1. The displacement BC on the axis is not necessary, but it doesn’t hurt.  If the pressure from the window caused the steel ring to squash down and the sides were to change shape slightly, you wouldn’t want any BC on the sides so it was free to do that.
  2. Large Deformation was used to update the nodal locations so the frictional contact could update the pressure being transferred and keep the contact pressure as accurate as possible.
  3. Maximum Principal Stress = the highest tensile stress and Minimum Principal Stress = highest compressive stress. You can find more information in a Solid Mechanics course.
  4. The mirrored appearance of the Minimum Principal Stress is because there is a small zone of contact on the rounded edge of the steel ring due the the diamond window bending slightly. The stiffness of the steel and diamond are similar so the stress pattern is similar.
  5. Equivalent stress is only positive and is used to evaluate the stress in ductile materials that might be approaching the yield point.  Diamond doesn’t yield, it fractures and comparing the Maximum Principal Stress with the Ultimate Tensile Strength of diamond is one way to decide if the window will fracture.