LOCALIZED AUTOFRETTAGE AS A DESIGN TOOL FOR THE FATIGUE IMPROVEMENT OF CROSS-BORED CYLINDERS

An investigation was launched into the feasibility of improving the fa tigue life of thick-walled cylinders with cross-bores by using a local ized autofrettage technique. This technique utilized the high stress c oncentration at the cross-bore to induce localized residual stresses u sing relatively low internal pressures. An elastic-plastic finite-elem ent analysis indicated that the resulting residual stresses in the vic inity of the cross-bore were predominately compressive and not suffici ent in magnitude to induce reverse plasticity. When the resulting resi dual stresses were used with an elastic fracture-mechanics assessment of a quarter-circular crack at the intersection of the cylinder and cr oss-bore inner diameter, a significant extension of fatigue life was s hown to be possible. In addition to prolonging the useful life of the cylinder, the localized residual stresses were shown to be possible at pressures below the yield threshold for the thick-walled cylinder Thu s, reverse plasticity permanent deformations, and the need for post-au tofrettage machining operations that could inadvertently lessen the be neficial results of a traditional autofrettage were avoided.