In mechanical engineering, ultimate failure describes the breaking of a material.
In engineering there are multiple types of failure based upon the application of the material.
Although this deformation or weakening of the material is not the technical definition of ultimate failure, the piece has failed.
In most technical applications, pieces are rarely allowed to reach their ultimate failure or breakage point, instead for safety factors they are removed at the first signs of significant wear.
When necking occurs the material will begin to stretch more in the middle and the radius will decrease.