STRESS-CONCENTRATION, STRESS INTENSITY, AND FATIGUE LIFETIME CALCULATIONS IN AUTOFRETTAGED TUBES CONTAINING AXIAL PERFORATIONS WITHIN THE WALL

Elastic, elastic-plastic and experimental stress analyses, and fatigue lifetime predictions are presented for thick cylinders containing mul tiple, axial holes within the wall. The holes are generally semi-ellip tical (including semi-circular), and the cylinders are autofrettaged a fter introduction of the holes and are subsequently subjected to cycli c pressurization of the bore. Two potentially critical failure locatio ns are identified; a fracture-mechanics based design methodology is pr oposed; elastic and elastic-plastic finite element (FE) analyses are u ndertaken. 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 l ocation and for nominal overstrains of 40 percent or greater, the resi dual compressive stress at the bore is reduced by approximately 15 per cent below the value for a plain tube of the same radius ratio. Two ex perimental investigations are reported, one based upon X-ray diffracti on, to measure residual stresses and stress gradients, and the other b ased upon radial tube slitting, to measure opening angle. They confirm most features of the residual stress profiles predicted from FE analy sis with the exception of high compressive residual stresses and stres s gradients immediately adjacent to the hole boundaries. Appropriate u se of the residual stress information permits prediction of tube lifet imes for cracks emanating from the bore and from the hole. For the geo metry and loading under consideration, the more critical location is p redicted to be the hole boundary the lifetime for failures originating from this point bring some 60 percent of the lifetime for cracks orig inating at the bore.