Dear For the issue of plotting a force vs displacement curve, if you plot the displacement of the plate, it doesn't matter that some parts of the sample lift off the plate, both the top and bottom plates have this effect. One approach for tracking the displacement of the part is to make a Named Selection of elements that are very close to the top plate. Then you can plot the Minimum Z Directional Deformation. That value would capture the elements that are still touching the plate. The elements in that Named Selection that lifted off the plate would be measured by the Maximum Z Directional Deformation.
The mesh sensitivity study shows a good trend. What is the thickness of the wall? How many elements are across the thickness at a mesh size of 0.01 mm? Linear Tet elements are known to be overly stiff in bending, so 1 element through the thickness must never be used, 2 elements through the thickness is unacceptable, 4 elements through the thickness is barely acceptable, 8 elements is great.
That guidance is for linear tet elements, and doesn't apply to quadratic tet elements, but those are not available in Explicit Dynamics.
Linear Hex elements perform better than linear Tet elements. If you could generate a gyroid surface, mesh that with quad elements, then extrude those into hex elements, like two layers on each side, that would be four hex elements through the thickness and would give you a better ability to increase the number of elements through the thickness without creating a huge number of nodes.
You will find that the Force-Displacement curve converges with element size faster than the Stress-Strain curve.