Improvement of fatigue performance by cold hole expansion. I: Model of fatigue limit improvement

The paper is concerned with localized fatigue performance improvement by means of a hole expansion technique. To explain this effect a theoretical model is proposed containing potentially important factors such as a residual stress sigma sub r (strain epsilon sub r ) induced by an uneven plastic deformation, stress--strain curve origin transformation sigma sub t , microstructural changes evoked by plastic deformation Delta S and secondary (technological) factors Delta sub 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 sub r,opt in the hole root material volume. A method is proposed for estimating its value, yielding the maximum fatigue life enhancement. Materials discussed include Al--4Cu--Mg, low carbon steel, Ni--Cr steel, manganese steel, and Cu.