the ability of a material to retain any deformation produced in it by a stress. thus for a perfectly plastic material, no strain disappears when it is relieved from stress. some important engineering materials go through a plastic stage at stresses beyond the elastic limit. they then exhibit the property of 'flow' in much the same way as does a viscous liquid, in that particles of the material may be physically moved from one position to another under the action of unequal stresses.