Three-Point Bending of Heat-Treatable Aluminum Alloys: Influence of Microstructure and Texture on Bendability and Fractu
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ALUMINUM alloys in the form of rolled sheets and extruded profiles intended for use in the automotive industry should possess an appropriate combination of strength and ductility. Sheets or profiles are typically formed into final shapes such as body panels or bumper beams by cold deformation. Hence, the bendability of the material is of great importance in fabrication as well as in use, where the ductility is required for optimal energy absorption. Uniaxial tensile testing is often used to determine the mechanical properties of metals. However, this type of test does not always give a good description of the deformation mode experienced by a component. In this case, the three-point bending test is a suitable method for describing the mechanical properties. The bendability is defined as the ratio between the minimum bending radius and the thickness of the test specimen without fracture occurring.[1–4] In 1960, Datsko and Yang[1] came up with a simple relationship between bendability and the reduction in area obtained by tensile testing. This IDA WESTERMANN, Postdoctoral Candidate, and ODD STURE HOPPERSTAD, Professor, are with the Structural Impact Laboratory (SIMLab), Centre for Research-Based Innovation, 7491 Trondheim, Norway and is also with the Department of Structural Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Contact e-mail: [email protected] KNUT ERIK SNILSBERG and ZEINAB SHARIFI, Master Students, KNUT MARTHINSEN and BJØRN HOLMEDAL, Professors, are with the Department of Material Science and Engineering, Norwegian University of Science and Technology. Manuscript submitted August 24, 2010. Article published online July 1, 2011 3386—VOLUME 42A, NOVEMBER 2011
empirical relationship, however, does not include any of the underlying microstructural mechanisms. Several phenomena influence the bendability, e.g., hardening behavior, constituent particles, shear band formation, microstructure, and texture/anisotropy. Experimental findings, e.g., References 5 through 7, indicate that the presence of constituent particles influences the bendability of aluminum alloys. What part the particles play in the damage of the material, however, is still uncertain. Sarkar et al.,[7,8] in agreement with Datsko and Yang,[1] have shown that an increasing amount of constituent iron-rich particles not only influences the bendability of an alloy, but also the tensile ductility. Ragab and Saleh[9] and Bettaieb et al.[10,11] took into account void growth and coalescence in modeling of bendability. Constituent particles in metals are found to be natural nucleation sites for microvoids.[6] Dao and Li[12] and Wilkinson et al.,[13] however, suggest that the particles also affect the formation of shear bands during bending, which essentially leads to fracture. Shear localization plays a major role in fracture initiation.[12,14–17] In anisotropic materials such as rolled and extruded metals, the texture is found to affect the shear band formation as well. Kuroda and Tvergaard,[14] in their finite eleme
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