Viscoelastic material modeling using stress-relaxation data (ANSYS)
Im having a bit of trouble trying to develop a viscoelastic material model to couple with my hyperelastic model. I already have experimental stress-time curves for my material. I then simply calculated the modulus at each stress value for a particular time, and then determined the shear modulus by dividing the modulus by 3 (general approximation). Then I put all of this data in the curve fitting tool for viscoelastic material modeling in ANSYS. The time I put in was basically in seconds with 5 seconds intervals. Only room temperature curve was obtained.
Now I am not clear about the results I am getting for the curve fit. No matter what options I choose (Shear, Bulk, Shift), I always get the plot as shown in the attached image:
Please advise where I could be going wrong. Is it the initial calculation? Or am I doing something wrong with the fitting?
If you are using shear modulus data from a uniaxial relaxation test then you do not have enough data to fit the bulk viscoelasticty and shift functions. Anything you do to fit these will probably not affect the results you are showing.
Your data is approximately linear over the range you show and I would think you could get a better fit. How many Prony terms are you using? This is a relatively narrow time range and you probably can use maybe 3-4 terms to get a reasonable fit. Fitting viscoelastic coefficients can be tough and require a lot of tweeking. This is because it involves exponential functions that often dont behave well in fitting algorithms. One typical trick is to fix your relaxation times to cover your time scale of interest (eg, tau1=1e1, tau2=1e2, tau3=1e3) and then let the fitting tool pick the modulus terms. You can then try manipulating and/or fixing the modulus terms to improve the fitting results.