Bauschinger effect design procedures for autofrettaged tubes including material removal and Sachs' method

Autofrettage is used to introduce advantageous residual stresses into press ure vessels and to enhance their fatigue lifetimes. The Bauschinger effect serves to reduce the yield strength in compression as a result of prior ten sile plastic overload and can produce lower compressive residual hoop stres ses near the bore than are predicted by "ideal" autofrettage solutions (ela stic/perfectly plastic without Bauschinger effect). A complete analysis pro cedure is presented which encompasses representation of elastic-plastic uni axial loading material behavior and of reverse-loading material behavior as a function of plastic strain during loading. Such data are then combined w ith some yield criterion to accurately predict elastic-plastic residual str ess fields within apr autofrettaged thick cylinder. Pressure for subsequent reyielding of the tube is calculated. The numerical procedure is further u sed to determine residual stress field's after removal of material from ins ide diameter (i.d.) and/or outside diameter (o.d.), including the effects o f any further plasticity. A specific material removal sequence is recommend ed It is shown that Sachs' experimental method, which involves removing mat erial from the i.d., may very significantly overestimate autofrettage resid ual stresses near the bore. Stress ranges and stress intensity factors for cracks within such stress fields are calculated together with the associate d fatigue lifetimes as such cracks propagate under cyclic pressurization; T he loss of fatigue lifetime resulting fr om the Bauschinger effect is shown to be extremely significant.