It is well known that the fatigue strength of certain engineering components can be enhanced by the application of overloads. Extended fatigue life can occur when components with stress gradients at critical locations (e.g., bending or the presence of a stress concentration, or both) are service loaded positively in one direction following an overload in the same direction. Localized plasticity can result in residual compressive stresses that combine with the applied service stress history to enhance fatigue life.
For a component with a stress concentration (notch), guidelines are presented for specifying an “optimal” overload level based on the elastically calculated severity of the notch and the axial stress-strain behavior of the material, characterized by a Ramberg-Osgood relation. An algorithm is presented based on standard notch strain analysis techniques (Neuber's Rule and the strain energy density approach) and the concept of a reverse plasticity criterion. Use of the algorithm is demonstrated by generating examples of design curves for engineering steels. One version of the algorithm, based on a Neuber analysis, suggests that the optimal overload level is a function of only the yield strength of the material and independent of the strain hardening exponent. The results presented in this paper approximate situations where notch root constraint is small or negligible. However, the approach can be extended to account for multiaxial constraint on notch root deformation. Although the algorithm and criterion attempt to maximize the local compressive stress, supplemental analyses are required to assure that fracture or gross plastic deformation, or both, are avoided. Limitations to the approach are discussed and modifications are suggested with regards to the Bauschinger effect. The approach does not attempt to address the maintainability of the residual stress during subsequent fatigue loading and experimental data for direct comparison to the approach do not currently exist.