I'm trying to get a constitutive model for a material based on some uniaxial test results obtained from one of my professors. However, I'm not able to obtain a working model with the data. I get a warning message about the model becoming unstable at the following: UNIAXIAL TENSION: 0.210E+00 UNIAXIAL COMPRESSION: -0.119E+00 EQUIBIAXIAL TENSION: 0.700E-01 EQUIBIAXIAL COMPRESSION: -0.880E-01 PLANAR TENSION: 0.140E+00 PLANAR COMPRESSION: -0.116E+00 From running a few simulations it seems clear the results are completely off, it's actually compressing when I apply a pulling force and other abnormal things like that. The data itself doesn't appear to be too bad; the curve fitting didn't give me any issues on Ansys. Except there appears to be a part of the curve that goes below zero, as can be seen here: Does anyone with experience with hyperelastic models know how I can try and make a stable hyperelastic model? Any tips and suggestions? Thank you. I should add that I also tried with some curve fits that were less than adequate to see if at least those would appear stable, but I had problems with them too.