Unexpected Strain Rate dependence of Mat 26 (MAT_HONEYCOMB / Crushable Foam)
I am trying to simulate a honeycomb-like material, and though that perhaps MAT_HONEYCOMB would work.
I went to test that this material model, and found unexpected (to me) behaviour: The stress is dependent on the strain rate. Looking through the manuals, I see that the material can be strain rate dependent (using the variable LCSR), but does not have to be.
To test the material, I made a single hexahedral element of the material, fixed one side, and applied prescribed motion to the other side. This used MAT_HONEYCOMB, for which I used a load curve with a constant value to define the various stress load curves. I did not set the strain rate dependent curve, LCSR.
After running this simulation, I find that the stress in the material is higher than the value set in the load curves. I find that the boundary force recorded to compress the element agrees with the reported stress, higher than what is configured in the model. By changing the rate of the boundary motion, I find that the amount the stress exceeds the limit I set in the model is proportional to the strain rate.
This is unexpected to me. According to equations 22.26.6 and 22.26.7 in the theory manual, I expect the stress to be limited to the value set in the load curve, and not dependent on strain rate.
Can anyone shed light on an effect I am missing, or do these materials simply not behave as documented?
I have made a simple example model to demonstrate the effect, which is attached. In this model, 6 single element parts are crushed by prescribed motion. Two cubes are MAT_HONEYCOMB, two are MAT_CRUSHABLE_FOAM, and two are MAT_SOIL_AND_FOAM. One piece of each material is crushed quickly, and one is crushed more slowly. Both MAT_HONEYCOMB and MAT_CRUSHABLE_FOAM show greater stress when crushed quickly, while MAT_SOIL_AND_FOAM behaves as expected, ignoring strain rate [side note: to make the SOIL_AND_FOAM work, I had to constrain its nodes to not move except in the direction of crushing, to stop the material simply spreading out]
PS Further experimentation indicates that the magnitude of this effect is proportional to the density assigned to the material.