Thermal-breaking characteristics and crack distribution of rock irradiated by laser beam

Laser-assisted rock breaking is a non-contacted technology used to break rock, and its effectiveness is mainly dependent on the laser power, irradiation time, and defocusing distance. Rock-breaking experiments with different irradiation parameters were conducted on three types of rocks, including limestone, sandstone and granite. The temperature field, perforation parameters, specific energy, thermal fracturing efficiency, crack distribution, mineral constituent and microstructures were investigated. The results demonstrated that the surface temperature of the three types of rocks irradiated by laser beam increased significantly and exceeded 2000 ℃, and the maximum temperature gradient could reach up to about 1500 ℃/mm. At the same radius, the granite sample exhibited the highest surface temperature and temperature gradient, followed by sandstone, and the limestone had the lowest values. The perforation diameter, depth and speed of the three types of rocks were closely related to the corresponding irradiation parameters, and the maximum perforation speeds of sandstone, limestone and granite were 3.18 mm/s, 2.68 mm/s and 0.8 mm/s, respectively. The variation of specific energy with the irradiation parameters was similar to that of thermal fracturing efficiency when the same irradiation parameters were applied. However, the value of specific energy was about 1-2 orders of magnitude greater than that of thermal fracturing efficiency. The ranking of specific energy for the three types of rocks was granite > sandstone > limestone. These rock samples displayed several prominent cracks that extended radially to the edge of the samples. Secondary cracks were extensively developed in limestone and granite specimens, while no significant secondary cracks were detected in sandstone. The irradiated rock samples exhibited obvious changes in both diffraction intensity and microstructures compared with the corresponding original specimens.