i have several but similar materials which i tested all at
compression (strain rates=0.666%/s , 13,3%/s / max strain=66.6%)
simple shear (strain rates=0.666%/s , 13,3%/s / max strain=33.3%)
Some typical results can be seen in the attached picture [I]compression and shear as measured[/I]
i imported the measured stress-strain curves as true stress / strain but they were measured as engineering and not corrected before. If i import them as engineering MCalibration deforms them dramatically. I guess it is because MCalibration implies a Poissons ratio of 0.5 which is absolutely not true with this soft foams. I tried to add a virtual test of the Poissons Ratio as it was described in one of the tutorial videos but it only worked for high PR (like 0.499) but not for small ones like 0.1.
Both shear curves are from the same sample and were measured directly after one another. Unfortunatelly they dont seem to have the same hyperelastic storage stiffness with just variing hysteresis around that as it is constitutive in the paper Constitutive modelling of the large strai time-dependent behavior of elastomers (if i got that right). MCalibration fits something between that two curves.
I tested all material models available to get a feeling and the Dynamic Bergstrm-Boyce made by far the best fits for the different load cases if considered separately.
[I]The curve fits can be seen in the pictures compression alone and shear alone.[/I]
Subsequently i tried to fit all four cures together. I tried the following strategies:
First a curve fit of just the compression data while the shear curves were deactiveted. Then activating the shear data and deactivating the compression for a second curve fit. (see figure final results)
Like above but the other way around (even worse than in the figure)
All four curves active from the beginning (see figure final results)
It seems to me that the Poissons ratio might be the biggest issue here.
Dr. Berstrm kindly answered some of my questions some month ago before i started to measure and he recommended the models:
Micro Foam Model (the fit was not really good)
BB Foam (i did not find this model ?!?)
as the probably most suitable for my material while the Dynamic Bergstrm-Boyce might just not be suitable for compressable foams.So there are some more questions:
Is there a BB Foam model somewhere? Or do i have to modify the BB model to make it a Foam model?
If i have to use the PN model could someone give me a hint of how to start the model structure?
many questions and i am thankfull for every advice, inquiries and hints
Greetings from Bremen / Germany