Multiaxial Fatigue Behavior of AA6016 and AA6016 to DX54D SPR Joints: Fracture Mechanisms and Data‐Driven Rupp's Model

ABSTRACT The fatigue and fracture behavior of self‐piercing rivet (SPR) joints between AA6016 aluminum sheets (1–2 mm) and DX54D steel (0.95 mm) was studied under 90°, 45°, and 0° multiaxial loading. Pure shear (90°) loading produced the highest fatigue strength, whereas multiaxial (45°) loading resulted in reduced performance, with pure tension (0°) loading showing minimum life. Failure location and mode depended on sheet thickness and material pairing: pierced sheets showed “eyebrow” cracks near the rivet head, and clenched sheets developed two radial cracks merging over the rivet protrusion. Fretting occurred at rivet–sheet and sheet–sheet interfaces, more pronounced under 90°, promoting microcrack nucleation with localized aluminum oxides. Cracks consistently initiated at the sheet–sheet interface. A data‐driven adaptation of Rupp's structural stress model, optimized using the generalized reduced gradient method, predicted fatigue life for various SPR geometries and material combinations. About 80% of experimental results fell within a ±3× scatter band, demonstrating robust predictive capability of the Rupp's model with optimized parameters.