Suppression of fatigue crack propagation of duralumin by hybrid peening

Peening is one of the mechanical surface modification techniques, which is used to improve the fatigue strength by the introduction of the compressive residual stress into the surface of specimen. Several peening techniques, such as shot peening, cavitation peening and laser peening have been proposed and demonstrated the improvement of fatigue strength. Mechanical properties on surface modification layer vary with the techniques because of the difference of mechanism of impacts and strain rate. Therefore, it is possible to improve the fatigue strength further by hybrid peening, which is a combination of different peening techniques, due to controlling the mechanical properties on surface modification layer. In this paper, in order to demonstrate the suppression of fatigue crack propagation by hybrid peening combining shot peening and cavitation peening or cavitation peening and laser peening, the fatigue crack propagation in duralumin treated by hybrid peening was investigated, and the crack propagation behavior of the specimen treated by hybrid peening and single peening method was compared. The relationship between the fatigue crack propagation and the mechanical properties was also evaluated. It was revealed that hybrid peening (former process: laser peening, latter process: cavitation peening and former process: cavitation peening, latter process: shot peening) was more effective to suppress the fatigue crack propagation than single peening method and the number of cycle to failure of the specimen treated by hybrid peening was the largest. The new indicator f(σR’, HV’, Rz’, β’) = σR･HV･β/Rz combined with four factors i.e. compressive residual stress σR, Vickers hardness HV, full-width at half maximum β and maximum height roughness Rz showed the effect of suppression of fatigue crack propagation by peening.