When a metal wire is repeatedly bent and straightened, it is gradually hardened. In the same way, a steel sheet becomes hardened and strong through work hardening (plastic deformation).

When a flat steel sheet is bent and straightened, the first bent area (A) remains and the new bent areas are formed on both sides of the sheet. It happens because the first bent area becomes hardened and strong through work hardening. In the same way, when a body panel is press-formed (plastic deformation), work hardening occurs.

On a collision damaged vehicle, the same work hardening occurrs where parts were buckled (buckling) due to the external impact force. Work hardening also takes place when the parts are repaired.