Although resistance spot welded joints are widely applied for fabrication of the monocoque body of cars, fatigue strength of the joints are less than the base materials in general. It is very important for satisfying the structural integrities of car bodies to evaluate the fatigue performance of spot welded joints. On the other hand, the S-N curves approach for the fatigue life prediction, which is commonly applied in the structural design stages, cannot give the fatigue crack growth history. Therefore, the fracture mechanics approach for estimating the fatigue crack growth history is expected. Improved effective stress intensity factor range based on the RPG load criterion and the numerical simulation method of fatigue crack growth which consider the fatigue crack opening and closing behaviour of the crack proposed by authors' group can be powerful method to estimate fatigue crack growth history under arbitrary loading sequences. This procedure could be applied not only a through thickness crack but also surface cracks from a sound stress concentration part. However, the changing manner of aspect ratio of a surface crack, which corresponds to the relationship between the aspect ratio and the dimensionless crack depth, must be given in advance. The aspect ratios of a fatigue surface crack occurred at the spot welded toe in SPCC270, SPCD270 and SPC440 steels, which are commonly applied for car bodies, during the growth were measured and confirmed the characteristics of behaviour of the aspect ratio for spot welded joints.
