Abstract: Isotropic hardening, kinematic hardening and mixed hardening are three frequently-used laws to describe hardening patterns in plasticity. However, they are just reasonable and idealized assumptions and these assumptions are not necessary as some materials do not accord with them during the hardening. A new plastic hardening pattern based on virtual strength parameters which does not need priori assumptions is presented. The main idea is that hardening yield functions can be acquired simply by replacing real strength parameters with virtual ones that are respectively constructed as the functions of the hardening parameter. With a group of stress-strain data of coal rock samples under several confining pressures, a hardening function based on Coulomb-Mohr criterion and a hardening function based on Drucker-Prager criterion were respectively investigated. It shows that meridian lines in both functions expand in stress space through translations and rotations. The viewpoint of virtual strength parameter provides with a logical bridge for extending failure function, by replacing its real strength parameters with virtual ones, to hardening function.

Key words: virtual strength parameters, plastic hardening, yield criterion, subsequent yield, failure function