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