Numerical simulation and experimental investigation on fracture mechanism of granite by laser irradiation

Numerical simulation and experimental tests are conducted to determine the stress distribution and fragment mechanism of granite during laser perforation. The mechanisms of material removal through laser perforation are spallation, melting and ablation. High intensity laser energy concentrates locally on the granite surface area and causes the local temperature to increase instantaneously. The closer the center of laser beam is, the higher the temperature is. With the laser power increasing the size of laser perforation hole is expanded. In addition, the high heating and cooling rates induced by laser irradiation brings out the generation of high tensile stress (481-536 MPa), which is far higher than the tensile strength (11 MPa) of granite sample, and fracture or spallation occurs. Fracture analysis showed the inner wall of the glassy layer after laser perforation is smooth with lots of pores. Moreover, there is a hackle region with regular strips of micron/nano scale at the edge to the glassy layer, and the secondary cracks propagate perpendicular to the regular strips. Compared with the fracture morphology of the glassy layer, the fracture surface of the interfacial transition zone is coarse, and obvious secondary cracks also generate. Obviously, the fracture mechanism of granite faced to laser irradiation is the typical brittle fracture.