I am attempting to model an cylindrical End Cap that is part of a titanium pressure vessel that has failed several times at pressures below what FEA predicted. The first parts were fabricated without fillets and failed catatrophically at the inside sharp corner. Of course the simulation results indicate a large stress concentration at this corner and I had indicated that a fillet would be required to avoid this concentration. I have now added a fillet in my model and still get very high stresses in the fillets and especially the tangent lines. What is the best method to model this type of feature in Simulation?