Post-failure deformation experiments were carried out for rocks and coal having compressive strength varying from 24 MPa to 366 MPa and analyzed for the two classes of post-failure behavior i.e.Class-1 and Class-II as reported in the literature. All the experiments were conducted under uniaxial compressive stress conditions using an MTS compression testing machine. Depending on the rock type, either lateral strain or displacement control was used as the feedback signal. Post-failure deformation was recorded for all the rocks and coal samples. Based on the post- failure deformation pattern and the fracture mode of the failed samples, three types of post-deformation patterns were identified. Type-1 is characterized by a steep fall in stress with tensile cracks, Type-2 showed a typical pattern of rise and fall in stress with a combination of tensile and shear cracks, while Type-3 showed a gradual decrease of stress with shear cracks. The Class-II failure as reported in the literature was not observed even for rocks having strength of 366 MPa. Based on this study it is inferred that all the rocks may undergo post-failure deformation, the amount of deformation depends on the failure mechanism which is a combination of tensile and shear cracks.
The concept of Class-I and Class-II post-failure behavior was originally proposed by Wawersik and Fairhurst(1,2) to classify the shape of the complete stress-strain curve for a particular rock according to its strain beyond the peak strength. If the strain increases monotonically throughout the failure process, the curve is designated as Class-I, and other curves as Class-II. Class-I behavior is characterized by ‘stable’ fracture propagation. But rock that exhibit Class-II behavior, the failure is unstable and the fracture of rocks cannot be controlled. The dividing line between Class-I and Class- II behavior is defined by the dashed line as shown in figure 1. In the present study six different rocks including coal having strength from 24 MPa to 366 MPa were investigated for the two types of post-failure behavior.
A total of six rock types including coal were investigated, table 1 gives their uniaxial compressive strength, Young's modulus and Poisson's ratio. Except for coal remaining samples were collected in the form of drilled cores. Coal samples were drilled from the blocks perpendicular to bedding plane to obtain NX size cylindrical cores. The diameter of the samples varied from 37 mm to 54 mm. The length to diameter ratio was maintained at 2.0. Samples were cut to the required length and the end faces were ground using surface grinder.
rock mechanics system was used along with their extensometers for conducting post-failure experiments. As reported in the literature (3, 4, 5, and 6) three feedback controls are commonly used for the post-failure tests and they are:
Lateral strain control
Axial strain control
Displacement control or also called as stroke control Load control is not used as a feed back as it produces sudden failure of the sample beyond the peak stress.