PSI - Issue 5

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

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Fig. 3. (a) The fatigue performance of flat and crenellated panels with different materials and (b) da / dN vs.  K plots of the four tested specimens.

3.3. Fracture surface observation

Fig. 4 compares the fracture surfaces of AA2139 and AA2198 panels at macroscopic scale. Both of the two AA2139 panels show flat crack surfaces with smooth single shear lips. The shear lips start at positions marked by the white arrows and are fully developed at positions marked by the black arrows, where the whole section of the fracture surface is 45° inclined. In the panels of AA2198, the fracture surface is much more tortuous due to the development of complex shear lip morphology. In the flat panel double shear lips are sometimes observed which are accompanied with deviations of crack path around 10°. In contrast with the gradual rotation of the crack plane in the shear lip development of the AA2139 panels, abrupt changes of the shear lip plane are usually observed in the AA2198 panels as marked by the squares in Fig. 4. In the crenellated panel, those abrupt changes are usually found associated with the thickness steps in the crenellations. In addition, very sharp shear lips are found along the crenellated side since the beginning of the fatigue tests. Those very sharp shear lips are less than 1 mm wide with the local crack surface about 70° inclined to the original crack plane. At large crack length (a > 95 mm) stable single shear lips are observed in both flat and crenellated panels as those observed in the AA2139 panels. The very sharp Brass texture {1 1 0}<1 1 2> observed in AA2198 is very typical for cold rolled FCC metals (Suwas and Ray, 2014). The S texture components {1 2 3}<6 4 3> near the surface of the plate are due to the increasing degree of deformation caused by the direct contact with rollers (Cai et al., 2009). The large amount of cold working in AA2198 is also reflected by the pancake grain structure. The distinct textures of the materials can significantly influence the morphology of shear lips that are developed. This has been reported by several previous researchers. Zuidema et al. (2005) suggested the formation of shear lips is related with the type and amount of texture in the material. Tchorzewski and Hutchinson (1978) also found the width of shear lips is strongly influenced by the orientation of the fracture plane in the textured Ti-6Al-4V alloy. The influence of texture on the shear lip morphology can be explained in terms of slip characteristic of the material. According to Schijve (2009), in the region where shear lips form, the initial tensile mode decohesion of the material is overruled by the shear mode decohesion along the plane of maximum shear stress, which is 45° inclined to the initial crack plane. Thus the formation of shear lips involve large amount of dislocation slip along the plane of shear decohesion and along the planes with close orientations. In the strong textured material AA2198, the easy sliding planes – the {111} planes of the grains are not randomly orientated but concentrate in specific orientations. If they have a good alignment with maximum shear stress plane, they can contribute a lot to the slip that is necessary for the shear lip formation. In such a case the morphology of shear 4. Discussion 4.1. Texture and the shear lip formation