Fatigue Behavior of Resistance Spot Welded Steel Sheets Fabricated Using Electrodes with Different Tip Diameters

Tensile shear test specimens were fabricated by a resistance spot welding (RSW) procedure using electrodes with two different tip diameters. The tip diameter was changed to adjust the corona bond length with keeping the nugget sizes the same. Furthermore, the nugget sizes were changed by controlling the welding process parameters, resulting in the nugget sizes of 3√t, 4√t and 4.7√t, where t was the sheet thickness. Microstructural analyses revealed that the corona bond length increased with increasing tip diameter at a given nugget size. Subsequently, tensile shear static and fatigue tests were conducted to investigate the effects of corona bond lengths and nugget sizes on the mechanical properties of the welds. The tip diameter had little effect on the tensile strength of the welds, while the strength increased with increasing nugget size and the strength level of steel sheet. However, the effects of the tip diameters, nugget sizes and strength levels of steel sheets on the fatigue strengths were hardly seen. That was because the fatigue crack propagation life was dominant in the total fatigue life of the welds. Under fatigue loading condition, fatigue crack tended to grow along the corona bond, which could be attributed to the lower bonding strength along the corona bond in the high strength steel. However, the fatigue crack propagation life along the corona bond was negligible in comparison with the total fatigue life.