Thanks a gain for your answer
Regarding contact between screw heads and plate holes, since in the reality there are threads around each screw head and plate hole, we assumed the bonded contact between with MPC formulation for screw head and plate hole. Because we didi not model screw and hole threads and modeled them as simple cylinders with MPC contacts.
Regarrding the element size, I check the max. von-Misess stress of plate and hole by increasing the element size of plate and refined element size of hole G but the Max stress values did not converged. Hence I think that this Max stress point is a singulaity. On the other hand, the average and unaverage von-Misess stress values are not the same, and it confirmed again that this Max. stress point is a singularity. Hence, stess evaluations should be perfomed some nodes a way from this singulaity in which average and unvarge von-Misess stress values are not significantly different. I am grateful in advance if you could help me in this regard.
Best regards, Shabnam