In air (Fig. 12), bifurcation, crumb formation and some zig-zag crack
propagation were sustained to high ∆K . In the SW , at
∆K above the 24 MPa√m, the crumb formation and crack branching
were almost absent (Fig. 19) and this resulted to a sudden increase in
the FCGR to the factor of 4. In Fig. 21(b), the crack growth
rates of the G8 and G10 denoted by the triangular data points, at 10kN,
5Hz are basically the same, confirming that the microstructures are
similar. In the Paris Region taken from 19.50 MPa√m, the CFCGR is
just higher than that of the air by a factor of about 1.5 measured from
their respective mean lines (not shown). There is no obvious trend in
the CFCGR s of the three tests in SW under sinewave in Fig.
21(b). We can reasonably say that
the CFCGR s are the same above about 20.50 MPa√m, but below this
value the 9kN, 0.2Hz test is somewhat higher. Fig.
21(c) shows the crack growth rates
for the two microstructures – J2N and TMCP in air and SW . It is
very clear that the crack path with the least crack diversion, branching
and crumb formation gave the highest FCGR . The extent of these
three phenomena is a function of the nature of the material
microstructure and crack tip condition. If the microstructure,
environment and loading condition are such that the three phenomena are
extensive then crack growth retardation occurs - otherwise the rate may
increase. High loading condition
in SW tends to limit the extent of crack diversion and branching.
This happens as a result of generation of relatively large plastically
deformed areas around and ahead of the crack tip by the high loading
condition and subsequent rapid dissolution of these areas by the
electrochemical process resulting to various degrees of crack tip
blunting. An incessant blunting of the crack front will limit the length
of the diversion and branched arm, forcing the crack to propagate in a
zig zag amplitude along the horizontal plane. Small zig zag amplitudes
will increase the crack growth rate. This perhaps explains why the crack
path found in J2N-A10F5 (Fig. 12) and J2N-S10F0.2 (Fig. 19) produced the
highest crack propagation rate in air and SW respectively as
shown in Fig. 21(c). In general, we observed that the crack path having
considerable diversion, bifurcation and crumbs will lower the rate of
the crack propagation. In Fig. 21(c), the CFCGR of the J2N is
higher than that of TMCP steel by a factor of about 2.0 and this
implies that the microstructure of the TMCP steel is superior to
that of J2N in resisting crack growth in α-P steel.
Technologically, this implies that the microstructure of steel can be
engineered to retard FCG in the Paris Region!
We mentioned earlier that a general stress ratio, R = 0.1 has been used
in this comparative study. This is an attempt to exclude the effect of
the so-called crack closure, as if such effect exited it would affect
the steels equally. In fact, the elimination of the asperities or rough
edges makes roughness induced crack closure in corrosive environment
unlikely. In the case of interlocking of deviated cracks, in SWthe corrosion process will be rapid in these areas too because the
mechanical rubbing action will exposure fresh surfaces for fast attack.
Thus, the superiority of TMCP steels over the NR inSW , even in air cannot be attributed to crack closure and is
unlikely for interlocking. A plausible reason in this study remains that
under stress ratio range of 0.1 – 0.67, temperature range of
0oC to room temperature and frequency range of 0.05 –
0.7Hz, the microstructure ofα-P steel strongly affected the rate at which the crack
propagated in air and SW in the Paris Region. This study
therefore asserts that, the existing theory that the microstructure does
not significantly affect the Paris Region appears to be incorrect. This
tends to be so, when the same grade of steel, but with different phase
morphologies are put into considerations. The major factor responsible
for this difference appears to be the ferrite and pearlite phase
morphologies and properties, which are function of the processing method
and chemical composition of the steel.
Conclusions
This study investigated the
influence of microstructure on the FCGR in advanced α-Psteels in the Paris Region, both in air and SW under sinewave
fatigue loading. Three phenomena - crack diversion, crack bifurcation
and metal crumb formation were identified as primary factors that
retarded crack growth in the steels. The angle of crack-tip diversion
and bifurcation affected the FCGR . The metal crumb affectedFCGR by wedging action. The extent of formation of the three
phenomena appears to be a function of the material microstructure,
environment and crack-tip condition. If the microstructure, environment
and loading condition are such that the three phenomena are extensive
then crack growth retardation occurs - otherwise the rate may increase.
That is, decrease in the crack diversion angle, branched-crack length
and number of metal crumbs formed is found to increase FCGR . High
angle crack-tip diversion, bifurcation and re-orientation of the metal
crumb resulting to a mismatch or wedging action, retarded FCG .
The three factors appear to retard the crack growth by reducing or
re-distributing the effective stress or driving force at the main active
crack tip. In SW , high angle crack diversion and bifurcation are
considerably reduced and this is a consequence of the repeated crack tip
blunting by the electrochemical dissolution process. If the branched
crack tips are blunted such that the length is short and the main active
crack tip is sharp, CFCGR can increase to about a factor of 4. If
the main active crack tip is blunted as well, the crack propagation
speed is considerably reduced. Microplastic zone appears to
control CFCGR in α-P steels. The microplastic zonesize and the extent of the electrochemical dissolution of the zone (or
crack tip blunting) appear to be additional primary factors influencing
crack growth in α-P steel. The microstructural phases and
morphology local to the main crack front and microplastic zone
size appear to determine which mode the crack growth will adopt. The
angle the crack front makes with the phases ahead of it tends to
determine if it would propagate in transgranular or byquasi-intergranular mode under low SIFR . This study,
therefore, asserts that the nature of the microstructures of the
α-P steels has a strong influence on the FCGR in the Paris
Region of the da/dN vs. ∆K sigmoidal curve in air andSW . However, considering the relatively small number of tests
involved, more experiments are needed to confirm and increase the
confidence in the results obtained in this work.