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).