Auxetic behavior of Beishan granite after thermal treatment: A microcracking perspective

Abstract

Majority of minerals or rocks commonly display a positive Poisson’s ratio between 0.05 and 0.45. However, granites that experience high temperature treatment can exhibit negative Poisson’s ratio under low compressive stresses. The underlying mechanism responsible for the auxetic behavior of thermally-treated granites is poorly understood. Here we present experimental results of uniaxial compression tests on thermally-treated Beishan granite, which show auxetic behavior with increasing thermal treatment temperature and the Poisson’s ratio is as low as −0.18 for a maximum thermal treatment temperature of 650 °C. To reveal the thermally-induced change in the microstructures of Beishan granite, particle discrete element models are built and calibrated using the experimental data. The numerical results show that intensively distributed microcracks appear in the Beishan granite specimens after high temperature treatments. The number and the widths of microcracks increase with increasing thermal treatment temperature. A number of isolated block clusters consisting of mineral grains form due to the connection of thermally-induced microcracks under high temperature thermal treatment. The deformation behavior of these block clusters is primarily responsible for the observed negative Poisson’s ratio of the thermally-treated Beishan granite during compression.