Engineering Vs. True stress strain curve in nonlinear analysis 1

[assafwei]

Hi,

When doing a non-linear analysis, I know that I should insert the true stress and strain curve to the material model (regular metal plasticity model using ANSYS for instance). The data I get from the material manufacturer (Young's modulus, yield point) is based on the engineering stress strain curve.

As I understand the yield point should be the same (engineering Y.P = true Y.P) so is the data before the yield point.

In Ansys the stress-strain curve first point is the yield point and from there on I should insert the true stress - strain data is this correct? the curve data is from the true stress-strain and the parameters (yield point, young's modulus) is based on the engineering data because up to the yield point true and engineering should be almost the same?

 

[FEA way]

If you convert your engineering (nominal) stress-strain data to true stress-strain using the formulas below you will see how the values for both measures differ depending on the stress/strain level:

ε_true = ln(1+ε_eng)
σ_true = σ_eng(1+ε_eng)

You should notice that the difference becomes visible starting from the yield point.

Depending on the software you may also have to convert total strain to plastic strain:

ε_pl = ε_tot - (σ/E)