Real‐Time Observation of Strain Evolution in Lap Shear Test of BH220 Steel Resistance Spot‐Welded Joints via a Digital Image Correlation Method

This article presents an investigation of the real‐time deformation and strain field changes of baked hardening (BH) 220 steel plate resistance spot welds in the lap tensile shear tests via digital image correlation (DIC) technology. 2D DIC analysis can be used to provide a quantitative assessment of the strain competition between the weld nugget and the surrounding metal. The data obtained from the DIC technique indicate that the shear strain is primarily concentrated in the outer metal of the weld, consistent with a nugget pull‐out failure (PF) mode. In contrast, if the welding parameters are inappropriate, for example, if the welding current is 7.2 kA, the welding time is 10 cycles and the electrode pressure is 0.35 MPa, a significant shear strain appears in the nugget of the BH220 weld. This subsequently causes the weld to fail in an unfavorable interface failure mode in the shear test. Calculations show that if the failure mode is nugget pull‐out, the mean failure strength is 13.5 kN, while the value for interface failure is 6.55 kN. The PF mode is characterized by ductile failure, whereby the material yields and necks through the base material. This failure mode is not associated with the notch region.