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.