{PDF} A definition of stable inelastic material D. C. Drucker

The definitions given previously for work-hardening and perfect
plasticity are broadened to cover viscous effects. As before, the system
considered includes both the body and the time-dependent set of forces
acting upon it. An external agency is supposed to apply an additional
I set of forces to the already loaded body., It is now postulated that
the work done by the external agency on the addition displacements it
produces is positive. Some of the restrictions thus imposed on permissible stress-strain relations are explored. Especial attention is paid to simple laws of creep* Uniqueness of solution also is studied.