**4. Temperature characteristics**

**Figures 3** and **4** show the temperature distributions on the surface of samples and along their straight lines [44]. As is seen, the surface temperature of samples was non-uniformly distributed, i.e. the longer the irradiation time, the more nonuniform the temperature distribution. A local high-temperature region occurred on the surface of the cylindrical samples. The temperature inside the samples was higher than that on the surface of the samples, and the maximum temperature occurred near the center of the samples. There were two high-temperature zones at the left and right sides of the lower part of the cylinder sample surface. The non-uniformity of temperature distribution led to the non-uniformity of thermal expansion within the samples, which will promote the cracking and fracturing of samples. The influence of irradiation time on the surface temperature of the basalt samples is shown in **Figure 5**. As is seen, after microwave irradiation for 10 s, 20 s, and 30 s, the maximum temperatures were 91.6°C, 164.8°C, and 228.2°C at a microwave power of 5 kW while the average temperatures were 66.6°C, 115.2°C, and

**Figure 4.**

*Temperature distributions along straight lines of samples passing the point with the highest temperature [44]. (a) the upper surface (line a). (b) Cylindrical surface (line B).*

**153**

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

163.4°C, respectively. When the microwave power is constant, the maximum and average temperature on the surface of the samples both linearly increased with the irradiation time (**Figure 5**). The longer the microwave irradiation time, the higher

*Surface temperature vs. irradiation time of* Φ*50 × 100 mm cylindrical basalt specimens [29].*

The surface temperatures of the cylindrical basalt samples increased with the increase of irradiation time at each of the three microwave powers (**Figure 5**) [29]. The surface temperature of the sample increased linearly with the microwave irradiation time, and the heating rate increased with the growth of microwave power. Samples burst at approximately 230°C and 320 s at 1 kW, 210°C and 100 s at 3 kW, and 160°C and 50 s at 5 kW. The broken pieces began to melt with increasing the irradiation time. The higher the microwave power input, the faster the rate of

heating, and the shorter the time needed for the sample to bursting.

cylindrical samples after microwave treatment for 60 s at 3 kW.

the energy needed for the thermal expansion of olivine [29, 39].

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

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

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

Under the conditions of 5 kW power and different microwave irradiation times,

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

the surface temperature of the samples.

**Figure 5.**

**5.1 Microscopic crack propagation**

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

#### **Figure 5.**

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

**Figures 3** and **4** show the temperature distributions on the surface of samples and along their straight lines [44]. As is seen, the surface temperature of samples was non-uniformly distributed, i.e. the longer the irradiation time, the more nonuniform the temperature distribution. A local high-temperature region occurred on the surface of the cylindrical samples. The temperature inside the samples was higher than that on the surface of the samples, and the maximum temperature occurred near the center of the samples. There were two high-temperature zones at the left and right sides of the lower part of the cylinder sample surface. The non-uniformity of temperature distribution led to the non-uniformity of thermal expansion within the samples, which will promote the cracking and fracturing of samples. The influence of irradiation time on the surface temperature of the basalt samples is shown in **Figure 5**. As is seen, after microwave irradiation for 10 s, 20 s, and 30 s, the maximum temperatures were 91.6°C, 164.8°C, and 228.2°C at a microwave power of 5 kW while the average temperatures were 66.6°C, 115.2°C, and

*Temperature distributions along straight lines of samples passing the point with the highest temperature [44].* 

*(a) the upper surface (line a). (b) Cylindrical surface (line B).*

**4. Temperature characteristics**

**152**

**Figure 4.**

*Surface temperature vs. irradiation time of* Φ*50 × 100 mm cylindrical basalt specimens [29].*

163.4°C, respectively. When the microwave power is constant, the maximum and average temperature on the surface of the samples both linearly increased with the irradiation time (**Figure 5**). The longer the microwave irradiation time, the higher the surface temperature of the samples.

The surface temperatures of the cylindrical basalt samples increased with the increase of irradiation time at each of the three microwave powers (**Figure 5**) [29]. The surface temperature of the sample increased linearly with the microwave irradiation time, and the heating rate increased with the growth of microwave power. Samples burst at approximately 230°C and 320 s at 1 kW, 210°C and 100 s at 3 kW, and 160°C and 50 s at 5 kW. The broken pieces began to melt with increasing the irradiation time. The higher the microwave power input, the faster the rate of heating, and the shorter the time needed for the sample to bursting.
