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Tensile Properties of Nugget Region in a Spot Weld at Various Strain-Rates (점용접 너겟부의 변형률속도에 따른 인장 물성에 관한 연구)
A resistance spot welding process has been essential in the joining process of automobile industry because it is economic and quality guaranteed joining process. A modern vehicle, for example, typically contains 7,000 ~ 13,000 spot-welded joints.
There are two fracture modes, button full out and interfacial fracture, in spot welded joints. In case of mild steel, interfacial fracture is considered as undesirable fracture because energy absorption ratio and load-bearing capacity do not satisfy the standard. In case of advanced high-strength steels (AHSS), however, interfacial fracture takes place with satisfying load-bearing capacity standard.
Strain-rates during automobile crash accidents are generally in the range of 0.001/s to 100/s. The mechanical properties of metals, especially UTS and yield stress are remarkably different on static and dynamic loading conditions.
This paper deals with the tensile properties acquisition of nugget region in spot-welded joint at various strain-rates using micro material testing machine. The driving and load measuring part of Static Micro Material Testing Machine (SMMTM) and High-Speed Micro Material Testing Machine (HSMMTM) were replaced by considering the load requirement. A jig and a gripping device are designed to meet the requirements of the specimen shape and strength.
Two kinds of specimens for the tensile tests were obtained each from the center of the welded spot and 1 mm away from the centerline at the spot welded joint of the DP980 by wire electric discharged machining(EDM). The thickness of the specimen was 0.3 mm and the gage length of the specimen was 1 mm. Digital image correction method (DIC) was applied to measure the longitudinal strain. The speckle pattern generation method was also improved for DIC method application. Quasi-static tests were performed with SMMTM at strain-rates of 0.001/s and 0.1/s. Dynamic tests were performed with HSMMTM at strain-rates of 1/s, 10/s and 100/s.
The Stress-Strain curves were obtained at each strain-rate.
A comparative evaluation was performed between the material properties of base metal and nugget region in the spot welded joint, based on the obtained stress-strain curves.