PSI - Issue 5

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

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lips developed should be quite different from AA2139 exhibiting only a very weak <100>//RD fibre texture with cubic texture component {0 0 1}<1 0 0>, where the {111} planes are much more randomly orientated .

Fig. 4. Fracture surface of (a) flat panels and (b) crenellated panels and the different shear lip shapes developed at different positions. (c) Orientation of {111} planes in the fatigue cracked AA2198 panels and the activated slip systems involved in the shear lip formation. Based on the measured sharp Brass texture in AA2198 panels and the alignment of fatigue crack growth direction with the rolling direction in the tests, the orientations of the {111} planes with respect to the sample frame can be determined, which are schematically sketched in Fig. 4(c). The (11-1) plane coincidences with the initial crack plane in tensile mode and the other three {111} planes form a regular tetrahedron above the crack plane with one corner pointing to the opposite of the crack growth direction. The Schmid-Factors of the 12 slip systems on the {111} planes were calculated (Table 1) presuming a tensile stress field perpendicular to the original crack plane. It can be seen that 6 slip systems of them are more advantageous to be activated. Since (-111) and (1-11) planes have the best alignment with the maximum shear stress planes, which are 45° inclined to the side surfaces of the panels, the activation of slip systems (-111) [01-1], (-111)[110], (1-11)[10-1] and (1-11)[110] will probably have the greatest contribution to the shear lip formation. If so, the co-activation of slip systems (-111)[01-1] and (-111)[110] will favor shear decohesions along the (-111) plane and will finally generate a left-slanted shear lip; the co-activation of slip systems (1-11) [10-1] and (1-11)[110] will favor shear decohesions along the (1-11) plane and will finally generate a right-slanted shear lip. As shown in Fig. 4(c) the (-111) and (1-11) planes are symmetric and equally advantageous in initiating shear lips. If shear lips are initiated along both planes but on different sides of the panel surface then a double shear lip forms as observed in the AA2198 flat panel. This equality of (-111) and (1-11) planes also implies there can be a competition of activated slips along those planes. Local disturbances (e.g. the thickness steps of crenellations) can probably lead to alternate activation of slip along those planes, during which the abrupt change of shear lip plane orientations can occur as observed in experiments (Figure 4(b), marked by squares). The texture of AA2198 also plays an important role in the formation of sharp shear lips along the crenellated side in the crenellated specimen. Normally the formation of shear lips in crenellated panel should have been delayed compared to the flat panel due to the reduced crack propagation rate (Zuidema et al., 2005) in the purposely introduced retardation region. However, the first shear lip observed in the crenellated panel was only 1-2 millimeters from the starting notch. This unexpected early formation of shear lips is probably attributed to the concentrated shear at the edge of a buckling cracking surface (Zuidema, 1995) due to the out-of-plane bending of the crenellated panel. The enhanced shear stress finally can activate sufficient slip along either (-111) or (1-11) plane to initiate the shear lip. Under this near threshold condition, since the slips are dominantly provided by the slip systems belongs to either (- 111) or (1-11) plane, the orientation of shear lip plane should show strong dependence on the orientation of those planes. In experiment, the angle between those sharp shear lip plane and the initial crack plane is around 70°, which