A fractographic investigation on shear mode fatigue crack growth in annealed mild steel has been made. Although it is well known that shear mode growth is seen in the initial stage of fatigue crack growth in smooth or mildly notched specimens and is called stage I growth, a fractographically similar mode of fatigue crack growth has been observed in several other cases, too. In this study, the crystallographic similarlity of these cases of shear mode fatigue crack growth was investigated by the use of etch pits.
The fractographically similar fatigue crack growth to the stage I was observed in the initial stage of fatigue crack growth from a sharp notch or precrack. It was also observed when ΔK was decreased to make very slow crack growth after the transition to the stage II growth, or in the Mode II fatigue crack growth under a specially made apparatus. The crack growth of this mode was named shear mode growth here. The fracture surface of shear mode growth usually showed cylindrical surface formed by parallel lamellae of the length of about one grain. It was shown by the use of etch pits that the direction of these parallel lamellae coincides with the direction of slip in iron <111>. Accordingly, it was concluded that the fracture surface of shear mode growth was the pencil glide surface known in the slip in iron under monotonic loading.
