It is observed that the complete stress-strain curve consists of three parts, i.e. the pre-peak part, the negative slope part and the residual part of the post-peak region. In the curve, the maximum stress that the rock sample can sustain indicates the peak strength. The stress in the residual part indicates the residual strength. The negative slope part should describe the strength evolution from peak strength to residual strength. Mohr-Coulomb (M-C) criterion is employed and the inter-relations between post-peak behavior of the complete stress-strain curve and strength parameter evolution of rocks are studied in the paper. First, a new approach for modeling strain-softening behavior is introduced. The stress–strain curve in the strain-softening stage is approximated in a stepwise way, and the strain-softening processes is modeled by a series of stress drop and plastic flow during the increase of plastic strain. Then several sets of parameters are studied and the corresponding stress-strain curves are obtained. The relations between the post-peak strength evolution and the shape of complete stress-strain curves are discussed, and the implications in geotechnical engineering are analyzed.
When rock (rock mass) is loaded, deformation is produced. What is the relationship between the applied force and the displacement? Tests are needed to answer the question. That is why a thorough knowledge of the complete stress–strain curve is of great significance for the researchers(Hudson et al.1971, Jaeger and Cook. 1976). In the early stage of the rock mechanics, the complete curve cannot be obtained due to the limitation of the testing techniques and methodologies. The samples are often very unstable and the tests have to be terminated after the peak strength is reached.
However, the so called post-peak behavior of rocks is more interesting than the pre-peak one because they can sustain applied load after attainment of the ultimate strength.
A typical record of rock compression test is shown in Fig. 1 where both the pre-peak and the post-peak region(strain-softening region) are included.