The strain hardening exponent (also called the strain hardening index), usually denoted , is a measured parameter that quantifies the ability of a material to become stronger due to strain hardening. Strain hardening (work hardening) is the process by which a material's load-bearing capacity increases during plastic (permanent) strain, or deformation. This characteristic is what sets ductile materials apart from brittle materials. The uniaxial tension test is the primary experimental method used to directly measure a material's stress–strain behavior, providing valuable insights into its strain-hardening behavior.
The strain hardening exponent is sometimes regarded as a constant and occurs in forging and forming calculations, as well as the formula known as the Hollomon equation (after John Herbert Hollomon Jr.), originally posited as:
J. H. Hollomon, Tensile deformation, Trans. AIME, vol. 162, (1945), pp. 268-290.
where represents the applied true stress on the material, is the true strain, and is the strength coefficient.
The value of the strain hardening exponent lies between 0 and 1, with a value of 0 implying a perfectly plastic solid and a value of 1 representing a perfectly elastic solid. Most metals have an -value between 0.10 and 0.50. In one study, strain hardening exponent values extracted from tensile data from 58 steel pipes from natural gas pipelines were found to range from 0.08 to 0.25, with the lower end of the range dominated by high-strength low alloy steels and the upper end of the range mostly normalized steels.
Tabulation
+ Tabulation of - and -values for several alloys Material n K (MPa) Aluminum 1100–O (annealed) 0.20 180 2024 aluminum alloy (heat treated—T3) 0.16 6905052-O0.13210 Aluminum 6061–O (annealed) 0.20 205 Aluminum 6061–T6 0.05 410 Aluminum 7075–O (annealed) 0.17 400 Brass, Naval (annealed) 0.49 895 Brass 70–30 (annealed) 0.49 900 Brass 85–15 (cold-rolled) 0.34 580 Cobalt-base alloy (heat-treated) 0.50 2,070 Copper (annealed) 0.54 325 AZ-31B magnesium alloy (annealed) 0.16 450 Low-carbon steel (annealed) 0.26 530Low-carbon steel (cold worked)0.08700 4340 steel alloy (tempered @ 315 °C) 0.15 640 304 stainless steel (annealed) 0.450 1275
References
External links More complete picture about the strain hardening exponent in the stress–strain curve on www.key-to-steel.com