Can I use a non-linear stress-strain curve that has a negative slope

Rick Kerner_20858

I have received errors before in Optistruct when I input a stress-strain curve that includes both positive and negative slopes.

Is there a workaround for this?  I am modeling a polycarbonate material and its stress-strain slope goes negative after yield and then returns slowly to a positive slope as shown in the attachment.

If RADIOSS provides a better platform to use material models like this, please let me know.

Thanks, 

Rick

PC stress-strain.PNG

PaulAltair

Someone more expert in polymer materials may chime in, but that doesn't look like true stress/strain data (notwithstanding the curve label)? There shouldn't be a negative slope like that in a true stress/strain curve, only in an engineering stress/strain curve, I think the dip must be due to necking?

Neither OS or Radioss allows a negative slope (actually, technically Radioss 'allows' it, but it is not recommended due to it being numerically unstable, stress should not decrease for increasing strain)

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