Issue 37

D. Angelova et alii, Frattura ed Integrità Strutturale, 37 (2016) 249-257; DOI: 10.3221/IGF-ESIS.37.33 255 smaller than that of Specimen 9 which is tested at higher stress range. The surface of Specimen 7 whole-working area shows (by observation after specimen's fracture) unusual roughness, probably due to the polishing of the specimen; it confirms the especially strong influence of the surface-condition factor on fatigue behaviour. (a) (b) Figure 5 : Images of failure development and fractured surfaces: (a) fractographic observations of the fractured surfaces of Specimen 7 (   =1200 MPa) from Steel A specimens; (b) crack development at   = 915 MPa, Steel B, Air. The curves "Crack length – N umbers of cycles" for B steel are plotted in Fig. 6b. They show a major crack and only a few secondary cracks in each family "Major crack - Secondary cracks" corresponding to a given stress range. An exception is the family "Major crack – Secondary cracks" at Δτ =900 MPa in corrosion environment with several secondary cracks; this can be seen clearly in Fig. 4a on the corresponding microstructure that is result of application of Δγ =1.2% (Δτ =900 MPa) at N/N f =0.902. The family "Major crack–Secondary cracks" at Δτ =817 MPa in corrosion environment has more secondary cracks than those at Δτ =404 MPa and Δτ=601 MPa, Fig. 6b. At the same time the two families "Major crack – Secondary cracks" at Δτ =817 MPa and 900 MPa having more secondary cracks are located very closely, which shows than above Δτ =817 MPa surface short cracks propagation and paths, Fig. 6b, look like those of the family "Major crack – Secondary cracks" of Specimen 7 (Δσ=1200 MPa), Steel A, Fig. 6a. This means that nature of material is of great importance, but varying with fatigue loading and kind of scheme, as well as with environment and surface condition, it is possible to observe (it can lead to) a similar short fatigue crack behaviour.