The holes made to accommodate the fasteners in many of the engineering applications as in aircraft structures are subjected to cyclic loads. The improvement in fatigue characteristics by the cold expansion process is the established practice. It has been the ongoing research trend to quantify the enhancement in the fatigue strength due to the induction of residual stresses by prestressing the drilled hole. The optimization of the fatigue strength and to establish corresponding percentage of expansion of the fastener hole is the focus of this paper. Expanding into Taylor series, the functional relation between cyclic stress and mean stress given by Soderberg and subsequent application of interface theory lead to the formulation of the objective function. The expansion is linear in the elastic zone governed by Hooke's law and non-linear in plastic zone obeying the Holloman's law. The elasto-plastic relation between stress and strain provides the constraint condition. The non-linear relation of optimization is solved by a novel method to get the optimum enhancement fatigue strength due to cold expansion of the hole. The analytical results are compared with the experimental analysis for justification as reported in previous literature.

Key words: Elasto-plastic analysis, Cold hole expansion, Fatigue life improvement.