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Fatigue Behaviors of Resistance Spot Welds for 980 MPa Grade TRIP Steel

1
Joining R&D Group, Korea Institute of Industrial Technology, Incheon 21999, Korea
2
School of Mechanical Design Engineering, Hanyang University, Seoul 04763, Korea
3
Department of Materials Science & Engineering, Seoul National University, Seoul 08826, Korea
*
Authors to whom correspondence should be addressed.
Metals 2019, 9(10), 1086; https://doi.org/10.3390/met9101086
Received: 23 September 2019 / Revised: 7 October 2019 / Accepted: 8 October 2019 / Published: 10 October 2019
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. View Full-Text
Keywords: resistance spot welding; fatigue behavior; failure mode; nugget size; fracture surface; advanced high strength steel resistance spot welding; fatigue behavior; failure mode; nugget size; fracture surface; advanced high strength steel
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Cho, H.; Nam, S.; Hwang, I.; Oh, J.H.; Kang, M.; Kim, Y.-M. Fatigue Behaviors of Resistance Spot Welds for 980 MPa Grade TRIP Steel. Metals 2019, 9, 1086.

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