Fig. 14: Effect of microstructure on FCGR in NR steel in air, for 10kN,
5Hz
Fig. 14(a, b & c) shows instances of the propagation of the crack that
initiated from the C(T) specimen notch of length 20.50 mm. In existing
theory, a long crack is regarded as a fully developed crack with
attendant plastic-induced crack closure effect and of length usually
greater than 0.5 mm [18], the plastic zone is larger than
one-fiftieth of the crack length (r > 1/50a )
and the rate of crack propagation is described by the Paris law. At a
length of 20.50 mm, the crack shown in Fig. 14 is a very long one. The
dotted yellow line shows the boundaries of the grains. The encircled
spaces by the yellow dotted lines are the ferrite grains while the red
dotted lines, where used, encircled the pearlite nodules. The white
solid line shows the movement of the fatigue crack through the
microstructural phases. The dots in Fig. 14(a) showed crack fronts that
stopped growing while the arrow indicates the mean active crack front.
In Fig. 14(a, b & c), the crack appears to have initiated from and then
propagated along what appears as the grain boundaries or
‘intergranularly’. A closer observation shows that the crack propagated
through the thin ribbon of the high alloy ferrite phase,αHA , that surrounds theαLR grain surfaces. This mode therefore is
properly referred here as quasi-intergranular cracking mode. At
the point marked X in Fig. 14(a), the thin ferrite phase ribbons, shown
by the blue triangular arrows is not favourably aligned to the crack
front – i.e., they are aligned at a very high angle, towards the
vertical, to the crack front. In this case, the crack will have to
advance transgranularly through the grain αLR in
front of it. In Fig. 14(b), the αHA is aligned
favourably or lies at low angle to the crack front, hence, the crack
advanced following this ferrite ribbon. Similar situation is seen in
Fig. 14(c), however, it appears that the grain ofαLR in front of the advancing crack front
resisted transgranular propagation of the crack in the direction 1 and
this caused the nucleation of another crack front that attempted to
follow the αHA (shown by the blue arrow) in
direction 2. This resulted to crack bifurcation and tends to suggest
that the αHA or theαLR/αHR boundary is the preferred
path for the crack growth, especially when the boundary is lying at low
angle to the crack growth plane. Note that in Fig. 14(a, b & c), theα phase dominated the local area with no P phase.
As the ∆K increased and the crack moved into local areas of mixedα and P phases as in Fig. 14(d to k). This region is
dominated by the αLR phase and some few Pcolonies. The αHR is almost absent in this
region. It appears that the P is equally likely to cause
crack-tip diversion and crack branching, but a closer examination showed
that these phenomena had been influenced to some extent by theαHA which appears to serve as the origin or
starting phsase for the P formation. Fig. 14(d) shows
intragranular crack growth and bifurcations, however, one could see that
some of the crack tip followed αHA phase. Fig.
14(e) shows the formation of metal crumb and this microstructural region
is also dominated by the αLR . Fig. 14(f to j)
show crack branching in the P phase. The tendency for crack
branching in the P may be attributed to the orientation of theP -colonies and αHA boundaries. In the
first case, this tendency may be because the pearlite has two
alternating phases – ferrite, α (iron, Fe ) and cementite,θ (Fe3C ) [64][65] with different
physical and chemical properties. Daeubler [66] studied the
influence of microstructure on the surface FCG behaviour of
pearlitic steels and presented a classical case where theP -colony orientation influenced the crack growth, especially when
the lamellae are favourably oriented to the load axis. In the present
study (PS ), it was observed thatαLR/αHA or andαHA/P boundaries are preferred paths ofFCG . The blue arrow in Fig. 14(i) shows a branched crack that
followed the αHA and the black arrow in Fig.
14(j) shows what appears to be a branched crack front followingαLR/P boundary. Fig. 14(k) shows another metal
crumb that formed in the vicinity of mainly αHRand αLR phases. It appears that the formation of
the metal crumb occurs in the αHR andαLR phase fields as shown in Fig. 14(e & k).
Fig. 15 shows crack path for G8 test in air, under 10kN, 5Hz stress
ratio of 0.1 and sinewave. The crack length measured about 10 mm
obtained under ∆K of about 18.50 to 34.25 MPa√m. The number of
high angle diversion and crack branching is by far more in theTMCP steel as compared with low angle crack-tip diversion and
crack branching that was consistently found in NR (see Fig. 14).
Measurements of the angle of crack branching in NR steel show
that they are generally below 45o. The only time high
angle diversion was found to be a little above 45o inNR steel is when the crack is avoidingαLR phase
or following the αHA or α/P boundaries
that is favourably oriented in its path. The α is shown in Fig.
15 by the blue triangular arrow. The space enclosed by the red dotted
line is the P while the yellow dotted line is the α .
Multiple formations of metal crumbs are also shown. These micrographs
show that the αLR dominated the microstructure
with extensive formation of the αHA . Many of theαHA appear
as sub-grain phases inside the αLR grains. The
presence of αHR is very small and theP -nodules are very small also - even some nodules are purely
single colonies. The crack path in the TMCP steel is completely
non-planar and more complex than that of the NR steel.