Comparative experimental investigation of direct tension and Brazilian splitting behaviours 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. The authors experimentally investigate the direct tensile and Brazilian splitting behaviours of crystalline rocks exposed to different heat-treated temperatures. The tensile curve of dog-bone-shaped samples shows 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 non-linearity. 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 their 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. The authors shed light on distinct negative Poisson’s ratio effects in thermally damaged crystalline rocks under direct tension and Brazilian splitting, clarifying their impact on DTS and BTS, and their disparities.