Issue 37

D. Angelova et alii, Frattura ed Integrità Strutturale, 37 (2016) 258-264; DOI: 10.3221/IGF-ESIS.37.34 260 R ESULTS AND DISCUSSION Data obtained for Steel A and published for Steel B - a and N - are presented in Figs. 1a, b,c respectively, and dependences “Crack length, a – Numbers of cycles, N ”, “ a – N ”, plotted in the same figures. The Steel A curves “ a – N ” for RBF, Fig.1a, show that both major cracks (for Δσ = 1200 and 1400 MPa) have originated first. It can be seen that the major crack in the family "Major crack - Secondary cracks", "MC-SC", at Δσ = 1400 MPa has initiated in an earlier stage in comparison with the initiation stages of all secondary cracks. At the same time the major crack and the secondary cracks in the family "MC-SC" at Δσ = 1200 MPa show small difference at the number of cycles to initiation, close propagation (in terms of cycles) and a kind of competition between the secondary cracks for merging their propagation paths with that of the major crack. The surface of whole working area shows (by observation after specimen's fracture) unusual roughness, probably due to specimen polishing [5,6], confirming the strong influence of the surface-condition factor on fatigue behaviour. The curves “ a – N ” for Steel B (TF) are plotted in Fig. 1b,c. They show a major crack and only a few secondary cracks in each family "MC-SC" corresponding to a given stress range. An exception is the family "MC-SC" at Δτ = 900 MPa in corrosion with several secondary cracks. The family "MC-SC" at Δτ = 817 MPa in corrosion has more secondary cracks than those at Δτ = 404 and 601 MPa. Then the two families "MC-SC" at Δτ = 817 and 900 MPa having more secondary cracks are located very closely (in terms of cycles), which shows that above Δτ = 817 MPa the surface short cracks propagation and paths look like those of the family "MC-SC" of Steel A at Δσ = 1200 MPa. This means that nature of material is of great importance, but varying with fatigue loading and kind of scheme, environment and surface condition, it is possible to observe a similar short fatigue crack behaviour of different materials. a b c Figure 1: Plots “Crack length, a – Numbers of cycles, N ” and lines d 1 and d 2 defining PSC regime for Steels: (a) A - RBF; (b) & (c) B – TF An interesting fact can be mentioned. During the transition regime of PSC some specific events take place: (i) As a crack grows longer at a constant cyclic stress range, its plastic zone size controlling crack growth rate, increases.