Comparative experimental investigation of direct tension and Brazilian splitting behaviors of thermally-damaged crystalline rocks: unique negative poisson's ratio effect

Typically, a rock's direct tensile strength (DTS) is lower than its Brazilian splitting tensile strength (BTS). This disparity becomes more pronounced when subjected to thermal damage. However, a comparative and in-depth analysis of the mechanisms underlying these disparities remains lacking. We experimentally investigated the direct tensile and Brazilian splitting behaviors of crystalline rocks exposed to different heat-treated temperatures. The tensile curve of dog-bone shaped samples showed declines in peak stress, slope, and failure displacement with increasing heat-treated temperature. The disc-shaped sample under Brazilian splitting exhibits similar curve trends, but with increased failure displacement and greater curve nonlinearity. DTS decreases exponentially with increasing heat-treated temperature, while BTS first slightly drops and then significantly declines. Notably, thermally-damaged crystalline rock exhibits a negative Poisson's ratio, causing dog-bone shaped samples to extend laterally under tension and disc-shaped samples to contract bilaterally near its compressive zones. Both phenomena, especially the latter, counteract tensile strength reduction due to thermal damage. With varying heat-treated temperatures, the DTS/BTS ratio decreases exponentially, and the difference between BTS and DTS follows a Gaussian distribution. We shed light on distinct NPR effects in thermally-damaged crystalline rocks under direct tension and Brazilian splitting, clarifying their impact on DTS, BTS, and their disparities.