Experimental: Plastic Deformation

The diagram shows a stress/strain curve for a
typical ductile metal sample tested at a constant
strain-rate in uniaxial tension. The inset pictures
show the shape change of the sample during the
test. Up to the point M (UTS) on the curve the
deformation is uniform. The linear initial slope is
the Elastic response of the sample. The slope
change indicates the start of Plastic deformation.

From: Callister, "Materials Science & Engineeering," Wiley (1994)

Plastic deformation produces a change in the properties of the sample. It does not return to its initial size when the load is removed, and the positive slope of the stress/strain curve up to the point M indicates that the metal is becoming harder as the plastic deformation is increased. It is this change in hardness (Work hardening or Strain hardening) that is to be measured. Use the Rockwell tester to measure the sample hardness before deformation and repeat these measurements after the material has been fractured.