This test method is useful for estimating the strain at the onset of necking in a uniaxial tension test (1). Practically, it provides an empirical parameter for appraising the relative stretch formability of similar metallic systems. The strain-hardening exponent is also a measure of the increase in strength of a material due to plastic deformation.

The strain-hardening exponent may be determined over the entire plastic stress-strain curve or any portion(s) of the stress-strain curve specified in a product specification.

Note 3-The strain interval 10-20% is commonly utilized for determining the n-value of formable low carbon steel products.

This test method is not intended to apply to any portion of the true-stress versus true-strain curve that exhibits discontinuous behavior; however, the method may be applied by curve-smoothing techniques as agreed upon.

Note 4-For example, those portions of the stress-strain curves for mild steel or aluminum alloys which exhibit yield-point elongation or Lüders bands may be characterized as behaving discontinuously.

Note 5-Caution should be observed in the use of curve-smoothing techniques as they may affect the n-value.

This test method is suitable for determining the tensile stress-strain response of metallic sheet materials in the plastic region prior to the onset of necking.

The n-value may vary with the displacement rate or strain rate used, depending on the metal and test temperature.