The development of fatigue cracks in galvanized low carbon and high-strength-low-alloy steel, tensile-shear-spot weldments was investigated using a d-c electrical-potential-drop technique which sensed the depth of fatigue cracks as they propagated through the thickness of the specimens. These resistivity changes were correlated with the actual crack depths measured on sectioned specimens. This correlation was used to identify lives at which certain crack depths were achieved: the duration of Stage I (initiation) corresponded to a crack depth of about 18% of the sheet thickness (≈0.16 to 0.48 mm), and the duration of Stage II (through-thickness propagation) corresponded to a crack depth equal to the sheet thickness (0.89 to 2.72 mm). The final failure of the specimen determined the conclusion of Stage III (cross-width crack propagation).
The effects of sheet thickness, specimen width, and nugget diameter were studied. Sheet thickness was found to have the largest effect on fatigue life, while the nugget diameter had the least. The fatigue life of the galvanized high-strength-low-alloy specimens was found to be dominated by through-thickness propagation (Stage II), while the fatigue life of the galvanized low carbon specimens was dominated by Stage I. The fatigue life of all specimens tested at a given R ratio could be correlated with the initial value of stress intensity factor for the combined effects of Mode I and II loading conditions.