Fatigue properties of spot joints of metal-plastic composites with DP 800 steel prepared by ultrasound resistance spot welding

The aim of this work is to analyze the properties of spot joints of metal-plastic composites (Litecor) with DP 800 steel. The joints were made using ultrasound resistance spot welding technology. A metallographic analysis of the joints was carried out, and the basic areas of the weld structure were determined. The separation and decomposition of the polymer core was also illustrated, with no observed diffusion between the Litecor covers and the polypropylene core. Fatigue tests were the main goal of this work, therefore a fatigue curve was determined and the mechanisms of fatigue failure at various levels of fatigue load were analyzed. The tests were carried out at a frequency of 30 Hz, the cycle asymmetry coefficient was R = 0.1 and the limit number of cycles was 2 × 106. Fatigue failure mechanisms specific to particular levels of fatigue load were demonstrated, which were: 2.2, 1.9, 1.5, 1.2, and 1 kN. For joints subjected to fatigue shear has been demonstrated that the boundary between low-cycle and high-cycle fatigue is located at a cyclic shear stress level of approximately 132 MPa. However, with the assumed limit number of fatigue cycles, the fatigue shear strength was 70.576 MPa. Macro- and microscopic fractographic analysis was carried out for joints after fatigue tests in order to demonstrate the mechanisms of failure at individual levels of cyclic load.