**6.1 Mechanical strength**

After microwave irradiation, the uniaxial compression strength decreases with the increase of microwave irradiation time at three microwave powers (1 kW, 3 kW, and 5 kW). The uniaxial compression strength decreases in an approximately linear manner with increasing microwave irradiation time. The greater the applied microwave power, the faster the decrease rate of uniaxial

compression strength [29]. Pyroxene is a highly microwave-absorbing mineral and has strong heating ability after microwave irradiation. Olivine is a strong thermal expansion mineral, which can produce strong thermal expansion under high temperatures. As a result, transgranular cracks mainly occurred in olivine particles, and intergranular cracks mainly occurred between olivines and plagioclases. With the increase of microwave irradiation time, the microcracks slip and connect, forming weak planes in the rocks. Compared with the rocks not irradiated by microwave, the strength of the rocks decreased to different extents. The longer the microwave irradiation time or the higher the microwave power, the more developed the weak surface and the greater the strength reduction.

**Figure 8** shows the relationships between the mechanical strength of a basalt and microwave irradiation time for different microwave power levels. As illustrated in **Figure 8(a-c)**, the uniaxial compression strength (a), the Brazilian tensile strength (b), and the point load strength (c) decreased with irradiation time at each power level. Overall, the magnitude of the reductions was least for uniaxial compression strength, and most for point load strength. According to the slope of the fitting curves, the higher the microwave power, the faster the rock strength decreased. For example, the Brazilian tensile strength was reduced by approximately 39% at 1 kW power after 300 s irradiation, 37% at 3 kW power after 90 s irradiation, and 46% at 5 kW power after 30 s irradiation. Similarly, the uniaxial compression strength was reduced by less than 11% at 3 kW power and by 27% at 5 kW power after 30 s irradiation. Under the three microwave powers, Uniaxial compressive, Brazilian tensile, and point load strength were reduced by up to 37%, 46% and 62% respectively. Microwave power and irradiation time are important parameters affecting basalt strength, and the reduction of strength has an approximately linear relationship with the irradiation time at each power level.

At the applied power of 5 kW, the relationship between conventional triaxial compressive strength of basalts and microwave irradiation time is shown in **Figure 9** [45]. Under four confining pressures (σ3 = 0 MPa, 10 MPa, 30 MPa, and 50 MPa), the conventional triaxial compressive strength reduces at different rates with increasing microwave irradiation time. At 30 s irradiation, the conventional triaxial compressive strength reduces by 27%, 7%, 2%, and 1% under the four confining pressures, respectively. It is worth noting that, with the increase in confining pressure, the conventional triaxial compressive strength of the basalts gradually reduces (The test value has a certain discreteness, and the overall effect is reduced.). Confining pressure closes part of the cracks induced by microwave irradiation and increases the frictional forces that prevent slippage of microcracks. Therefore, confining pressure inhibits the reduction of conventional triaxial compressive strength and discreteness. The discreteness of conventional triaxial compressive strength is mainly caused by the heterogeneity of microcracks, while the confining pressure leads to the closure of microcracks in the samples and reduction of microcracks that produce slippage. Therefore, the confining pressure can decrease the discreteness of conventional triaxial compressive strength. Geological factors such as in situ stress should be considered when microwaveinduced fracturing is used in underground geotechnical engineering [45].

Due to rocks are heterogeneous materials, differences between samples can lead to the discreteness of test results. The existence of microwave sensitive minerals and strong thermal expansion minerals results in the random distribution of microcracks in different directions within the rocks after microwave irradiation. After microwave irradiation, microcracks occurred within the rock, which can further increase the dispersion of the test results. The higher the microwave power

**157**

**Figure 8.**

*Experimental Investigation on the Effect of Microwave Heating on Rock Cracking…*

*Relationships between mechanical strength of basalt and microwave irradiation time for three power levels: (a) uniaxial compressive strength, (b) Brazilian tensile strength, and (c) point load strength [29].*

*DOI: http://dx.doi.org/10.5772/intechopen.95436*

*Experimental Investigation on the Effect of Microwave Heating on Rock Cracking… DOI: http://dx.doi.org/10.5772/intechopen.95436*

#### **Figure 8.**

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

compression strength [29]. Pyroxene is a highly microwave-absorbing mineral and has strong heating ability after microwave irradiation. Olivine is a strong thermal expansion mineral, which can produce strong thermal expansion under high temperatures. As a result, transgranular cracks mainly occurred in olivine particles, and intergranular cracks mainly occurred between olivines and plagioclases. With the increase of microwave irradiation time, the microcracks slip and connect, forming weak planes in the rocks. Compared with the rocks not irradiated by microwave, the strength of the rocks decreased to different extents. The longer the microwave irradiation time or the higher the microwave power, the more developed the weak surface and the greater the strength reduction.

**Figure 8** shows the relationships between the mechanical strength of a basalt and microwave irradiation time for different microwave power levels. As illustrated in **Figure 8(a-c)**, the uniaxial compression strength (a), the Brazilian tensile strength (b), and the point load strength (c) decreased with irradiation time at each power level. Overall, the magnitude of the reductions was least for uniaxial compression strength, and most for point load strength. According to the slope of the fitting curves, the higher the microwave power, the faster the rock strength decreased. For example, the Brazilian tensile strength was reduced by approximately 39% at 1 kW power after 300 s irradiation, 37% at 3 kW power after 90 s irradiation, and 46% at 5 kW power after 30 s irradiation. Similarly, the uniaxial compression strength was reduced by less than 11% at 3 kW power and by 27% at 5 kW power after 30 s irradiation. Under the three microwave powers, Uniaxial compressive, Brazilian tensile, and point load strength were reduced by up to 37%, 46% and 62% respectively. Microwave power and irradiation time are important parameters affecting basalt strength, and the reduction of strength has an approxi-

mately linear relationship with the irradiation time at each power level.

compressive strength of basalts and microwave irradiation time is shown in **Figure 9** [45]. Under four confining pressures (σ3 = 0 MPa, 10 MPa, 30 MPa, and 50 MPa), the conventional triaxial compressive strength reduces at different rates with increasing microwave irradiation time. At 30 s irradiation, the conventional triaxial compressive strength reduces by 27%, 7%, 2%, and 1% under the four confining pressures, respectively. It is worth noting that, with the increase in confining pressure, the conventional triaxial compressive strength of the basalts gradually reduces (The test value has a certain discreteness, and the overall effect is reduced.). Confining pressure closes part of the cracks induced by microwave irradiation and increases the frictional forces that prevent slippage of microcracks. Therefore, confining pressure inhibits the reduction of conventional triaxial compressive strength and discreteness. The discreteness of conventional triaxial compressive strength is mainly caused by the heterogeneity of microcracks, while the confining pressure leads to the closure of microcracks in the samples and reduction of microcracks that produce slippage. Therefore, the confining pressure can decrease the discreteness of conventional triaxial compressive strength. Geological factors such as in situ stress should be considered when microwaveinduced fracturing is used in underground geotechnical engineering [45].

At the applied power of 5 kW, the relationship between conventional triaxial

Due to rocks are heterogeneous materials, differences between samples can lead to the discreteness of test results. The existence of microwave sensitive minerals and strong thermal expansion minerals results in the random distribution of microcracks in different directions within the rocks after microwave irradiation. After microwave irradiation, microcracks occurred within the rock, which can further increase the dispersion of the test results. The higher the microwave power

**156**

*Relationships between mechanical strength of basalt and microwave irradiation time for three power levels: (a) uniaxial compressive strength, (b) Brazilian tensile strength, and (c) point load strength [29].*

**Figure 9.** *Reductions in compression strength of basalts under different confining pressures [45].*

and the longer the irradiation time, the more discreteness the test results will be. When significant crack propagation or weak surface occurs in the rock, the bearing capacity of the rock will be significantly reduced, leading to the decrease of the rock strength. It is revealed that the confining pressure inhibits discreteness of basalt strength and the strength differences induced by microcracking gradually decrease with increasing confining pressure.

### **6.2 Elastic modulus and Poisson's ratio**

The average elastic modulus and Poisson's ratio are calculated by linear fitting of the stress–strain curve. The elastic modulus and Poisson's ratio are calculated by linear fitting of the stress–strain curve, and the calculation results are shown in **Figures 10** and **11**. The basalt has a compact structure and the average elastic modulus before microwave irradiation is 97 GPa. After microwave irradiation,

**Figure 10.** *Relationships between elastic modulus of basalts and microwave exposure time under uniaxial compression [45].*

**159**

after microwave treatment.

Poisson's ratio.

**Figure 11.**

resistance of the rocks to elastic deformation.

**6.3 Cohesion and internal friction angle**

*Experimental Investigation on the Effect of Microwave Heating on Rock Cracking…*

the elastic modulus and Poisson's ratio both decrease at three power levels (1 kW, 3 kW, and 5 kW). The decrease rate of elastic modulus is greater than Poisson's ratio at the three power levels, and elastic moduli are decreased by 22%, 25%, and 22%, while the Poisson's ratios decrease by 11%, 19%, and 22%, respectively. The elastic modulus and Poisson's ratio decrease linearly with the microwave irradiation time. According to the slopes of the fitting curves, it is known that the higher the microwave power, the greater the degree of reduction of elastic modulus and

*Relationships between Possion's ratio of basalts and microwave exposure time under uniaxial compression [45].*

After microwave treatment, the decrease of elastic modulus indicates that microwave irradiation can reduce the stiffness of rocks, that is to say, it can reduce the bearing capacity of rocks. As is known, in the compression test, the elastic deformation of rocks is mainly determined by the skeleton of mineral particles. Transgranular fracture and intergranular cracks can be induced in and between mineral particles by microwave treatment. With the increase and penetration of these microcracks, new cracks will be formed. Therefore, microwave irradiation changes the skeleton structure of mineral particles in the rocks and weakens the

In the field of rock mechanics, cohesion refers to the attraction between molecules on the surface of adjacent mineral particles. After microwave treatment, the peak strength of the samples at different irradiation times presented monotonically increasing relation with confining pressure, which was in accordance with the Coulomb strength criterion. The cohesion *c* and internal friction angle φ of basalt samples were calculated according to the Coulomb strength criterion (**Table 1** and **Figure 12**). After microwave treatment, the cohesion of the samples decreased linearly with the increase of microwave irradiation time. When the microwave power is 5 kW and the microwave irradiation time is 10 s, 20 s, and 30 s, the cohesion decreases by 14%, 13%, and 25% respectively. After microwave treatment, transgranular and intergranular cracks were generated within and between mineral particles, which reduced the cementation between mineral particles and thus reduced the overall cohesion. While the internal friction angle increases slightly

*DOI: http://dx.doi.org/10.5772/intechopen.95436*

*Experimental Investigation on the Effect of Microwave Heating on Rock Cracking… DOI: http://dx.doi.org/10.5772/intechopen.95436*

#### **Figure 11.**

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

and the longer the irradiation time, the more discreteness the test results will be. When significant crack propagation or weak surface occurs in the rock, the bearing capacity of the rock will be significantly reduced, leading to the decrease of the rock strength. It is revealed that the confining pressure inhibits discreteness of basalt strength and the strength differences induced by microcracking gradually decrease

*Reductions in compression strength of basalts under different confining pressures [45].*

The average elastic modulus and Poisson's ratio are calculated by linear fitting of the stress–strain curve. The elastic modulus and Poisson's ratio are calculated by linear fitting of the stress–strain curve, and the calculation results are shown in **Figures 10** and **11**. The basalt has a compact structure and the average elastic modulus before microwave irradiation is 97 GPa. After microwave irradiation,

*Relationships between elastic modulus of basalts and microwave exposure time under uniaxial compression [45].*

with increasing confining pressure.

**Figure 9.**

**6.2 Elastic modulus and Poisson's ratio**

**158**

**Figure 10.**

*Relationships between Possion's ratio of basalts and microwave exposure time under uniaxial compression [45].*

the elastic modulus and Poisson's ratio both decrease at three power levels (1 kW, 3 kW, and 5 kW). The decrease rate of elastic modulus is greater than Poisson's ratio at the three power levels, and elastic moduli are decreased by 22%, 25%, and 22%, while the Poisson's ratios decrease by 11%, 19%, and 22%, respectively. The elastic modulus and Poisson's ratio decrease linearly with the microwave irradiation time. According to the slopes of the fitting curves, it is known that the higher the microwave power, the greater the degree of reduction of elastic modulus and Poisson's ratio.

After microwave treatment, the decrease of elastic modulus indicates that microwave irradiation can reduce the stiffness of rocks, that is to say, it can reduce the bearing capacity of rocks. As is known, in the compression test, the elastic deformation of rocks is mainly determined by the skeleton of mineral particles. Transgranular fracture and intergranular cracks can be induced in and between mineral particles by microwave treatment. With the increase and penetration of these microcracks, new cracks will be formed. Therefore, microwave irradiation changes the skeleton structure of mineral particles in the rocks and weakens the resistance of the rocks to elastic deformation.

#### **6.3 Cohesion and internal friction angle**

In the field of rock mechanics, cohesion refers to the attraction between molecules on the surface of adjacent mineral particles. After microwave treatment, the peak strength of the samples at different irradiation times presented monotonically increasing relation with confining pressure, which was in accordance with the Coulomb strength criterion. The cohesion *c* and internal friction angle φ of basalt samples were calculated according to the Coulomb strength criterion (**Table 1** and **Figure 12**). After microwave treatment, the cohesion of the samples decreased linearly with the increase of microwave irradiation time. When the microwave power is 5 kW and the microwave irradiation time is 10 s, 20 s, and 30 s, the cohesion decreases by 14%, 13%, and 25% respectively. After microwave treatment, transgranular and intergranular cracks were generated within and between mineral particles, which reduced the cementation between mineral particles and thus reduced the overall cohesion. While the internal friction angle increases slightly after microwave treatment.


#### **Table 1.**

*Conventional triaxial compressive strength test results of basalt samples at different exposure times [45].*

**Figure 12.**

*Relationships between cohesion and internal friction angle of the basalts with microwave exposure time [45].*

## **7. Summary and conclusions**

Microwave power level had a significant positive relationship with basalt heating rate: the higher the applied power, the faster the basalt temperature rises. The surface temperature of the sample is not uniformly distributed, which is conducive to the generation of cracks. The higher the microwave power, the more serious the crack propagation. As the number of fractures increased, visible cracks were generated, leading to significant strength reduction. The Uniaxial compressive, Brazilian tensile, and point load strength all decreased after irradiation at each of the three

**161**

*Experimental Investigation on the Effect of Microwave Heating on Rock Cracking…*

power levels; the higher the power level, the faster the strength decreased. Under the three microwave powers, Uniaxial compressive, Brazilian tensile, and point load strength were reduced by up to 37%, 46%, and 62% respectively. The conventional triaxial compressive strength decreased linearly with microwave irradiation time, and the higher the confining pressure, the smaller the reduction in the strength of basalt samples after microwave treatment. At 30 s irradiation, the conventional triaxial compressive strength reduces by 27%, 7%, 2%, and 1% under the four confining pressures (σ3 = 0 MPa, 10 MPa, 30 MPa, and 50 MPa), respectively.

The elastic modulus and Poisson's ratio of basalts also decreased in a quasi-linear

Uniaxial compressive, Brazilian tensile, and point load strength are important parameters that affect the service life and the penetration of mechanical rockbreaking tools. The reduction of rock strength can increase the service life and the penetration of mechanical rock-breaking tools. Microwave irradiation weakens the mechanical properties of rock, which effectively reduces the resistance of the rock to the mechanical rock-breaking tool, which can reduce the wear of the mechanical rock-breaking tool and improve the rock-breaking efficiency. Microwave-assisted rock-breaking has significant potential application to in-situ mining, tunneling,

Financial support for this work by the National Natural Science Foundation of China (No: 42002281), by the Natural Science Foundation of Henan Province (No: 202300410002, 212300410325), by the Science and Technology Research and Development Plan of China Railway Group Limited (No: 2020-Zhongda-06) and the Science and Technology Innovation Plan of China Railway Tunnel Group

manner with elapsed microwave irradiation time. At the three applied powers, the decrease rate in elastic modulus always exceeds that of the Poisson's ratio, and elastic moduli are decreased by 22%, 25%, and 22%, while the Poisson's ratios decrease by 11%, 19%, and 22%, respectively. The confining pressure results in the closure of the microcracks caused by microwave irradiation, so the influence of microwave treatment on strength and deformation is reduced, leading to a decrease in the influence on the elastic constants. The cohesion decreases with the increase of microwave irradiation time and presents an approximate linear decreasing relationship. At microwave power of 5 kW and irradiation times of 10 s, 20 s, and 30 s, the cohesion is reduced by 14%, 13%, and 25%, respectively. In the basalt samples, new microcracks in various directions generated by microwave irradiation can increase the discreteness of test results, while the discreteness of test results caused by

microcracks gradually reduces with increasing confining pressure.

rock breakage, and comminution.

(No: Suiyanhe 2020-11) are greatly appreciated.

**Acknowledgements**

*DOI: http://dx.doi.org/10.5772/intechopen.95436*

#### *Experimental Investigation on the Effect of Microwave Heating on Rock Cracking… DOI: http://dx.doi.org/10.5772/intechopen.95436*

power levels; the higher the power level, the faster the strength decreased. Under the three microwave powers, Uniaxial compressive, Brazilian tensile, and point load strength were reduced by up to 37%, 46%, and 62% respectively. The conventional triaxial compressive strength decreased linearly with microwave irradiation time, and the higher the confining pressure, the smaller the reduction in the strength of basalt samples after microwave treatment. At 30 s irradiation, the conventional triaxial compressive strength reduces by 27%, 7%, 2%, and 1% under the four confining pressures (σ3 = 0 MPa, 10 MPa, 30 MPa, and 50 MPa), respectively.

The elastic modulus and Poisson's ratio of basalts also decreased in a quasi-linear manner with elapsed microwave irradiation time. At the three applied powers, the decrease rate in elastic modulus always exceeds that of the Poisson's ratio, and elastic moduli are decreased by 22%, 25%, and 22%, while the Poisson's ratios decrease by 11%, 19%, and 22%, respectively. The confining pressure results in the closure of the microcracks caused by microwave irradiation, so the influence of microwave treatment on strength and deformation is reduced, leading to a decrease in the influence on the elastic constants. The cohesion decreases with the increase of microwave irradiation time and presents an approximate linear decreasing relationship. At microwave power of 5 kW and irradiation times of 10 s, 20 s, and 30 s, the cohesion is reduced by 14%, 13%, and 25%, respectively. In the basalt samples, new microcracks in various directions generated by microwave irradiation can increase the discreteness of test results, while the discreteness of test results caused by microcracks gradually reduces with increasing confining pressure.

Uniaxial compressive, Brazilian tensile, and point load strength are important parameters that affect the service life and the penetration of mechanical rockbreaking tools. The reduction of rock strength can increase the service life and the penetration of mechanical rock-breaking tools. Microwave irradiation weakens the mechanical properties of rock, which effectively reduces the resistance of the rock to the mechanical rock-breaking tool, which can reduce the wear of the mechanical rock-breaking tool and improve the rock-breaking efficiency. Microwave-assisted rock-breaking has significant potential application to in-situ mining, tunneling, rock breakage, and comminution.
