Unlike elastic deformation, plastic deformation is an irreversible process.

When the applied stress exceeds the yield strength of a material, the stress-strain curve becomes nonlinear. In simpler terms, the material is no longer acting like a spring.

This is generally caused by slipping of atoms along planes within the crystal lattice. Dislocations allow the slipping to occur one row of atoms at a time, which is much easier than trying to simultaneously break all the bonds in the given plane. This results in yield strengths as much as an order of magnitude lower than the theoretical values for many materials. Fortunately, this movement of dislocations is slowed or stopped by other crystal defects, so the yield strength may sometimes be increased by work hardening (aka strain hardening), precipitation hardening (aka age hardening), or by solid-solution hardening.