Fig. 15: Fatigue crack growth in TMCP, G8, 10kN, 5Hz in air
It
was observed that the crack-tip diversion and branching in Fig. 15 were
caused by the crack tip following path of least resistance created
mainly by the αHA or theαLR/αHR boundaries - but mostly
by the αHA ribbons. The crack bifurcation is
quite extensive with long arms as can be seen in Fig. 15(a to e) and the
diversion of the main active crack front as shown in Fig. 15(f) was
measured to be about 43ᵒ to the horizontal plane. The
branched arms are measured to be in the range 38ᵒ -
46ᵒ. It is interesting to see that the path of least
resistant caused many of the branched arms to deviate almost at an angle
within this range. This simply implies that both tensile and shear
stresses continuously acted on the crack as it propagated in theTMCP , even in the Paris Region! The metal crumbs as noted in Fig.
15 are by far more in the TMCP than the NR steel. It was
observed also that the αHA does not only have the
potency to divert and branch out crack front, it can as well facilitate
re-joining of branched crack fronts into one main crack front resulting
to the crumb formation. This crumb is found to be a tore-off of the
steel grain or block of grains. In other words, the fatigue crack front
in α-P steel appears not to have a horizontal crack-tip edge, but
rather series of fluctuating wavelike crack front, whose nature depends
on the steel grains and the phases surrounding the grain surfaces. It is
important to note that this metal crumb along the crack path has not
been reported elsewhere in the literatures or considered in any
theoretical formation or treatment. The P- colonies in theNR steel are large, blocky and dense. In the TMCP steels,
the P appears to grow in two forms – small-blocky colony and a
thin, elongated or needle-like morphology colony along what may be
described as the α grain boundaries as shown, in some cases, by
the space enclosed with the red dotted line in Fig. 15. The effect of
thermomechanical control rolling process appears to be the disruption of
the P banding and blocky morphology into randomly distributed
thin P needles. From the observation, it appears that the
interface between the α and P is an additional effective
preferable path for the crack growth if the P -nodule is elongated
and aligns favourably to the crack front. In Fig. 15, the branched
cracks did not just move about linearly but tends to trace theαHA phase. This suggests that the energy needed
to propagate through the αHA is much less than
that for traversing the αLR grain. The influence
of the phase morphology persisted to a length of 9 mm and at 32 MPa√m.
The existing theory says that the crack path in the Stage II of theda/dN vs. ∆K sigmoidal curve is generally across the
grains - transgranular [17] and the material microstructure has
little or negligible effect in this region
[3][5][6][18]. It can be seen that the crack path in theNR in Fig. 14 and that of TMCP in Fig. 15 are not the
same. The extensive crack diversion, bifurcation and metal crumbs
formation retarded the FCGR and explains why the FCGR in
the TMCP is lower than that of the NR in Fig. 13. This
finding appears to show that microstructure has a strong influence on
the rate at which fatigue crack grows in α-P steel in air. This
assertion is contrary to the existing theory that microstructure of a
steel has little or negligible effect on the FCGR in air. Again,
both transgranular and intergranular modes of propagation were observed
in air. What appears as the intergranular cracking mode predominated in
the TMCP steel. However, what appears as an intergranular mode is
actually the growth of the crack through the thin layer ofαHA in between the grains ofαLR or the P . Hence this mode is
identified here as the quasi-intergranular mode. The morphology
and chemistry of the microstructural phases and load level appear to
determine which mode the crack growth will adopt.
Another factor is that the angle
the crack front makes with the phases ahead of it tends to determine if
it would propagate in transgranular or quasi-intergranular mode.
However, in Fig. 10(f) ductile
striations were found inside the secondary crack – which actually
appears as a branched crack when viewd from the crack line. This clearly
shows that the mode is not strictly intergranular, but the crack moved
in the αHA phase ribbon immediately adjacent to
the αLR or the P interface, justifying the
use of word - quasi-intergranular . In general, TMCP steel
is more resistant to crack propagation than NR steel of the same
steel grade. This is made possible by its refined microstructure that
branched (with long arms) and diverted the crack front at large angles
as compared to the small angle path diversion found in the NRsteel.
It
must be noted that the ability of the αHA to
deviate crack front appears to lie in its alloying composition relative
to the surrounding phases. TheαHA clearly
has more of carbon, silicon and oxygen contents than other phases (see
Table 3). The increase in the amount of these elements might have
embrittled the phase, making it the least resistant path.