Joinery tester - Mortise and Tenon 2 layer joints.
This is a couple tests I had done for a 2 layer mortise and tenon style of joinery for strong joints. It works well, though it requires a closed box design for max strength. My theory from doing stress tests is that they are weak to rotational twisting along the joint due to the two flat edges on the inside layer.
I have since done tests with locking L shaped tabs as the tenons to great success.