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Thanks for the reply. What I would like to simulate is the AZ31B magnesium alloy. The material parameters are cited from the paper linked: A constitutive and fracture model for AZ31B magnesium alloy in the tensile state - ScienceDirect
The fracture material parameters are defined below:
which are very similar to the formula in LS-DYNA user manual. In real experiment, the parameters are calculated by fitting the data measured, and the result for the paper is shown:
So when using the negative D3 value, we can generate the correct stress-strain curve. However, in LS-DYNA manual, it indicates that D3 must be positive (if negative, use the absolute value). I don't know why we need to do that.
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Besides, I also get the same result by using both positive and negative values for the material property for calculation (same absolute value, keeping all other JC parameters the same).