Sergey Zhmaev

hi Peter. I have Mechanical 2021R2 license

The real life testing of these two geometries shows necking of the BOX thread relief, in other words by torquing them BOX thread relief stretches very much. The thing I don't understand from FEA is why at the TRUE SCALE I see this picture (the part I apply moment to is just expending radially without seeing the other part). 

When I run 2D axisymmetric analysis and apply only axial force, the resulting picture is more realistic. See below. I'm expecting to see the similar image when I do 3D torque, since high torque generates axial force trying to neck both PIN and BOX thread reliefs.