ANALYSIS OF THE POST-PEAK POISSON'S RATIO OF ROCK SPECIMENS IN UNIAXIAL COMPRESSION

The ratio of lateral strains to axial strains at the strain-softening stage in uniaxial compression, which is termed as the post-peak Poisson's ratio, is studied analytically. It is assumed that plastic deformation stems from the localization of shear strain. After the localization is initiated at peak strength, axial and lateral strains are each decomposed into two parts. One is elastic strain described by elastic mechanics; the other is plastic part determined by gradient-dependent plasticity and geometrical relations. At the strain-softening stage, lateral strain-axial strain relation, lateral strain-axial strain relation, and axial stress-axial strain relation were verified by existing experiments. At the peak strength, the localization is just initiated and the post-peak Poisson's ratio is equal to the Poisson's ratio at the elastic stage. When compressive stress reaches zero, a critical value of the post-peak Poisson's ratio is achieved. Whether the critical value is greater than the Poisson's ratio at the elastic stage or not, the constitutive parameters of rock material, the geometrical size of the specimen and inclination angle of the shear band play an important role. It is possible for linear, upwards concave or upwards convex relations between the post-peak Poisson's ratio and the flow compressive stress. Generally, a post-peak Poisson's ratio depend on the flow compressive stress and is concerned with geometrical size. Therefore, the Poisson's ratio cannot be seen as a material constant, which is different from the Poisson's ratio at the elastic stage.