IMPROVEMENT OF FATIGUE PERFORMANCE BY COLD HOLE EXPANSION .1. MODEL OF FATIGUE LIMIT IMPROVEMENT

The paper is concerned with localized fatigue performance improvement by means of a hole expansion technique. In order to explain this effect a theoretical model is proposed containing potentially important factors such as a residual stress sigma(r) (strain epsilon(r)) induced by an uneven plastic deformation, stress-strain curve origin transformation sigma(t), microstructural changes evoked by plastic deformation DELTAS and secondary (technological) factors DELTA(tech). Based on Haigh's diagram and the material stress-strain curve it is concluded that the expanding process has an optimum that can be characterized by the optimal residual strain epsilon(r,opt) in the hole root material volume. A method is proposed for estimating its value, yielding the maximum fatigue life enhancement.