Process characterization of vibrostrengthening and application to fatigue enhancement of aluminum aerospace components-part II: Process visualization and modeling

In a companion paper [1], vibrostrengthening was introduced as a fatigue life enhancement process that can be used as an alternative to shot peening. Vibrostrengthening is a modification of a typical vibratory finishing process; it maintains the workpiece fixed inside a vibrating tub filled with a granular (typically ceramic or metal) medium. The resulting interaction between granular medium and workpiece produces improved fatigue resistance. Experimental results indicate that the vibrostrengthening process produced significant improvement in fatigue resistance on baseline (as machined) samples and performance as well, if not better than shot peening. The enhanced fatigue life produced by vibrostrengthening is thought to be the result of the combined effects of the improved surface finish and the compressive residual stress field that the process produces on the workpiece's surface. Curiously, compared to shot-peening, the velocities (and hence kinetic energy) of an individual particle impinging on the workpiece's surface is much smaller. Yet, the levels of subsurface stresses are comparable to those seen in shot-peened specimens. This paper makes two contributions to the understanding of the vibrostrengthening process. Through an experimental study using high-speed video recordings of the granular medium and motion analysis to correlate the instantaneous dynamics of particles impinging on the workpiece, it provides evidence that a single impingement event transfers the momentum of a number of particles from the granular medium to the workpiece, thus providing a plausible explanation for the plastic deformation and the resulting compressive stress field observed after the process. It also gives estimates of the effective number of particles participating in such an event. The study of the motion of the particles relative to the workpiece also explains variations in the effectiveness of the process in producing a fatigue strength enhancement with changes in location of the workpiece within the media tub. The second contribution the paper makes is in the development of a model to predict the expected fatigue life of a workpiece, given the surface finish and the residual stress levels introduced by the vibrostrengthening process. In the companion paper [1], we concluded that the fatigue life enhancement is the result of vibrostrengthening improving the surface finish and introducing a compressive residual stress field near the surface. Therefore, we introduce a model that, if given measured values of the surface roughness and the residual stress field, can predict fatigue crack propagation and hence the life of the specimen.