix within the heating range affects the P-wave velocities in theXandY-directions.The reduction in the P-wave velocity in the upper part of theZ-direction is greater than that in the lower part,which is inconsistent with the crack distribution on the Y1 and Y2 surfaces.This is because cracks on the Y1 surface are visible to the naked eye,while those on the Y2 surface are not.This indicates that the crack width has a more pronounced influence on the P-wave velocity reduction than the number of cracks.Under the same conditions,the greater heating depth increases the range of destressing.

Fig.10.Fracture characteristics and wave velocity reduction of the basalt specimens after microwave-induced borehole fracturing:(a)The basalt specimens after fracturing;and(b)P-wave velocity reduction in three directions.

5.Discussion

5.1.Intelligent fracturing of hard rock

Fracturing efficiency is the index chosen to evaluate the value of microwave-assisted mechanical rock breaking.When the heating time is too short,the rock mass remains inadequately fractured(Fig.12a).On the contrary,excessive fracturing (Fig.8c) occurs,leading to a reduction in the utilization rate of microwave energy.Microwave pre-treatment should properly reduce the mechanical properties of the rock mass rather than completely breaking it.Principles from intelligent systems have been applied to many rock engineering scenarios (Zhao et al.,2020).In the context of intelligent fracturing,microwave parameters are dynamically adjusted according to the state of fracturing within the rock mass.Therefore,it becomes essential to determine a reasonable fracturing time for practical application.Fig.11 shows the proportion of the AE activity during each 30-s interval.Overall,the AE activity,which corresponds to fracturing,first increases and then decreases,with the optimum fracturing occurs between 150 and 180 s.The fracture ratio between 30 and 180 s is 78.2%,much higher than that between 180 and 292 s.This finding indicates that the fracture efficiency is much higher in the early stage than in the late stage,with a critical heating time in between.The extent of deterioration in the mechanical properties of the rock in each time interval can be estimated based on the AE activity.

Fig.11.The proportion of the AE activity in each time period (15 kW,292 s).

Fig.12.(a) The basalt specimens after microwave-induced surface fracturing (15 kW,80 s,2.45 GHz);and (b) P-wave velocity reduction distribution (15 kW,80 s,2.45 GHz).

In addition to microwave parameters,the heating is also affected by many other factors,e.g.the distance between the heater and the rock mass,and the reflected power(Hassani et al.,2016).It is difficult to achieve the expected target of microwave fracturing with fixed microwave parameters.It is necessary to determine the state of fracturing within the rock mass,but real-time monitoring and dynamic feedback on the mechanical properties of the rock mass are difficult.The heating rate and temperature are internal factors that drive thermal fracturing of the rock mass.According to engineering conditions,the relationship between heating rate,temperature,and the degree of fracturing of a rock mass can be established,which may reduce its complexity.Therefore,it is feasible to use the heating rate and temperature as feedback parameters.In practical applications,the relation can be integrated into the control system,allowing for timely adjustments to microwave parameters (Fig.5) or the applicator can be moved according to the received dynamic feedback.The microwave system developed in the present work can promote the development of microwave-assisted mechanical rock breaking technology.

5.2.Applicability of microwave frequency

Low-frequency microwaves penetrate deeper through rock materials,but this characteristic can be both an advantage and a disadvantage.Generally,the thermal response of rock-forming minerals to microwaves is different,including strong microwave absorbers,weak microwave absorbers,and non-microwave absorbers (Lu et al.,2017;Wei et al.,2019).In strong microwave absorbers (e.g.basalt),low-frequency microwave irradiation can produce a better fracturing effect and greater fracture depth when more power is applied.Fig.12 shows basalt specimens and the distribution of P-wave velocity reduction after microwave-induced surface fracturing(15 kW,80 s,2.45 GHz).The fracture form of rock specimens is mainly spalling,with a small number of cracks.The spalling pit reaches a depth of 20 mm,while the damaged zone on the side extends to only about 50 mm.The fracture depth is not sufficient for auxiliary mechanical rock breaking.The fracture depth of the rock reaches 160 mm under low-frequency microwave irradiation.In weak microwave absorbers (e.g.granite: ε=5.25 -0.17i (Zheng et al.,2017)),experience excessive penetration depth(about 703.2 mm) with low-frequency microwaves,leading to a lack of concentration of microwave energy and reduced the fracturing effect.The heating effect of high-frequency microwaves in weak microwave absorbers(rocks)may be better under an applied power of 30 kW,which may be more conducive to auxiliary TBM rock breaking.When considering rockburst protection,the fracture range is a crucial factor for destressing.Extending the heating time can potentially enhance the degree of fracturing.Therefore,the applicability of low-frequency microwaves for weak microwave absorbers(rocks)may surpass that of high-frequency microwaves.In summary,the applicability of microwave frequency requires comprehensive consideration of rock microwave sensitivity,fracture depth,and application scenarios.

6.Conclusions

A high-power microwave fracturing test system for hard rock was developed in Northeastern University,China.The key problems of the system composition were described,and preliminary tests were conducted to verify its stability.The following conclusions can be drawn:

(1) By using a parallel resonant circuit and rapid power cutting,the power supply is found to have strong anti-ignition ability;an intelligent impedance tuner was developed by establishing the relationship between the position of three pins and the load impedance.Together with reflected power threshold protection,the problem of the instability and insecurity of the system during the high-power microwave output was solved.

(2) Through the cut-off waveguide and self-designed temperature sensor,the problem that the temperature cannot be measured in a strong microwave field was solved,and active control of microwave parameters was achieved.By reducing contact surfaces,assembling errors and optimizing structure,etc.,the problem of the discharge breakdown between coaxial lines during high-power microwave output was solved.By solving the above problems,the system can adapt to the complex and changeable environment encountered in the field.

(3) Based on the experimental comparisons,high-power microwave irradiation takes less time to fracture hard rock.Specifically,the fracture range (160 mm) of a 915-MHz microwave source is about three times that of 2.45 GHz.After microwave-induced borehole fracturing,many tensile cracks occur on the surface and in the borehole,and the maximum reduction in the P-wave velocity is 12.8%.It shows that a high-power microwave source operating at 915 MHz is more beneficial to auxiliary mechanical rock breaking and destressing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No.41827806)and the Liaoning Revitalization Talent Program of China(Grant No.XLYCYSZX1902).The authors also thank Yuying You,Xinyue Wang,and Dr.Kong Rui for their kind help.