Analysis of uniform arrays of 3-D unequal-depth cracks in a thick-walled cylindrical pressure vessel

The effect of crack depth unevenness 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 "two-crack-depth level model" previously proposed and is performed via the finite element (FE) method using the submodeling technique and, employing singular elements along the crack front. The distribution of K_IP - the stress intensity factor (SIF) due to pressurization, for numerous uneven array configurations bearing n₁=n₂= 4-60 cracks, a wide range of crack depth to wall thickness ratios, a₁/t=0.01-0.3, and various crack ellipticities (a₁/C₁=0.3-1.5) are evaluated for a cylinder of radii ratio R_o/R_i=2. The "interaction range" for different combinations of crack arrays and crack depths is then determined. The results clearly indicate that unevenness as reflected in K_IP distributions depends on both the number of cracks in the array as well as on the cracks' depths. The "interaction range" between adjacent cracks is determined by the relative depth of the cracks and the density of the array and varies with the parametric angle along the cracks front. Furthermore, it is found that both the submodeling technique and the improved displacement extrapolation have a considerable impact on the accuracy of the results.