Strain Hardening Exponent
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Strain Hardening Exponent

The strain hardening exponent (also called strain hardening index), noted as n, is a material's constant which is used in calculations for stress-strain behavior in work hardening. It occurs in the formula known as Hollomons equation after John Herbert Hollomon Jr. who originally proposed it[1]:

? = K ? n,
where ? represents the applied stress on the material,
? is the strain,
and K is the strength coefficient.

The value of the strain hardening exponent lies between 0 and 1. A value of 0 means that a material is a perfectly plastic solid, while a value of 1 represents a 100% elastic solid. Most metals have a n value between 0.10 and 0.50.


Tabulation of n and K Values for Several Alloys [2][3]
Material n K (MPa)
Aluminum 1100-O (annealed) 0.20 180
2024 aluminum alloy (heat treated--T3) 0.16 690
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) 0.54 315
AZ-31B magnesium alloy (annealed) 0.16 450
Low-carbon steel (annealed) 0.26 530
4340 steel alloy (tempered @ 315 °C) 0.15 640
304 stainless steel (annealed) 0.450 1275


  1. ^ J. H. Hollomon, Tensile deformation, Trans. AIME, vol. 162, (1945), pp. 268-290.
  2. ^ Callister, Jr., William D (2005), Fundamentals of Materials Science and Engineering (2nd ed.), United States of America: John Wiley & Sons, p. 199, ISBN 978-0-471-47014-4
  3. ^ Kalpakjian, S (2014), Manufacturing engineering and technology (2nd ed.), Singapore: Pearson Education South Asia Pte, p. 62

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