PSI - Issue 5

Nikolai Kashaev et al. / Procedia Structural Integrity 5 (2017) 263–270 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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is nearly the same as the angle between different {111} planes (70.72°). When the fatigue crack grows longer (> 80mm in our case) and the stress concentration near the crack tip becomes high enough, this tendency of shear lip formation along (-111) or (1-11) plane disappears. 45° inclined shear lips become dominant in the end. In summary the tortuous shear lip morphology in AA2198 panels either the due to the abrupt change of shear lip plane or due to the formation of sharp shear lips is closely related with sharp Brass texture of the material. Table 1. Schmid-factors of different slip systems in the strong textured AA2198 panels subjected to tensile stress field that is perpendicular to the crack line Slip system (111)[-110] (111)[-101] (111)[01-1] (-111)[110] (-111)[101] (-111)[01-1] Schmid-factor 0 0.272 0.272 0.272 0 0.272 Slip system (1-11)[110] (1-11)[-101] (1-11)[011] (111)[-110] (111)[101] (111)[011] Schmid-factor 0 0.272 0.272 0 0 0 The tortuous shear lip morphology in the AA2198 panels can influence the fatigue behavior in two different aspects. Firstly the sudden change of shear lip plane and the occurrence of double shear lips in the AA2198 panels can lead to crack deflection. In such a case the mode I crack driving force is reduced due to the deviation from maximum tensile stress plane as described by Ritchie (1988). This can be one major additional toughening mechanism compared with the material AA2139. Without considering the regions of shear lips, the mechanism of fatigue cracking in both materials is the same, which results in similar smooth and flat crack surface perpendicular to external loading. As a result before the development of tortuous shear lips and after the formation of stable single shear lips in AA2198, the fatigue crack in AA2198 develops at a similar speed to that in AA2139 as shown in Figure 3(b). This agrees with the observation of Bergner et al. (2001), that many aluminium alloys show similar da / dN at ΔK 0 = 10 MPa ⋅  m, and the additional fatigue resistance shown in some alloys is due to the increased fracture surface roughness. The second influence is the heightened crack closure level in the AA2198 panels. The enhanced out of plane bending in the crenellated panel increases the chance of surface interference. This plus the tortuous crack surface morphology provides additional sliding crack closure. It is the same situation as the roughness induced crack closure observed in low ΔK range, which requires both a rough surface and relative displacement between the two crack surfaces (Suresh and Ritchie, 1982). The AA2198 flat panel has a rough fracture surface but with a much lower relative displacement between crack surfaces compared to the crenellated panel. As a result the flat panel of the AA2198 alloy shows a slightly enhanced crack closure level whereas the F op of AA2198 crenelated panel is significantly heightened, which leads to a further extended fatigue life. The materials used in the crenellated structure can significantly influence the effectiveness of crenellations. The fatigue life improvement via crenellations doubles in the strong textured material AA2198 compared to AA2139 with a rather weak <1 0 0>//RD fibre texture. The strong texture leads to tortuous shear lip morphology. The out of plane movement of crenellated structure under service-related loading conditions (biaxial loads in our case) favors the formation of sharp shear lips along the crenellated side and leads to an enhanced crack surface interference at those shear lips. The resulted sliding crack closure contributes to the additional fatigue life improvement observed in the AA2198 crenellated panel. 4.2. Shear lip morphology and the fatigue behavior 5. Conclusion

Acknowledgement

The authors would like to thank Mr. Jürgen Knaack, Mr. Karl-Heinz Balzereit and Mr. Falk Dorn from the department Joining and Assessment of the Institute of Materials Research, Materials Mechanics of the Helmholtz Zentrum Geesthacht for their support in the experimental work.