Fatigue strength of self-piercing riveted joints in lap-shear specimens of aluminium and steel sheets

The self-piercing riveting (SPR) process is gaining popularity because of its many advantages. This study investigated the fatigue strength of SPR joints in tensile-shear specimens with dissimilar Al-5052 and steel sheets. A structural analysis of the specimen was conducted. For this specimen, the upper steel sheet withstood applied load in a monotonic test and played a major role in the low-cycle region. In the high-cycle region, however, the harder surface of the upper steel sheet reduced the fatigue strength by enhancing fretting crack initiation on the opposite softer aluminium surface. Therefore, the fatigue endurance of the specimen was reduced. The fatigue endurance of a SPR joint with the combination of steel and aluminium sheets was found to be governed by the strength of the lower sheet, which is more vulnerable to the applied loading. Thus, it is desirable to use a stronger metal sheet as the lower sheet with regard to the fatigue performance. Scanning acoustic microscopy was effectively used to reveal and prove the formation and growth of subsurface cracks in SPR joints. The structural stress can predict the fatigue lifetimes of the SPR joint specimens within a factor of three.