IMPROVED FATIGUE LIFE OF AN AUTOFRETTAGED THICK-WALLED PRESSURE-VESSEL WITH AN EXTERNAL GROOVE

An autofrettaged thick-walled pressure vessel with an external groove suffers a decreased fatigue life due to cracking at the exterior. In order to improve the fatigue life of an autofrettaged thick-walled pressure vessel containing an external groove subjected to pulsating internal pressure, a shape change and a shot peening were done at the external groove, which is the most vulnerable location to fatigue cracking. The groove shape was changed by using an optimization technique in order to minimize the stress concentration at the groove root. Stress concentration calculation during the optimization stage was carried out by the finite element methods. Autofrettage residual stresses were simulated by the thermal stresses from the thermal loading analogy. Shot peening was performed on the changed groove region in order to induce the surface compressive residual stresses. Fatigue tests that simulate an autofrettaged thick-walled pressure vessel under pulsating internal pressure loading conditions were performed using C-shaped specimens taken from the autofrettaged thick-walled pressure vessel. Fatigue life improvement of the pressure vessel was achieved by design change of the groove shape to relieve the high stress concentration, and also by surface enhancement at the groove root using shot peening. Simulation fatigue tests resulted in improved fatigue life for the changed and shot peened groove by a factor of 3.5 to 13, compared to the conventional groove.