Small crack formation in a low carbon steel with banded ferrite-pearlite structure

The phenomenon of the formation of small cracks in a banded plain carbon steel has been studied on dumble-shaped plate type specimens under varied cyclic stress amplitudes at the load ratio of R = 0. The locations at which the cracks were found to nucleate could be classified as: (i) ferrite-pearlite interface (FPI), (ii) ferrite-ferrite grain boundary (FFGB), (iii) ferrite grain body, and (iv) inclusion-matrix interface. The most preferred site for such crack nucleation in the investigated steel was found to be the ferrite-pearlite interface. The orientation of the initiated small cracks was found to vary widely between 0° and 90° with respect to the loading direction unlike some earlier reported results. It is reported here for the first time that the angle between the direction of banding and the loading axis has pronounced effect on the orientation of such small cracks. The lengths of these cracks at FPI and FFGB are found to be larger than the ones nucleated inside ferrite grain body. The preferred site of crack nucleation and the influence of the banding on the size and the orientation of the small cracks have been explained using inhomogeneous distribution of stress/strain in the microstructure and incompatible strains along the interfaces.