Mechanism of Shot Peening Enhancement for the Fatigue Performance of AA7050

Author:  Daniel J. Chadwick, Siavash Ghanbari, David F. Bahr, Michael D. Sangid
Source:  ICSP-13
Doc ID:  2017064
Year of Publication:  2017
Abstract:  
Introduction: Shot peening has long been an industrial standard for airframe material, predominantly Al alloys. With an ever-present goal to increase efficiency, the aerospace community has continued to employ lightweighting strategies, which often translates to thinner components. The fatigue benefit of shot peening is often a compromise between the induced compressive residual stress field and the surface and near sub-surface damage. For thinner components, those with a greater ratio of surface area to volume, additional investigation is warranted to understand this mechanism of fatigue enhancement and associated engineering trade-offs. Sharp and Clark investigated the effect of peening on the fatigue life of 7050 aluminum alloy, with the intention of ‘establishing a lifeimprovement-factor’ [1] for components subjected to shot peening for structural use on the F/A-18 Hornet aircraft, of the Royal Australian Air Force. Particular concern in their work was the increased sensitivity of airframe structure and the subsequent fatigue life from surface features such as corrosion and mechanical damage [1] which could be imposed during the shot peening process. 7050 aluminum will most often be used in a condition with significant intermetallic precipitation; these precipitates impart the strength to the alloy but also are more brittle than the surrounding metallic matrix and hence could fracture during the shot peening process. In unpeened material these intermetallic particles, specifically Al7Cu2Fe, Mg2Si, and Al2CuMg, were the primary sites for crack initiation [2].