Experimental Investigation on Hard Rock Breaking with Fiber Laser: Surface Failure Characteristics and PerforatingMechanism

)e rock-breaking characteristics and the influence factors of laser perforation are investigated in this study. A series of fiber laser perforation experiments on basalt, sandstone, and granite were conducted. Experimental measurements of rock failure morphologies and composition tests showed that the main surface features are thermal cracks and glazed layer formed by the melting and condensing of rock in laser perforation. It is also found that higher quartz content could help reduce the glazed degree of rock. Comprehensive results showed that the laser rock perforation is mainly formed by thermal fractures, the decline of molten pool, and the evaporating and splashing of the special melted rock components. )e depth of rock perforation and SE usually increase with the laser irradiation time, while the ROP gradually decreases. With the increase of laser power, the perforation depth and ROP gradually increase, while the SE initially decreases and then increases indicating that there is an optimal power that maximizes perforation efficiency. It is believed that the strength of rock is the main factor affecting laser perforation efficiency, and the lower rock strength resulted in deeper perforation depth, higher ROP, and lower SE.


Introduction
e demand of development of deep mineral resources and underground engineering is increasing fast worldwide, while the traditional drilling technology is difficult to solve the drilling problem in the deep complex strata effectively. In recent years, laser technology has shown very good application prospect in efficient hard rock breaking and become a research hotspot, with the advantages of low cost, high efficiency and reliability in oil and gas reservoirs and geotechnical engineering [1][2][3][4][5][6][7][8][9].
Many scholars have carried out experimental studies on the efficiency and effect factors of laser rock breaking. Graves [10] compared the specific energy (SE) of high-power laser drilling with traditional drilling methods and studied the impacts of rock structure, sample shape, and test conditions on sandstone on the SE. Laser perforation experiments on different types of rock showed that there was an optimal set of laser parameters for each rock to minimize the breaking SE, which was generally located in the peeling zone before melting [11]. Besides, the impacts of laser power, frequency, mode of irradiation, auxiliary gas pressure, characteristics, and the SE of rock were studied experimentally, and the surface cracks of rocks were quantitatively characterized [12][13][14][15][16][17][18].
In order to reveal the mechanism of laser rock breaking, the relevant mechanical properties of rocks after laser irradiation were also studied. Graves and Batarseh et al. studied the impacts of high-power laser on rock strength and mechanical properties, and the results showed that the high temperature increased the porosity and permeability of the rock and reduced the strength of the rock [19,20]. e impacts of complex changes of rock vapor, plasma, and molten pool on rock properties during the laser perforation process were studied with the highspeed video imaged [21]. Jamali and Han et al. carried out an experimental study on the feasibility of the laser heat treatment to weaken the rock structure, and the results showed that the rock strength had obviously weakened after laser irradiation, and the drillability of rock was greatly reduced [22,23]. In addition, the impact on porosity, cracks evolution, and strength performance of rock after laser irradiation was studied by computed tomography (CT) [24]. However, due to the limitation of experimental methods, most studies are limited to the comparative analysis of rock state before and after laser irradiation; there is a lack of the research about the detailed thermal breaking process and the evolution mechanism.
Hu et al. attempted to conduct numerical analysis on the temperature distribution of rock surface after laser irradiation and found that the laser perforation efficiency decreased with the increase of irradiation time [25]. A numerical model for predicting the transient thermal behavior of rock during laser perforation was proposed, and the influence rules of laser power and irradiation time on the SE of volume removal were revealed [26]. Based on numerical simulation, it was found that large thermal stress was generated at the location of sudden temperature change in rock during the perforation process, and the initial zone of microcracks was predicted [27]. Li and Ndeda et al. established the finite element model; the causes of microcracks and the impact of thermal stress on the cracks extending were studied based on the analysis of the thermal stress during the granite perforation [28][29][30]. However, the current numerical simulation studies lack a complete description of the phase transition process of rock under the laser irradiation, which could not systematically reflect the specific process of rock laser perforation.
In summary, current experimental studies mainly focus on the impact of technological parameters on rock breaking efficiency. Furthermore, relevant theoretical studies are limited to the structural and performance changes of rocks before and after laser irradiation, and the complex phase transition process and fracture mechanism of rock have not been studied.
is study carried out laser perforation experiments on basalt, sandstone, and granite, combined with scanning electron microscopy and X-ray diffraction to tests. By analyzing the breaking morphologies of laser perforation surface and comparing the changes of mineral composition before and after laser irradiation, the fracture mechanism of rock surface is summarized. In addition, based on the analysis of the impacts of laser parameters on perforation depth and the whole evolution process of laser perforation formation, the mechanism of rock laser perforation is revealed.

Experiment Procedure
e rock samples are basalt, sandstone, and granite processed with the size of 4.5 mm × 4.5 mm × 3.5 mm and characterized for uniaxial compressive strength, shear strength, and thermal conductivity tests (Table 1). A fiber laser perforator with 50 W is adopted in the experiments ( Figure 1). e fixed defocusing amount is 160 mm, and the radius of irradiation zone is 2 mm. e parameters of the irradiation time and laser power are changed in the laser perforation experiments on three kinds of rocks (Tables 2  and 3).
When the laser begins to act on the rock surface, the perforation area reacts violently with sizzling sounds. e rock gradually melts, accompanied by white smoke and many tiny splashing molten drops, and some white flocculent. After the irradiation, much small rock particles remain around the rock. After completion of laser perforation, the macromorphologies of the perforation and the surrounding zone are observed with a magnifying glass, and the depth of perforation are measured by a depth meter. e micromorphologies of perforation and characteristics of perforation edge cracks are observed by a scanning electron microscope (SEM). In addition, XRD is used to detect the components of the rock samples before and after the experiments.

Failure Mechanisms Contributing to the Rock by Fiber
Laser Irradiation

Failure Characteristics of Basalt by Fiber Laser
Irradiation. e surface morphologies of basalt after laser perforation are shown in Figure 2. e original basalt rock is compact in texture with many pores (Figure 2(b)). A smooth glazed layer is formed on the inner wall of perforation, where there also are many holes with many cracks around and a small number of broken vacuoles in local areas (Figure 2(c)). Besides, perforation edge cracks are also obvious ( Figure 2(d)). XRD test results of basalt samples before and after laser perforation are shown in Table 4. It is shown that there are no new mineral formations; the main reaction during laser perforation is the mutual transformation of feldspar minerals.
According the analysis of experimental results, there are many tight pores in the original rock which seal a small amount of gas. With the rock melting, some of the gas escapes and blows up to be vacuoles, which grow up and break with the heating process. e molten material condenses to form a smooth glazed layer, which remain on some holes and vacuoles. In addition, the thermal stress generated by the intense temperature difference forms stress concentration around the holes, causing the cracks to appear and extend outwards, and then forming a complex cracks net.

Failure Characteristics of Sandstone by Fiber Laser
Irradiation.
e surface morphologies of sandstone after laser perforation are shown in Figure 3. e original sandstone is relatively loose with many loose pores ( Figure 3(b)). A smooth glazed layer is generated on the surface after perforation, and there also are many holes on the inner wall of the perforation (Figure 3(c)). Cracks exist on the surface of the glazed layer and perforation edge ( Figure 3(d)). XRD compositions test results of sandstone samples before and after laser perforation are shown in Table 5. , and a small amount of gas in the original rock pores escape from the glazed pool. Due to the high content of quartz in the sandstone, the viscosity of the molten glaze is reduced, the vacuoles are not easy to appear, and the gas directly escapes, resulting in many small holes in the glazed layer after condensation. Under the concentration of thermal stress around the holes, microcracks appear and extend to perforation edge.

Failure Characteristics of Granite by Fiber Laser
Irradiation.
e surface morphologies of granite after laser perforation are shown in Figure 4. e original granite is relatively compact with fewer primary pores (Figure 4(b)). Compared with basalt and sandstone, granite generates less glazed material which is distributed in local clusters (Figure 4(a)). A certain number of holes can be seen on the glazed clusters (Figure 4(c)). And there are also obvious cracks in the perforation edge (Figure 4(d)). XRD test results of granite samples before and after laser perforation are shown in Table 6.
It can be inferred from the experimental results that the high content of quartz (melting point 1750°C) in the granite reduce the glazed degree, and the holes on the glazed clusters are mainly caused by CO 2 produced by thermal decomposition of calcite (CaCO 3 ) for fewer pores in the original granite. e cracks on the perforation edge are also caused by the thermal stress. e quartz content and glazed degree of basalt, sandstone, and granite in perforation are shown in Figure 5. It is shown that the higher the quartz content in the rock, the lower the glazed degree in the laser perforating. In fact, the melting of quartz needs to absorb a large amount of heat for its high melting point, which increases the difficulty of rock glazing, and the low viscosity of molten quartz also hinders the bonding of the glazed layer. erefore, the higher quartz content in the rock could help to reduce the glazed degree in the perforation.
In summary, the smooth glazed layer and the formed holes and vacuoles are the main surface characteristics after the laser perforation. e glazed layer is mainly formed by rock melting and condensing, while the formed holes and vacuoles are related to the pore structure and mineral compositions of the original rock. Specially, the quartz content has an important impact on the glazed degree of the laser-acted rock, and the increase of quartz content could reduce the glazed degree.

Failure Process of the Rock by Fiber Laser Irradiation.
e formation process of rock surface cracks under laser perforation is shown by Figure 6, which could be summarized into four stages. Initially, the temperature of rock increases gradually after the laser irradiation, and the affected zones could be divided into irradiation zone, transitional zone, and heat-affected zone (Figure 6(a)). en the temperature in the irradiation zone rises sharply to the melting point of the rock, and the rock begins to melt. And a small amount of bubbles appear in the molten pool ( Figure 6(b)), due to the escape of the sealed gases in the original pores and the thermally decomposed gases of special mineral components (silicate, etc.). With the increase of irradiation time, the temperature continues to rise and the molten pool gradually boils, the number of bubbles increases, and bubbles constantly grow and break. e temperature of heat-affected zone further increases, and the area of the transitional zone largens and appears to some microcracks (Figure 6(c)). After the laser irradiation, the molten pool condenses to form a smooth glazed layer where some bubbles remain and partly transform to holes. Meanwhile, the microcracks appear around the holes and  Figure 1: Experimental diagram of fiber laser perforation system.  rapidly extend outward to form a complex cracks net by the thermal stress ( Figure 6(d)).

Effect of the Laser Irradiation Time in Laser
Perforating. e laser perforation surface morphologies of basalt, sandstone, and granite under different irradiation time observed by magnifying glass are shown in Figure 7. It is shown that the perforation depth of basalt, sandstone, and granite gradually deepen with the increase of the irradiation time, and the smooth glazed layer in the hole gradually increases, but the glazed degree in the hole of basalt is significantly higher than sandstone and granite.
As shown in the Figure 8, the rock perforation depth increases with the laser irradiation time. And in the same irradiation time, the perforation depth of granite is the largest, while the basalt is the smallest. is is mainly caused by the difference of rock strength. Higher rock strength means more energy for rock breaking, so the perforation depth is smaller for the harder rock under the same laser energy. Combining with the experimental results of glazed surface, it reveals that the laser perforation depth is also closely related to the glazed degree of the rock, and lower glazed degree results in higher perforation depth. In addition, we also find that granite and sandstone enter the stagnation stage of perforation depth earlier. is is mainly due to the high viscosity and poor thermal conductivity of the molten glaze in granite and sandstone, which influences the further conduction of laser heat into the rock. Furthermore, the higher quartz content in the granite and sandstone improves the heat absorption, which induces them into the earlier depth stagnation.
As we know, specific energy (SE) is an important parameter during laser perforation, which is defined as the amount of energy required to remove a unit volume of rock [11,14]: where E is the input energy and V is the removed volume of the rock sample.
where t is the laser irradiation time, and W is the laser power. e relationship between specific energy (SE) and laser irradiation time is shown in Figure 9. It is showed that the specific energy increases with the irradiation time. e reason is that more glazed layer is produced with the irradiation time, which prevents the laser energy further transmitting to the perforation bottom, and more energy is required for breaking same volume of rock. In addition, under the same irradiation time, the specific energy of basalt is higher than that of sandstone and granite; this is mainly due to the higher strength and of basalt, which requires more energy for breaking.
Another parameter introduced during laser perforation is the rate of penetration (ROP), which contributes to determining the efficiency of laser perforating and calculating cost [11,14]. ROP is quantified as where h and t are the depth of laser perforation and the laser irradiation time, respectively. Figure 10 exhibits the correlation between laser perforation rate and irradiation time. It can be seen that ROP decreases gradually with the extension of laser irradiation time. is is because the rock gradually melts and produces more glazed layer, which blocks the laser energy transfer and causes a considerable ROP reduction. Furthermore, it is also shown that ROP declines rapidly in the early stage and then decline slows down. At the beginning of perforation, the laser irradiates the rock directly and most laser energy is absorbed by the rock to induce a high perforation rate. en, more rock is melted to be glazed which hinders the transmission of laser energy. erefore, ROP decreases rapidly in the early stage, while after the glazed layer forming, the heat absorption tends to be stable, and ROP decreases slowly. In  Advances in Civil Engineering   addition, it is also shown that ROP of granite is higher than that of sandstone and basalt, which is mainly due to its lower strength.

Effect of the Laser Power in Laser
Perforating. e morphologies of basalt, sandstone, and granite under different laser power are shown in Figure 11. When the laser power is low (<30 W), the perforation is shallow, the shape is irregular, and the glazed layer is hardly produced. With the increase of the laser power, the perforation becomes deeper, and the glazed layer in the hole increases more obviously. Figure 12 shows that both the laser perforation depth and the ROP increase with the laser power. e greater the laser energy, the higher the breaking rate of the rock and the larger the crushing volume. In addition, it is shown that the perforation depth of basalt is lower than that of sandstone and granite. It is mainly due to the higher strength of basalt, which required more laser power for breaking. Moreover, the higher content of quartz in sandstone and granite with lower glazed degree causes less hindering effects to the transmission of laser energy by the glazed layer; thus, the ROP and perforation depth of sandstone and granite is higher than that of basalt. e correlation between SE and laser power is shown in the Figure 13. With the increase of laser power, SE initially decreases and then increases with a V-shaped distribution. At lower power, the rock temperature is not enough to make the rock melt, and the main destructional form of the rock is thermal stress failure, so the energy utilization rate is high,  and SE gradually decreases. When the power increases to a certain value, the rock begins to melt and produces glazed layer, which hinders the laser energy transmission, and the decrease of energy utilization leads to the increase of SE. It can be inferred from the "V" pattern that there exists an optimal power to maximize the perforation efficiency.

Evolutionary Process of the Rock Perforation by Fiber
Laser. According to the above analysis of experiment results, the formation process of rock laser perforation is shown in Figure 14, which can be summarized as four stages. Initially, high temperature on the rock surface is generated under the laser irradiation, which gradually conducts to the inside to form a heat-affected zone (Figure 14(a)). Laser irradiates the rock continuously, and the rock melts gradually. During the melting, some original pores are filled, then the molten pool drops, and the perforation is preliminarily formed (Figure 14(b)). e molten pool is boiling under the continuous irradiation of laser, where the sealed gas in the

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( Figure 14(c)). e molten pool condenses when the laser irradiation is completed. e rock volume shrinks, and the bubbles in the molten pool expand, which coagulate to remain some slightly larger holes or vacuoles in glazed layer. Cracks appear around the holes and extend to outwards by the thermal stress. A divergent cracks net is formed around the glazed layer after the condensation (Figure 14(d)).

Conclusions
Fiber laser rock perforation experiments on basalt, sandstone, and granite were conducted. Based on the analysis of rock surface morphologies and composition tests, the surface failure characteristics of the lased rock are summarized. Based on the analysis of the perforation depth test results and hole characteristics under different laser parameters, the mechanism of rock laser perforation is also studied. e following conclusions could be drawn: (1) e main features of rock failure in fiber laser perforation are thermal cracks and glazed layer. e surface cracks of the lased rock are mainly generated by thermal stress for the large temperature difference, and the glazed layer is mainly formed by rock melting and condensing. e holes and vacuoles in the glazed layer are related to the escape of the sealed gases in the original pores and the pyrolytic gases by special mineral components. It is also found higher quartz content could help reduce the glazed degree of rock. Comprehensive results showed that the laser rock perforation is mainly formed by thermal fractures, the decline of molten pool, and the evaporating and splashing of the special melted rock components. (2) e depth of rock perforation and SE (specific energy) usually increase with the laser irradiation time, while the ROP (rate of penetration) gradually decreases. With the increase of laser power, the perforation depth and ROP gradually increase, while the SE shows a V-shaped profile which initially decreases and then increases. erefore, there is an optimal power for each rock type that maximizes perforation efficiency. (3) e strength of rock is the main factor affecting laser perforation efficiency. Lower rock strength results in higher ROP, deeper perforation depth, and lower SE. In addition, the glazed degree also influences the perforation depth and ROP of the lased rock. e higher quartz content in the rock could reduce the overall glazed degree of the perforation and then help to improve the perforating.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding the publication of this paper.