Hi,
I have cyclic uniaxial compression data of a PU. When I look at nominal stress vs nominal strain then the relation is clearly nonlinear (increasing stiffnes with increasing strain). When I convert to true stress vs true strain I still have a nonlinear relation, however, not as strong and the stiffness decreases with increasing strain.
When I look at true stress vs stretch, however, the relation seems to be almost (I'm only talking about the first loading cycle) linear. Nominal stress vs stretch is still nonlinear.
I'm currently using the 8-chain model (Arruda Boyce based on first invariant and stretch) to simulate hyperelasticity. Obviously, any hyperelastic material model is only useful when the response is nonlinear. I am quite confused at the moment and don't understand whymy material is highly nonlinear when looking at nom. values and quite linear when looking at true stress vs stretch.
Has anyone had similar problems?
Andreas