If you want to add specific physics on the boundary depends on te physics or application modes mostly you must use assembly mode to define the two boundary surfaces, each with opposed normals. The latter is mostly the default, as it means you have continuity of flux over this boundary, once you have "geomanalysed" your geometry, you have two bodies but one common boundary with "continuity". You should learn the diferences of asembled (two bodies in contact but with each it's own boundary overalapping) and union mode where both bodies have a common boundary. HOWEVER, here is the very odd result: note that the deformed shape and the z displacement output seem to be saying that the screw bends toward the negative z direction! And note the scale for the displacements - in the balpark of 1 e12 !! What is wrong? Am I missing something very basic in the model setup, leading to this absurd result? On the one hand, the ex strains look reasonable for the bending of the small screw's shaft as expected, e.g., tension on the one side and compression on the other. The puzzle is that the model appears to run fine when I am applying a distributed positively-directed z load on the top surface of the little screw I fix the boundaries of most of the bottom of the outside structure into which the small screw is placed. I did not apply any contact pairs or identity pairs where the threads meet inside. I'm doing an example stress analysis of a small screw inside of a larger structure (mainly just to make sure this problem is indeed running OK). The attached images give an idea of the example problem I'm running. I brought these new files into Comsol as stl files and assigned the properties of the two subdomains.
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