Failure process and fracture surface analysis of aluminum-steel tubular joints fabricated by magnetic pulse crimping under fatigue loading

Fatigue failure of joints is a critical factor affecting structural safety design. This study investigates the failure mechanism of aluminum (Al)-steel tubular joints fabricated via magnetic pulse crimping (MPC) using multiscale characterization techniques. Shear effects caused the thinning of the Al tube, which induced the formation of a notch on the inner surface. The mechanical interlocking of the material and the notch effect led to stress concentration, thereby promoting fatigue crack initiation (FCI) at the notch. Fracture morphology indicated that FCI was a mixture of intergranular and transgranular fracture. Interestingly, under higher stress conditions (58.5 MPa), the fatigue crack propagation (FCP) rate was extremely fast. The crack propagation process lacked a steady-state stage, with no distinct fatigue striations forming before fracture occurred. The features of the dimples were the main characteristic of final fracture (FF). Electron backscattered diffraction (EBSD) results showed that the differences in plastic deformation degree, dislocation density, and grain Schmid factor (SF) value were the main causes of intergranular and transgranular fracture in FCI.