Thermal fracture characteristics of granite irradiated by continuous and millisecond-pulsed laser

Laser rock-breaking technology presents promising applications for non-blasting excavation of extremely hard rock. To clarify the thermal fragmentation characteristics of continuous-wave (CW) and millisecond-pulsed laser on granite and compare their applicability in rock drilling, we conducted laser ablation experiments on granite. The physicochemical failure processes of granite were analyzed comparatively. We investigated the temperature rise and cooling characteristics of the granite surface using infrared thermography. An instantaneous heat transfer model for the rock surface under laser irradiation was established, incorporating the nonlinear temperature dependence of thermophysical parameters. Through CT scanning and three-dimensional reconstruction, we comparatively analyzed the morphology, structure, and distribution of internal three-dimensional cracks in granite. The results indicate that: (1) the phase transition damage zone in granite consists of a borehole and a molten layer, with approximate ratios of 4:6 under CW irradiation and 7:3 under millisecond-pulsed laser irradiation. The primary phase destruction mechanisms induced by the two laser types are melting and thermal cracking, respectively. (2) The surface temperature evolution over irradiation time can be divided into three stages: rapid rise, fluctuation decline, and sustained decline. Reducing the duty cycle or increasing the repetition frequency enhances the temperature difference during the fluctuation decline stage, resulting in greater thermal stress and intensified thermal cracking reactions. (3) Calculations based on the instantaneous heat transfer model indicate that for CW laser parameters (<italic>P</italic> = 3 000 W, <italic>t</italic> = 0.04 s), accounting for the nonlinear temperature dependence of thermophysical parameters yields a calculated surface temperature of 1 296.8 ℃, with a 6.99% error compared to the measured value. (4) The instantaneous temperature rise curve under millisecond-pulsed laser irradiation exhibits distinct “pulse heating-intermittent cooling” phenomena, with the initial heating rate under high-duty-cycle conditions significantly exceeding that of low-duty-cycle conditions. (5) As the laser ablation depth increases, the scale of shear cracks outside the phase transition zone enlarges. The damage capability of the CW laser outside the phase transition zone diminishes with increasing specimen depth, whereas that of the millisecond-pulsed laser strengthens with greater depth.