Stress Concentration, Stress Intensity, and Fatigue Lifetime Calculations in Autofrettaged Tubes Containing Axial Perforations Within the Wall

Abstract

Elastic, elastic-plastic and experimental stress analyses, and fatigue lifetime predictions are presented for thick cylinders containing multiple, axial holes within the wall. The holes are generally semi-elliptical (including semi-circular), and the cylinders are autofrettaged after introduction of the holes and are subsequently subjected to cyclic pressurization of the bore. Two potentially critical failure locations are identified; a fracture-mechanics based design methodology is proposed; elastic and elastic-plastic finite element (FE) analyses are undertaken. The elastic FE analysis predicts hoop stresses at the bore resulting from internal pressurization which are some 7 percent higher than those for the equivalent plain tube. For a given hole size and location and for nominal overstrains of 40 percent or greater, the residual compressive stress at the bore is reduced by approximately 15 percent below the value for a plain tube of the same radius ratio. Two experimental investigations are reported, one based upon X-ray diffraction, to measure residual stresses and stress gradients, and the other based upon radial tube slitting, to measure opening angle. They confirm most features of the residual stress profiles predicted from FE analysis with the exception of high compressive residual stresses and stress gradients immediately adjacent to the hole boundaries. Appropriate use of the residual stress information permits prediction of tube lifetimes for cracks emanating from the bore and from the hole. For the geometry and loading under consideration, the more critical location is predicted to be the hole boundary, the lifetime for failures originating from this point being some 60 percent of the lifetime for cracks originating at the bore.