Swage autofrettage analysis - Current status and future prospects

Swage autofrettage (SA) is far more complex than hydraulic autofrettage (HA). Any overall correlation between SA and HA residual stresses is coincidental. HA always produces compressive near-bore axial stress (NBAS) but SA can produce dominant and highly tensile NBAS. SA stresses are significantly influenced by material behavior (specifically Bauschinger effect), by swage flat length/bore radius (Lm/Ri) and by plastic break-thru (PBT), when plasticity extends throughout the wall. Prior to PBT, with modem swage profiles (Lm/Ri less than 15%), higher wall ratios do not produce tensile NBAS whilst lower wall ratios do. PBT causes hoop stress to collapse and creates severely disadvantageous near bore axial and hoop stress combinations. SA modeled via a moving pressure band does not match SA modeled via an equivalent flat length using displacement boundary conditions. The classic but perplexing 1960s Sachs experimental residual stress measurements on SA tubes may be explained on the basis of combinations of high (100%) Lm/Ri, dominant PBT and the failure to employ a post-autofrettage heat soak prior to incremental material removal. Further Sachs experiments should be conducted using modern swage flat lengths and incorporating heat soak. Design codes should provide specific commentary on swage autofrettage.