A New Perspective Based on Overcoming Sample Heterogeneity for the Estimation of Thermal Damage Inflicted on Volcanic Rocks Using Non-destructive Tests

Studies investigating the effect of high temperatures on physical and strength properties of natural rocks face the problem of sample heterogeneity even if the samples are prepared from the same block. The general approach available in the literature involves using different groups of samples for each temperature level during heat treatment and testing. In this study, a different approach where entire heat treatment and testing is conducted on the same samples was introduced. Disc-shaped samples were prepared from three volcanic rocks, and six groups containing five samples were created. One group from each rock type were treated at temperatures of 200, 350, 500, and 680 °C in successive steps. Two non-destructive tests (NDT), Leeb hardness, and P-wave velocity measurements, were conducted on the samples after each heat treatment step. After the last heat treatment step, ultimate tensile strengths of the samples were determined through Brazilian tests. Five groups from each rock type were tested according to the general approach. Results indicate that the suggested approach better represents the thermal damage inflicted on the samples because irrelevant and inconsistent data caused by sample heterogeneity is not present. Evaluation of thermal treatment coefficients, scanning electron microscope images, and X-ray diffractometers indicated that the heating–cooling cycles applied during the suggested approach does not cause significant exhaustion on the samples. An estimation model was established to estimate the thermal damage inflicted on the samples using NDT. Tensile strength of the samples for a desired temperature can also be predicted by the suggested estimation model.