**5. Effect of microwave heating on crack propagation of rock samples**

#### **5.1 Microscopic crack propagation**

Under the conditions of 5 kW power and different microwave irradiation times, the crack propagation on the surface of the 50 mm cubic samples was observed with an ultra-depth field microscope, as shown in **Figure 6**. Compared with the untreated cubic samples, more intragranular and intergranular cracks were observed around and along the olivine granules. With the increase of microwave irradiation time, cracks became wider and more pronounced. Relative to untreated samples, intergranular and intragranular microscopic cracks occurred within cylindrical samples after microwave treatment for 60 s at 3 kW.

After microwave irradiation, more intergranular and transgranular cracks occurred within the basalt samples. In particular, the intergranular cracks mainly occurred between plagioclase and olivine, while the intragranular cracks mainly occurred within the olivine grains. With the increase of intergranular cracks and intragranular cracks, macroscopic cracks mainly occurred across the area where olivine and enstatite grains gathered. This is because enstatite provides the energy needed for the thermal expansion of olivine [29, 39].

**155**

**Figure 7.**

**6.1 Mechanical strength**

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

power, the length and number of cracks increased gradually. Microwave power and irradiation time are important parameters affecting crack generation. The higher the microwave power, the shorter the time needed to generate cracks of the same degree.

*Typical pattern of macroscopic crack propagation on the surface of cylindrical specimens induced by* 

*microwave irradiation at three power levels and seven exposure times [29].*

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

**6. Effect of microwave heating on mechanical properties of rock**

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

#### **Figure 6.**

*Microscopic crack propagation on the surface of 50 mm cubic basalt specimens viewed under an ultra-depth of field microscope at 300× magnification before (left) and after (right) microwave treatment at 5 kW power and three exposure times [39].*

#### **5.2 Macroscopic crack propagation**

The typical pattern of macroscopic crack propagation on the surface of the cylindrical samples is shown in **Figure 7** [29]. It can be seen that the macroscopic cracks of the microwave-treated cylindrical samples propagated on the two end surfaces and the cylindrical curved surface of the cylindrical samples. The main crack on the cylinder was approximately parallel to the bus of the cylinder and connected to the crack on the end surfaces. After a long time of microwave irradiation, about three cracks propagated on the end surfaces of the sample, converging to a point near the center of the circle. The sample with low microwave power or short irradiation time had less crack propagation. With the increase of microwave irradiation time or microwave

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

**Figure 7.**

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

**154**

**Figure 6.**

*three exposure times [39].*

**5.2 Macroscopic crack propagation**

The typical pattern of macroscopic crack propagation on the surface of the cylindrical samples is shown in **Figure 7** [29]. It can be seen that the macroscopic cracks of the microwave-treated cylindrical samples propagated on the two end surfaces and the cylindrical curved surface of the cylindrical samples. The main crack on the cylinder was approximately parallel to the bus of the cylinder and connected to the crack on the end surfaces. After a long time of microwave irradiation, about three cracks propagated on the end surfaces of the sample, converging to a point near the center of the circle. The sample with low microwave power or short irradiation time had less crack propagation. With the increase of microwave irradiation time or microwave

*Microscopic crack propagation on the surface of 50 mm cubic basalt specimens viewed under an ultra-depth of field microscope at 300× magnification before (left) and after (right) microwave treatment at 5 kW power and*  *Typical pattern of macroscopic crack propagation on the surface of cylindrical specimens induced by microwave irradiation at three power levels and seven exposure times [29].*

power, the length and number of cracks increased gradually. Microwave power and irradiation time are important parameters affecting crack generation. The higher the microwave power, the shorter the time needed to generate cracks of the same degree.
