Dear Jorgen and all,
I am modeling a viscoelastic material. I got the uniaxial tensile relaxation test data and determined the Prony coefficients. Then I tried to simulate the tensile relaxation test (apply 1% tension strain(static general) and keep the strain constant and calculate the stress (visco)) . I did the followings:
1. Prony coefficients of tensile relaxation were converted to those for shear relaxation modulu and input into ABAQUS.
2. Set instantaneous modulus to 2100MPa, which is the upper asymptote of my tensile relaxation curve (corresponding loading time is about 1e-5 second). The simulation results match the test data very well up to about 2000 seconds. After that the simulated stress becomes smaller than the measured stress. At 1e5 second, the simulated stress is 0.15 Mpa, while the measured data is 0.5MPa.
3. I set the long-term modulus to 50 MPa, which is the lower asymptote of my tensile relaxation curve (corresponding loading time is about 1e5 second). In this case, the stress right after tension strain was applied is 0.5MPa and materials does not relax. At 1e5 second, stress only relax to 0.498Mpa.
4. I used instantaneous shear modulus (8700MPa), results are similar to case 3. Material does not relax, and stress is 87MPa from beginning to 1e5 seconds.
It seems case 2 is the right way, but long term simulation is not good. Please help me.