Fig. 11: Crack path (a) in TMCP steels in air: G8, 10kN, 5Hz, sinewave at a scale of 20 µm, crack length between about 20.33 mm (18.52 MPa√m) to 30.09 mm (34.25 MPa√m), (b) schematic of the crack path, (c) magnification of the crack front
(direction: from right to left)
The crack tip maintained its sharpness throughout the Paris Region in the range 18.52 to 34.25 MPa√m. For the fact thatαHA could influence crack front diversion in the Paris Region is very important in the design of fatigue resistant steel. The fractographs of fatigued surfaces as shown in Fig. 10 can only show mechanism of the growth but cannot show these phenomena found in Fig. 11 specially the formation of the metal crumbs, which has not been reported in any literature. Due to many branched and arrested cracks, it is expected that the fractography of the fractured surface for this sample will likely show many occurrences of SC s. This is evident in Fig. 10(e & g) where many secondary cracks are present.
Fig. 12(a) shows the crack path observed in NR steel in this study. Relative to the crack morphology in the TMCP (see Fig. 11(a)), a low-angle crack diversion and low-angle, short-length branching were seen. The red arrows show the low-angle crack diversion, the black arrows the low angle crack branching and the white arrows show some metal bits or crumbs. Fig. 12(b) is the schematic of the crack path showing clearly the enumerated features. This NR crack path is clearly different from that of the TMCP (Fig. 11(b)) and it is a function of the microstructural features. In comparison with the crack growth path in J2N, the crack path in the TMCP is more complex with extensive high angle crack diversion and crack branching. The lengths of the branched cracks are considerably longer than that of J2N. There was also severe formation of relatively large metal crumbs in air in TMCP than in J2N. Several small metal crumbs were also seen as the ∆K increased. The crack-tip diversion, branching and creation of metal crumbs can reduce the effective stress at the local crack tips resulting in retardation of the FCGR . Fundamentally, the degree of crack tip stress shielding will vary with the angle of crack-tip diversion and the nature of metal crumbs created. The metal bit will be effective in retarding the crack growth if it re-orients and causes mismatch or wedging action. The tendency to re-orient may increase if the crumbs are smaller in rough-round shape rather than in plate-elongated form. These two natures of metal crumbs will be a function of the steel microstructure