Issue 37

D. Angelova et alii, Frattura ed Integrità Strutturale, 37 (2016) 249-257; DOI: 10.3221/IGF-ESIS.37.33 254 In Fig. 3 are also shown fractographic images of the fractured surfaces of some tested specimens; the chosen specific areas of the fractured surfaces are: crack initiation zone, crack propagation zone and zone of final fracture. The specimens tested at stress range Δσ = 1500 MPa (Specimens 10 and 12) show several crack initiation zones which correspond to the shorter lifetimes of these specimens in comparison with the rest from the same group A, [7]. Marked numbers of specimens and the corresponding explanations of the fractografic images of their fractured surfaces shown in Fig. 3 are presented in Tab. 2 in terms of providing a better understanding of the whole picture. Image Marked number of specimens Δσ, MPa Fractographic characterization Image Marked number of specimens Δσ, MPa Fractographic characterization 1 12 1500 Crack initiation zone 8 6 1200 MnS inclusion at crack initiation zone 2 12 1500 Crack initiation zone/Crack propagation zone 9 7 1200 Crack initiation zone 3 10 1500 Zone of final failure 10 2 1000 Microcracks – zone of final failure 4 10 1500 Crack initiation / propagation zone 11 3 1000 Crack initiation zone 5 8 1400 Zone of final failure 12 3 1000 Crack initiation zone 6 8 1400 Microcracks – zone of final failure 13 4 1000 Crack propagation zone/Zone of final failure 7 5 1200 Fracture surface 14 4 1000 Crack initiation zone Table 2 : Stress-Cycles-Fractographic characteristics connected with the Wöhler curve of Steel A. By analogy the corresponding Wöhler curves for Steel B in air and aggressive environment are shown in Fig.4a together with characteristic areas with surface short cracks; these cracks are observed on the hourglass specimen surfaces, corresponding to given numbers of cycles from cracks originating through their propagation to the final stage of near- failure moments before complete specimen fracture. A comparison between fatigue behaviour of Steels A and B in stress terms (although presented in Δσ and in Δτ) Fig. 4b, shows a pronounced influence of material nature and of different kind of fatigue condition - RBF and TF. At the same time the strongest factor is obviously the surrounding environment, which is clearly focused on the corresponding Wöhler curve - the lowest one in Fig. 4b. Cracks development and paths can be observed in Fig. 5. Fractured surfaces of Specimen 7 are presented in Fig. 5a. By performing closer observation and analysis of the shown fractured surface a few crack initiation zones are found and three of them are chosen and indicated by black dashed-line ovals. The magnified images of these zones are shown separately in the black boxes in the same figure. It has been found that in the early stages of fatigue process the three cracks (originated from different spots) propagated independently from each other. However, in the further stages of fatigue development an interaction of the three cracks is observed, causing exhaustion of local plasticity and crack-merging process. The merging process gradually involves other cracks and finally a main crack is formed leading to the complete fracture of Specimen 7. The observed cracks interaction on the fractured surface of Specimen 7 also can be indirectly confirmed by replica monitoring of surface propagation of short fatigue cracks, Fig. 6a. Figs. 4a, 5b, 6b show fatigue cracks growth in the specimens of Steel B, tested under different stress ranges. The major crack propagation at Δτ = 915 MPa and in-air environment can be microstructurally seen in Fig. 5b; in comparison to it the observed corrosion fatigue in the specimen under the highest stress range of Δτ=900 MPa (Steel B), Fig. 4a, shows intense interaction between all the surface short cracks. Data from replica monitoring of surface short cracks propagation (in Specimens 7 (Δσ = 1200 MPa) and 9 (Δσ = 1400 MPa) obtained by use of metallographic microscopy) are plotted as “Crack length, a -Numbers of cycles, N ” and shown in Fig. 6a. In both cases the major cracks from the families "Major crack - Secondary cracks" have originated first. It can be seen that the major crack of Specimen 9 (tested at higher stress range) has initiated in an earlier stage in comparison to the initiation stages of all the secondary cracks. At the same time the major crack and all the secondary cracks in Specimen 7 show small difference at the number of cycles to initiation, almost parallel propagation and a kind of competition between the secondary cracks for merging their propagation paths with that of the major crack; also the major crack length is