That's a nice post with a good example! To further your point, I would like to add a few more notes:
- In FEA, forces and displacements are calculated at nodes and the stresses and strains are calculated at the integration points (internal to elements). The stresses are then either interpolated or copied to the nodal locations.
- Therefore, each node receives a stress value from each element it is connected to. An average of all these values is reported as the stress at that location. This is valid only when all these elements have the same material assignment.
- In a multi-body part, the bodies share nodes at the interface and they may have different material assignments. This is the reason the option to average across bodies is turned off by default. As stated in the above post, if all the bodies in the multi-body part have the same material assignment, this option can be manually turned ON.
Regarding the mesh, it might help to use multi-zone mesh method on the 3-body mesh to get a structured hex mesh as in the 1-body case. This is a common recommendation for modeling bolts.
P.S: You may want to mark the sentence "The point of this post..." in bold.