Elastic unloading

Reduction of an applied load with purely elastic strain increments while accumulated plastic strain is fixed. The superposition principle applies to the increment, so the current stress equals the previous stress plus the linear elasticity response to the load change. The resulting stress must still satisfy the yield surface constraint; reverse yielding ends the purely elastic stage.

Renewed plastic deformation during unloading or reversed loading when the stress reaches the opposite part of the yield surface. For the spherical elastoplastic cavity with maximum pressure $P_m>4{\tau }_Y/3$, elastic unloading first reaches the opposite Tresca yield criterion at the cavity wall after a pressure decrease $8{\tau }_Y/3$. Purely elastic return to zero pressure is possible when $P_m\le 8{\tau }_Y/3$, with equality reaching the yield surface exactly at the endpoint.