Abstract: According to curves acquired from cyclic triaxial loading-unloading tests on marble, the spatial plastic principal strains are obtained, and some plastic hardening and softening functions in classic plastic theory are embedded into the current yielding function. On this basis, the dynamic evolutionary process of strength parameters which meet Mohr-Coulomb criterion against the variation of plastic parameter throughout the process of initial yielding, pre-peak hardening and post-peak softening is researched. Processing and analysis of test data revealed that internal friction angle and dilatancy angle witnessed an increase at first and then a decline in plastic hardening period; whereas cohesion decreased both during hardening and softening. Furthermore, through nonlinear function fitting, the models of strength parameters (internal friction angle, dilatancy angle and cohesion) varying with plastic parameters are obtained. The stress-strain relationship of marble in in-door test was simulated by applying the models into FLAC^3D program. Research results show that the constitutive relation derived from triaxial loading and unloading control testing could reasonably describe the characteristic of plastic hardening and softening mechanical behavior, and therefore is of theoretical significance and application value.

Key words: cyclic loading and unloading, plastic hardening and softening, plastic parameter, Mohr-Coulomb yield criterion, strength parameters