Abstract:

Loading and unloading triaxial tests for coal samples under various confining pressures are conducted. The tests show that when the confining pressure is relatively low, the loading and unloading loci are almost coincident. The coal samples show the characteristics of linear elasticity and brittle failure. While under the relatively higher confining pressures, the stress-strain curve is nonlinear, and the unloading path does not come back along the loading locus any more. Moreover, the residual deformation appears, this shows the plastic characteristics of the coal so as to describe the hardening plasticity quantitatively; a new computing method for the hardening parameter based on its geometric meaning is presented in principal strain space. The accumulated plastic strain is chosen and computed with the test data of 27 MPa confining pressure. This hardening parameter increases with the increase of axial strain, the hardening parameter value computed with this method is higher than that of another traditional method. But the method presented is more rational.

Key words: coal, triaxial test, loading and unloading, plastic hardening