The beneficial effect of autofrettage on the fracture endurance of a cracked spherical pressure vessel

The distributions of the combined 3-D Stress Intensity Factor (SIF) due to both internal pressure and autofrettage along the front of an inner radial crack emanating from the bore of an overstrained spherical pressure vessel are evaluated. The 3-D analysis is performed by the finite element (FE) method employing singular elements along the crack front. A novel realistic autofrettage residual stress field incorporating the Bauschinger effect is applied to the vessel, and is simulated using an equivalent temperature field. SIFs for three vessel geometries (R0/Ri=1.1, 1.2, and 1.7), a wide range of crack depth to wall thickness ratios (a/t=0.01-0.8), various ellipticities (a/c=0.2-1.5), and three levels of autofrettage (ε =50%, 75%, and 100%) are evaluated. The results indicate the favorable effect of autofrettage in reducing the prevailing effective stress intensity factor i.e., delaying crack initiation, slowing down crack growth rate, and thus substantially prolonging the fatigue life of the vessel.