Thermal-breaking characteristics and crack distribution of rock irradiated by laser beams
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摘要: 激光辅助破岩是一种非接触式破岩技术，破岩效果主要受激光功率、照射时间和离焦距离等参数影响。选择石灰岩、砂岩和花岗岩3类岩石，开展了不同照射参数的破岩试验，分析了温度、射孔参数、比能、热裂效能、裂隙分布、矿物组分和微观结构等变化规律。结果表明，激光照射下3类岩石表面温度均超过2000℃、温度梯度最高可达1500℃/mm，相同位置处花岗岩表面温度和温度梯度最高、砂岩次之、石灰岩最低；3类岩石射孔直径、射孔深度和射孔速率均与照射参数密切相关，砂岩、石灰岩和花岗岩最大射孔速率分别为3.18，2.68，0.8 mm/s。相同照射参数下，比能较热裂效能高1~2个数量级，3类岩石比能排序为：花岗岩 > 石灰岩 > 砂岩。照射后岩样均存在数条主裂隙，沿径向延伸至试样边缘，石灰岩和花岗岩次生裂隙发育，砂岩未发现明显次生裂隙。照射后岩石矿物衍射强度和微观结构发生显著改变。Abstract: The laser-assisted rock breaking is a non-contact technology employed for the rock fragmentation, with its efficiency contingent upon the laser power, irradiation time and defocusing distance. To explore the effects of various irradiation parameters, the rock-breaking tests are conducted on three distinct rock types, including limestone, sandstone and granite. The temperature field, perforation parameters, specific energy, thermal-cracking energy, crack distribution, mineral composition and microstructural characteristics are studied. The results demonstrate that the rock surface temperature subjected to laser irradiation experiences a substantial increase, with the maximum temperature and temperature gradient of 2000 oC and 1500 oC/mm, respectively. The granite sample exhibits the highest surface temperature and temperature gradient, followed by sandstone, and the limestone displays the lowest values. The perforation diameter, depth and speed of rocks are closely associated with the corresponding irradiation parameters. The sandstone, limestone and granite exhibit the maximum perforation speeds of 3.18 mm/s, 2.68 mm/s and 0.8 mm/s, respectively. The relationship between specific energy and irradiation parameters mirrors that of thermal-cracking energy under the same irradiation parameters. However, the specific energy values are approximately 1-2 orders of magnitude higher than the corresponding thermal-cracking energy values. The specific energy ranking for the three rock types is as follows: granite > sandstone > limestone. The rock samples exhibit a notable presence of radial cracks that extend to the edges. The limestone and granite samples display extensive development of the secondary cracks, whereas the sandstone shows no significant presence of the secondary cracks. The irradiated rock samples demonstrat evident alterations in both diffraction intensity and microstructures when compared with their corresponding original samples.
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图 2 不同辐射参数下岩石表面径向温度
Figure 2. Radial temperatures of rock surfaces under different irradiation parameters
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图 4 不同照射参数下射孔深度与射孔速率
Figure 4. Hole depths and rates of perforation under different irradiation parameters
表 1 岩样物性参数
Table 1 Physical parameters of rock samples
物性
参数密度/(kg·m-3) 比热容/(J·kg-1·K-1) 导热系数/(W·m-1·K-1) 含水率/% 石灰岩 2714 720 3.06 0.46 砂岩 2661 781 1.78 0.40 花岗岩 2630 714 2.60 0.15 
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表 2 XRF结果
Table 2 XRF results
化学组份 Na2O MgO Al2O3 SiO2 K2O CaO Fe2O3 CO2 其它 石灰岩/% 0.13 2.30 1.17 6.21 0.08 51.23 0.37 38.39 0.12 砂岩/% 3.99 6.16 16.10 50.35 1.25 7.87 11.96 — 2.32 花岗岩/% 3.53 0.69 13.91 68.55 5.16 1.63 2.46 — 4.07
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表 3 试验参数
Table 3 Test parameters
参数设置 激光功率/W 照射时间/s 离焦距离/cm Ⅰ 400/600/800/1000 20 6 Ⅱ 1000 5/10/15/20 6 Ⅲ 1000 20 6/8/10/12 说明：改变激光功率和离焦距离时，花岗岩试样照射时间均为30 s。
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表 4 测试精度和不确定度
Table 4 Test accuracies and uncertainties
变量 温度/℃ 质量/g 长度/mm 体积/cm3 ES, ET, C/(kJ·cm-3) 测试精度 ±2% ±0.01 ±0.02 — — 不确定度 ±2% 2%~5% 0.11% 0.08% 2%~5%
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表 5 热裂效能
Table 5 Thermal-cracking energies
照射参数 热裂效能/(kJ·cm-3) 石灰岩 砂岩* 花岗岩 激光功率/W 400 0.122 1.51 1.628 600 0.103 1.36 1.450 800 0.090 1.08 1.138 1000 0.078 0.68 1.072 照射时间/s 5 — 1.51 — 10 0.039 1.65 0.459 15 0.043 2.04 0.570 20 0.078 3.18 0.838 离焦距离/cm 6 0.078 0.67 0.860 8 0.092 1.02 1.072 10 0.097 1.26 2.490 12 0.137 1.51 2.680 注：*表中砂岩热裂效能等于比能。
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