Fatigue properties and failure mechanisms of self-piercing riveted joints in titanium and aluminum alloy dissimilar sheets

A static test and fatigue test of dissimilar single-lap self-piercing riveted joints were conducted for pure titanium (TA1) and aluminum-lithium alloy (AL1420). The fatigue performance of the joints was analyzed on the basis of the S-N curves; specifically, a fitting method was used to construct the S-N curves of the joints using three empirical parameters. Scanning electron microscopy was used to study the fatigue fracture surfaces of the joints and the microfatigue failure mechanism of the joints. The results indicate that the static performance and fatigue characteristics of the joints are inconsistent:the TA1-AL1420(TA) joint exhibits better static performance, whereas its fatigue characteristics are worse than those of the AL1420-TA1(AT) joint. The fatigue failure mode of the TA joint for the lower sheet is fracture; the fatigue crack appears in the sheet area around the rivet tail and extends along the width of the sheet, leading to complete fracture of the lower sheet. The loss of efficacy of the AT joint in the short-life region is related to fracture failure of the rivet, which exhibits brittle fatigue fracture. A mixed failure mode of rivet fracture and lower sheet fracture occurs in the middle-long life region, and a fatigue crack appears from one side of the lower sheet, extends along the width of the sheet to the other direction, and eventually lead to the failure of the lower sheet.