Detecting meso-damage and subsurface cracks in a hard rock using frequency-modulated thermal wave imaging (FMTWI)

Mines, tunnels, and hillside roadways that are subjected to high levels of stress are prone to massive and violent occurrences of rock failures. It results in a multitude of irreversible consequences, including the loss of human lives. Nevertheless, preceding rock failures, the development of micro and macrocracks, which are sometimes not discernible on the surface, takes place. Subsurface cracks indicate the degradation of rock and can be employed as a means to anticipate occurrences of rock failures and bursts. Therefore, the utilization of subsurface imaging techniques for rocks facilitates the estimation of the true strength of the rock mass. Nevertheless, in many instances, rock masses are not easily reachable, posing difficulties for standard techniques such as ground-penetrating radar or computed tomography (CT) scan imaging, to identify the cracks. Hence, this research endeavours to explore the feasibility of employing frequency-modulated thermal wave imaging (FMTWI) for identifying subsurface cracks and their coalescence in hard rocks through the utilization of numerical simulation and experimental methods. A model was constructed using the finite element method wherein artificial cracks were intentionally introduced into a cylindrical granite specimen based on the CT scan data acquired during the meso-damage analysis. The thermograms obtained were subjected to pre-processing and post-processing techniques, and afterwards compared with the CT scan images. The FMTWI tests were conducted in the laboratory to calibrate and validate the simulation results. The findings derived from the analyses of temperature profiles and thermograms indicate that this particular technology is a promising one and offers several advantages in comparison to alternative methods for detecting micro- or macrocracks in deep mines and tunnels.