Propagation of part-through cracks (semi-elliptical surface cracks) by alternating bending stresses was studied with carbon steels of different strength level (S38C-N, S50C-QT). In the high stress intensity range, the propagation rate of part-through cracks could be correlated with that of through-thickness cracks in terms of the effective stress intensity based on the crack closure, irrespective of stress ratio and materials. In the low stress intensity range near threshold, on the other hand, no correlation could be found between the two, because the propagation rate-effective stress intensity relation in this range varied depending on several factors such as the geometry of crack, stress ratio, material property, etc.. These results seem to raise a question on the validity of using the threshold stress intensity as a design criterion.
The crack opening ratios were essentially the same for the two materials of different strength levels. This does not agree with the result reported in the literature, and the discrepancy was attributed to the difference of crack closure on the surface and in the interior of the specimen.