Materials such as the semi-crystalline polymer polyethylene undergo a reorganization of their molecular configuration during plastic deformation. The form of the stress/strain curve is shown below together with inserts that show the shape of the sample at several deformation stages.

In the initial elastic region, the cylindrical sample remains cylindrical and its dimensions change following Hooke's law and Poisson's ratio considerations. At the yield point, the first maximum on the curve, stable local deformation begins with the molecules in this region alligning with the tensile stress direction as the material is strained. The length of this strain crystallized region increases until it fills the test region and the sample fractures.

The molecular processes taking place in this deformation are illustrated on the following page.

From: Callister, "Materials Science and Engineering,"
Wiley (1997)