I have uniaxial compression data (nominal) from PU and I have been trying to use the 8-chain model (without amplified strain for now) to predict the deformation behaviour.
I have implemented the uniaxial, incompressible version of the model in Matlab in order to fit the material coefficients. When I try to fit true stress I do not get satisfying results. However, when I use nominal stress the fit is excellent. In my data, the slope of the nominal stress is increasing with increasing stretch (material gets stiffer and stiffer), whereas the slope of the true stress (calculated from nominal values) seems to level off or decrease slightly with increasing stretch. So what happens when I try to fit true stress, is the predicted slope increases and therefore diverges from the experimental data as stretch increases.
Another question I have, is how I know that my estimated coefficients make physically sense? Obviously, Matlab goes to what ever values it needs in order to get a fit. So I need upper and lower bounds. How do I define these?
Has anyone had similar problems with the 8-chain model?