Abstract
Keywords - Spot Weldment, Fatigue Life, DC Potential Drop Method, Nugget Diameter, Automobile Steel Sheet
Abstract - The initiation and propagation lives of fatigue crack were studied for spot weldments composed of cold rolled steel sheets (SPC×SPC) and galvanized steel sheets (GA×GA) using DC potential drop method (DCPDM). Through the various test results, it was known that the fatigue crack initiation and propagation behaviours in all specimens could be definitely detected by DCPDM. The fatigue crack initiation life (Ni) detected by DCPDM in SPC×SPC and GA×GA spot weldments increased as the welding current and the nugget diameter (Nd) increased. The fatigue crack propagation life (∆Nf-i) defined as the difference of Ni and the fatigue fracture life (Nf) also increased according to the decrease of fatigue load
(∆P) and the increase of nugget diameter. In the same spot weldments, the increase of nugget diameter came to increase fatigue crack propagation life owing to decrease of stress concentration in front of nugget, especially the increasing extent for GA×GA spot weldment was very high. In the welding current 6kA, Nf for GA×GA spot weldment decreased more than that of SPC×SPC specimen due to zinc layer coated in steel sheet and undersized nugget diameter. On the other hand, in 8kA and 10kA, the almost all GA×GA spot weldments showed higher Nf in spite of lower Ni than that of SPC×SPC specimens.