Fig. 12: Fatigue crack path (a) in J2N steel in air (10kN, 5Hz at a scale of 20 µm) (b) schematic of the crack path
(direction: from right to left)
Fig. 13 shows the FCGR s in two samples of the NR (the circle data) and three samples of the TMCP (the square data) steels. As noted, the tests were performed in air under similar test conditions at 10kN and 2Hz, 5Hz for the three materials - J2N, G8 and G10. Note that the microstructures of G8 and G10 are generally the same (see Fig. 4). The FCGR s at 10kN, 2Hz (J2N-A10F2) and that of 10kN, 5Hz (J2N-A10F5) are generally the same, though that of J2N-A10F2 is slightly higher. This may be because the maximum fatigue loading, Pmax can be fully delivered during test at reduced frequency. Generally, these tests show that the effect of frequency onFCGR in air is negligible within the same steel subgrade. An important feature of the crack growth is that generally the tips of the propagating cracks in the TMCP steel are always sharp throughout the Paris Region (see Fig. 11(c)). High angle crack bifurcation and diversion are generally absent, and the formation of metal crumbs are few in the NR steel. This explains the reason for the higherFCGR in the NR as compared with the TMCP steel. This implies that, the longer the branched arms; the more the diversions and branching angles; the more the crack growth is retarded due to re-distribution and reduction of the driving force at the main crack tip. Where the three phenomena are few, one would expect a highFCGR .