If failure occurs at the screw and/or screw hole it will be important to model that accurately enough. Did you model as a straight cylinder, or did you incude threads in the geometry? Are the threads small compared to the bolt diameter? If so, you may not need to model the threads. Do you know how failure occurs at the bolt from a physical test? Does the bolt shear? In the contact settings, at the bottom, there is a setting for bolt threads. This allows you to model with a straight cylinder and have Ansys simulate bolt threads. It's accuracy will be better than modeling and solving as straight cylinder, but not as good as actually modeling the threads in the geometry with appropriately small mesh sizes.
Did you use bonded contact between the entire bolt shank and hole? If so, it all acts as a solid mass through the hole/screw and will be pretty strong. Does the bolt have a threaded and shank portion where you can just just use bonded contact at the threads? You can use frictional contact if you set the "contact geometry correction" to "bolt thread" or geometrically model the threads or even if you use a straight cylinder as long as you set some constraint to keep the bolt from moving in the axial direction. What about the effects of bolt pretension?
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