What constitutive model to use for very stiff rubber
In our particular application, our rubber material will only see compression (essentially uniaxial compression).
The strain rates will range from .5s^-1 to 5s^-1. Both the loading and unloading behavior are important. The strain value will never exceed 25%.
Years ago, someone performed what were essentially drop tests on the material. With that data available to me, I had planned to use the Arruda-Boyce model to fit the drop test data. I am using LS-DYNA, where the Arruda-Boyce model is built-in, and incorporates viscoelasticity via a Maxwell model.
Only recently were quasi-static (monotonic) baseline tests performed [attached]. I was surprised to see that this material softens in compression (perhaps it would eventually roll-over, but we only care about strains up to 25%).
Is this common with very stiff rubbers?
Am I correct that the Arruda-Boyce model cannot capture this behavior?
I may use a metal constitutive model instead, or a MAT_181 in DYNA, which I believe is based on the Ogden model, but modified to be able to grab stresses and strains from any tabulated [uniaxial] stress-strain curve. Or I may go with something else entirely, I am open to suggestions.
edit: the attached is force vs. time, but it was a displacement controlled compression test on a small (few inches) cube specimen that went to about 25% strain
I am not quite sure I understand what you mean by softens in compression. Are you saying that the stress magnitude is not monotonically increasing? Or perhaps that the tangent stiffness is not monotonically increasing?
By softening I meant that the tangent stiffness is not monotonically increasing (the tangent stiffness is monotonically decreasing).
Unless Im mistaken, I think that my first step should be to find a model that can match the attached curve, which I am treating as my quasi-static baseline. This is where Im already running into issue with the Arruda-Boyce model, and so am looking into MAT_181, as mentioned.