Why the n parameter should be lower than 1 in Johnson-Cook model?

Altair Forum User

Hi all,

In the comments of the /mat/law2 Johnson-Cook, it shows The plastic hardening exponent, n must be less than or equal to 1. Why this limit? From my test data, I do the fitting, and found that the n was large than 1. 

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Altair Forum User

Hi Roy,

The value of the strain hardening exponent lies between 0 and 1. A value of 0 means a perfectly plastic, while a value of 1 represents perfectly elastic. Most metals have an n value between 0.10 and 0.50. In general,as the n-value increases, the material's resistance to necking increases, and the material can be stretched farther before necking starts.

 

Many literatures are available on this parameter and you can explore the same for more.

 

Altair Forum User

Hi George,

I use the least squares fitting to get the b,n value based on the tensile testing data. And the result shows the n value is larger than 1. 

From the theory formula, we also can get the stress-strain curve with different n value as show below. When n is greater than 1, the slope of the curve may larger.

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