Stress Intensity Factors for Closely and Densely Packed Cracks in Partially Autofrettaged Pressurized Thick-Walled Cylinders

Due to acute temperature gradients and repetitive high-pressure impulses, extremely dense internal surface cracks can be practically developed in highly pressurized thick-walled vessels, typically in gun barrels. In our previous studies, networks of typical radial and longitudinal-coplanar, semi-elliptical, internal surface cracks have been investigated with an ideal or realistic autofrettage level of 100 percent. We have shown that the combined SIFs are considerably influenced by the three-dimensionality of the problem and the Bauschinger effect (BE) along with dependence on other parameters, such as radial crack density, longitudinal crack spacing, crack depth, crack ellipticity, and the autofrettage level. When pressure is considered solely, radial crack density and longitudinal crack spacing were found to have opposing effects on the prevailing stress intensity factor, K_IP. Furthermore, the addition of the negative stress intensity factor (SIF), K_IA, resulting from the residual stress field due to autofrettage, whether ideal or realistic, tended to decrease the combined SIF K_IN = K_IP − |K_IA|. Therefore, to assess the fracture endurance and the fatigue life of a cylindrical, autofrettaged, pressure vessel containing such a network of cracks, it is necessary to determine the K_IA’s and the K_IN’s. However, to assess the SIFs accurately, significant computational efforts and strategies are necessary, especially for networks with closely and packed cracks. In this study, our effort will focus on the K_IA and the K_IN distribution for numerous configurations of closely and densely packed semi-circular and semi-elliptical networked cracks affected by pressure and partial-to-full autofrettage levels of 30–100%, which is practically seen in autofrettaged thick-walled pressure vessels. The 3-D analysis will be performed via the finite element (FE) method and the submodeling technique employing singular elements along the crack front and the various symmetries of the problem. The network cracks will include up to 128 equally spaced cracks in the radial direction; with relative, longitudinal crack spacing, 2c/d, from 0.1 to 0.99; autofrettage level of 30–100 percent; crack depth to wall thickness ratios, a/t, from 0.01 to 0.4; and, cracks with various ellipticities of crack depth to semi-crack length, a/c, from 0.2 to 2.