Cracks often appear in the surface of the VC coated steel specimens subjected to rotating bending fatigue tests. In order to investigate the cause of formation of these cracks, detailed observation was made on the surfaces of the rotating bending specimens after the fatigue test and during the static test, and of the two dimensional model specimens during the plane bending fatigue test.
The results obtained are as follows:
(1) In the rotating bending fatigue test, wide aperture cracks were observed on the fracture origin side, while narrow cracks and sometimes netty cracks were observed on the opposite side. These cracks became more significant with increasing ductility of the substrate steels.
(2) Even in the last stage of the fatigue process, no such surface crack appeared in the unbroken specimens. Therefore, it is considered that the surface cracks are generated by the deformation around the fracture point in the separation process of fracture.
(3) The static bending test results suggest that the wide cracks on the fracture origin side are formed by compressive stress, while the narrow and netty cracks on the opposite side are formed by tensile stress. Consequently, it is considered that the cracks mentioned at (1) are formed through the following process: During the final fracture of the rotating bending fatigue specimens, separation occurs by tensile stress, which creates narrow and netty cracks. Then, the cracked parts of the origin side strike each other, producing compressive stress, which creates wide cracks.