# Is the restoring force equal to the deforming force in the case of plastic deformation?

I've been told by my professor that the restoring force (a reaction force) is always equal to the deforming force due to Newton's third law, and this is why stress is calculated using deforming force.

My understanding is that in the case of elastic objects like a spring, the restoring force = the deforming force at equilibrium position. But before that, the deforming force will be greater than the restoring force, which is when deformation occurs. Once the equilibrium is reached, no more deformation occurs. This restoring force drives the spring back to its original position once the deforming force is removed. Is this correct?

Also, if restoring force (which must be generated due to Newton's third law) = deforming force at equilibrium for plastic objects like mud, why don't they attain their original dimensions after the deforming force is removed? Is it that plastic objects never attain equilibrium (i.e. the object goes on deforming until the object physically separates) and then fracture at some point? But that would mean that the two forces are unequal.

Apologies for the silly question, and my sincere thanks for any help.