Abstract
The fatigue life of the resistance spot weld of 980 MPa grade transformation induced plasticity (TRIP) steel was investigated and failure modes and fracture surfaces according to the fatigue load were analyzed. The fatigue life according to the nugget size was observed by using two electrodes with face diameters of 8 mm and 10 mm. When an electrode face diameter with 10 mm was used, the nugget size was large, and the fatigue life was further increased. After the fatigue test, three types of failure modes were observed, namely pull-out, plug, and heat affected zone (HAZ) failure, depending on the fatigue load. The fracture surfaces in each failure mode were analyzed. In all failure modes, a crack was initiated in the HAZ region, which is the interface between the two materials in all failure modes. In the case of pull-out failure, the crack propagates as if it surrounds the nugget at the outer edge of the nugget. In the case of HAZ failure, the crack propagates in the thickness direction of the material and outward in the nugget shell. Plug failure occurs with pull-out failure and HAZ failure mixed. The propagation patterns of cracks were different for each failure mode. The reason why the failure mode and the fracture surface are different according to the fatigue load is that the propagation speed of the fatigue crack is fast when the fatigue load is relatively large and is slow when the fatigue load is low.
Highlights
Environmental regulations are strengthened with interest in issues such as fine dust and carbon dioxide emission
Zhao et al carried out tensile shear tests on resistance spot welds of DP600 steel and studied the effects of electrode force on tensile shear strength and fracture energy absorption, failure mode, and microstructure [13]
The papers that observe fatigue behavior and fracture surface were limited to the analysis of the mechanical properties of the weld and failure mode by observing the microstructure
Summary
Environmental regulations are strengthened with interest in issues such as fine dust and carbon dioxide emission. Many studies have been carried out on the failure mode and fatigue behavior of resistance spot welds for various materials. Vural et al conducted a fatigue test after resistance spot welding of austenitic and galvanized steels, and derive S-N curves [12] They observed fatigue performance, crack length, and fracture surface according to weld combination and nugget size. Zhao et al carried out tensile shear tests on resistance spot welds of DP600 steel and studied the effects of electrode force on tensile shear strength and fracture energy absorption, failure mode, and microstructure [13]. The papers that observe fatigue behavior and fracture surface were limited to the analysis of the mechanical properties of the weld and failure mode by observing the microstructure.
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