The output of a tensile test is a graph giving the load versus the resulting extension of the test piece. These curves are dependent on the test piece size. If the test piece is larger it will require a larger load to give the same extension.

Therefore, in order to normalise across a range of different size test pieces, the load and extension are converted to engineering stress and engineering strain respectively (we will simply refer to these as stress and strain).

Stress and strain equation

σ = stress

F = force applied (load)

A = cross-sectional area

ε = strain

δl = change in length

= original length

 

Stress and strain are then plotted instead of load and extension, giving an engineering stress-strain curve, which is not dependent on the area or size of the test piece.

These curves can then be compared between different test pieces and different materials.