The favorable effect of autofrettage on the mode I stress intensity factor (SIF) distributions along the fronts of radial, semi-elliptical surface cracks pertaining to large uniform arrays of unequal-depth cracks emanating at the bore of a pressurized thick-walled cylinder is studied. The analysis is based on the, previously proposed, “two-crack-depth level model”. SIF values are evaluated by the finite element method (FE) using the ANSYS 6.1 code. In the FE model singular elements are employed along the crack front and an equivalent temperature load simulates the autofrettage residual stress field. The distribution of KIN = KIP + KIA, the combined stress intensity factor due to pressurization and full autofrettage, for numerous uneven array configurations bearing n = n1 + n2 = 8 to 128 cracks, a wide range of crack depth to wall thickness ratios, a1/t = 0.01 to 0.4, and various crack ellipticities, a1/c1 = 0.3 to 1.5, are evaluated for a cylinder of radii ratio Ro/Ri = 2. The accuracy of the evaluated SIFs is increased using an improved displacement extrapolation. The results clearly indicate the favorable effect of the residual stress field on the fracture endurance and the fatigue life of autofrettaged cylindrical pressure vessel bearing uniform arrays of 3-D unequal-depth cracks emanating from its inner bore. This favorable effect is governed by Ψ = σo/p — the ratio of the vessel’s material yield stress to its internal pressure. The higher ψ is the more effective autofrettage becomes. The “interaction range” for the various configurations of uneven crack arrays is evaluated. The range of influence between adjacent cracks on the maximal combined SIF, KNmax, is found to be dependent on the density of the array, as reflected in the inter-crack aspect-ratio, as well as on the cracks’ ellipticity.
Three dimensional stress intensity factor for large arrays of radial internal surface cracks in a cylindrical pressure vessel
