Tensile testing of metals determines characteristics such as yield strength, yield point elongation (YPE), ultimate tensile strength, plastic strain ratio (r) value and the strain-hardening exponent (n) value. One common requirement needed for all these calculations is accurately measuring the strain of the material in question.

In practical terms, strain is a measurement of the deformation in a defined, uniform section of a test specimen-known as the gage section-as it is subjected to a load. Engineering strain is the extension divided by the original gage length; it is usually expressed as a percentage.