Fatigue crack growth measurement and microstructural characterization of circumferentially part through cracked pipes under constant-amplitude pure bending

In the present work, an analytical solution for measuring the indirect crack length in the radial and circumferential directions of 3D TP 316L stainless steel pipe samples under pure bending with different notch angles (35°, 45°, and 50°) was developed. The fractographical features of crack initiation and propagation were also studied. Part-through circumferential prenotched pipes with notch angles at the center were considered for calibrating the crack opening displacement (COD) gauge. The analytical relationship between the COD reading and crack geometry directly reflects the crack length in the pipe under pure bending. The coefficient of calibration was calculated via polynomial regression of the COD readings with the experimental crack length. The crack initiated in the radial direction (R direction) and then propagated in the circumferential direction (C direction). Fatigue crack retardation takes place as the crack spreads through the internal wall of the pipe at all notch angles. The fatigue-fractured surface morphology indicated a granular microstructure, crack initiation sites, secondary cracks, striations, and microvoids corresponding to different stages of fatigue crack growth.