Fig.
8: Typical microstructures of low alloy α- P steels
One interesting note here is that
even though the steels have similar micro-constituents, the FCGR s
are significantly apart, especially at low SIFR . Often, many
researchers associate this variation in the Paris Region with material
variability or crack closure (R-factor), but we could see that the
margin of separation is very much significant – up to a factor of about
4 from the two extremes. An important observation also is that theFCGR of the two microstructural variants (NR &TMCP ) of the EH36 steel studied by Cheng [40] (NR ) and
Tsay [57] (TMCP ) under similar experimental conditions are
different. The FCGR data of the EH36 (NR ) denoted with
black circle in Fig. 9 partitioned into the NR domain while that
of EH36 (TMCP ) separated into the TMCP domain as shown by
the black square. Note that within this region the effect of crack
closure is insignificant or negligible. In general, Fig. 9 tends to
suggest that microstructure affected the FCGR in air.
Microstructure and FCG path in
air
Post-failure examination of the fatigued surfaces in air revealed
features shown in Fig. 10. The fractographs Fig. 10(a & b) were
obtained from the NR steel (J2N), while that of Fig. 10(c & d)
were obtained by Adedipe [63] for the same steel grade. Fig. 10(e)
is the fractograph obtained in air for the TMCP steel. The part
in a red box in Fig. 10(e) was magnified as shown in Fig. 10(f) and the
area in the red box in Fig. 10(g) was magnified to that shown in Fig.
10(h). From all the fractographs, it is very obvious that the failure in
air had occurred by ductile striation mechanism (DSM ) with
secondary crackings (SC ) as shown by the arrows. Few SC in
the normalised steel, J2N and more SC in the TMCP were
observed. The occurrence of the SC increased with increase inSIFR . Striations were seen even inside the secondary cracks as
can be seen in Fig. 10(f & h). This shows that the groove was made by a
moving crack front and not due to brittle cracking or cleavage. TheDSM as seen in the fractograph is typical of the failure
mechanism in the Paris Region for low carbon steel. These fractographs
do not give significant clue to the observed difference in theFCGR in Fig. 7 and Fig. 9. An altervative approach is to examine
the crack path and the features influencing crack growth in the
materials.
To understand the disparity in the FCGR between NR and theTMCP steels as presented in Fig. 7 and Fig. 9, a crack path
metallography was carried out and the result is presented in Fig. 11 for
the TMCP steel, G8. Fig. 11(a) shows the general crack paths seen
in the TMCP steels in air for 10kN, 5Hz, stress ratio of 0.1,
under sinewave. The yellow double line arrow in the figures shows the
direction of the applied fatigue load. The long crack path was obtained
for ∆K of about 18.52 to 34.25 MPa√m, i.e. fully in the Paris
Region. It is very clear that the crack path is non-planar and complex.
The traces of the crack path are given in Fig. 11(b). The crack path has
extensive high degree of large angle crack diversions, multiple
bifurcations and crumb formations as noted clearly in Fig. 11(b).