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

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.