Fatigue damage evolution of micro friction stir blind riveting joint through acoustic emission analysis

This research explored the fatigue behavior for a newly developed multilayer dissimilar material joining technique, micro friction stir blind riveting (viz. µFSBR), through integrating acoustic emission (AE) data with cyclic loading data. Multilayer µFSBR joints consisting of one Cu layer and three Al layers connected through Al rivet were investigated, where the fatigue behavior was encapsulated through examining failure modes, damage mechanism and in-situ damage evolution with AE monitoring. It was revealed that the µFSBR process mechanics influenced the failure modes as the resultant effects of friction stirring, leading to the failure in the stirred region for Cu layer at high loads and failure in base metal for Cu layer at low loads. The damage mechanisms were identified through hysteresis loops, where cyclic creep or ratcheting strain occurred in the low cycle fatigue scenarios while fatigue damage was the governing damage mechanism in high cycle fatigue paradigms. In-situ damage characterization was performed by introducing a novel analysis scheme using AE hit datasets, which effectively identified three distinct stages, including (1) initial deformation, (2) cyclic hardening at low cycle or softening at high cycle fatigue, and (3) crack initiation and growth till fracture. The stage of crack initiation and growth till fracture was further explained by leveraging the cumulative AE hit count rate. Furthermore, the relationship between the life fraction-load-AE amplitude was developed to characterize the damage source mechanisms in the entire fatigue life at different load levels by integrating both load and AE datasets.