Hey BrUnO XaVIeR,
I set up a test bed to return the Physical Material Type of a Cube Mesh when overlapping a trigger volume, and to Print the surface type to the screen. My approach is a bit basic, and does not involve Sub-Material situations or Skeletal Mesh, but hopefully the approach can be applied in your case.
Here is how I set up my Level Blueprint to return the Physical Material. You do mention you are using Hit Traces which I am not implementing in my example, so if this is not what you are looking for, that is fine also.
Physical Material Level BP
After looking at your question a bit more this seems directly related to the ‘Switch On EPhysicalSurface’ node. I will try to re-create an example from the image you provided and return here with my findings.
Cheers,