It is well known that Luders bands form in Al-Mg alloy sheets by tensile deformation. The Luders bands are classified in two types; type-A and type-B. In this study, the influence of stress ratio of biaxial tensile test on the type-B Luders band formation in an Al-5.5mass percent Mg-0.3mass percent Cu alloy sheet was investigated. The type-B Luders bands formed at uniaxial and plane strain tension, while they did not form at balanced biaxial tension. The mechanism of Luders bands formation was considered as below. In the case of uniaxial tension, the slip planes were limited easily because of pinning of dislocations by magnesium atoms. The deformation did not occur uniformly, then the type-B Luders bands occurred. On the other hand, multiple slip occurred easily in the case of balanced biaxial tension. Then, the sessile dislocations formed easily and the deformation occurred uniformly. Moreover, the type-B Luders bands formed at an angle of 57 degrees to the applied load in the case of uniaxial tension, while 90 degrees in the case of plane strain tension. It was considered that the deformation occurred at maximum shear stress planes by uniaxial tension, while it occurred preferentially at {110} plane components by plane strain tension.
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