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 theαLRHR 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 thatαLRHA 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.