13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of Plastic Anisotropy on Shear Localization and Fracture in Automotive Sheets Jidong Kang1,*, Raja K. Mishra2, David S. Wilkinson3 1 CanmetMATERIALS, Natural Resources Canada, Hamilton L8P 0A5, Canada 2 Chemical and Materials Laboratory, General Motors R&D Center, Warren 48090-9055, U.S.A. 3 Department of Materials Science and Engineering, McMaster University, Hamilton L88 4L7, Canada * Corresponding author: jkang@nrcan.gc.ca Abstract Tensile instability, as characterized by the Considère law, is one of the factors governing formability of metallic sheets. During tensile deformation, material thins in a narrow band due to shear localization, prior to final fracture. Strain rate value within the localized necking band tends to be higher than outside it and final fracture is governed both by the nature of the shear localization as well as the strain rate differential between the neck and the material outside the neck. This paper reports the dependence of shear localization and fracture on plastic anisotropy of the material. Three types of automotive sheet materials, namely IF steel (BCC structure), AA5754 aluminum alloy (FCC) and AZ31 magnesium alloy (HCP) are examined. Digital image correlation is used to follow the development of deformation pattern during tensile tests. The results show that both narrowing and thinning of the tensile sample occur in IF steel, while only thinning occurs in AA5754 and only narrowing occurs in AZ31. These differences arise from the differences in the plastic anisotropy of the three materials, as measured by their r-values. Even though all three materials exhibit ductile fracture, the damage and fracture processes in the three materials differ from each other. Keywords Shear localization, Plastic anisotropy, automotive sheets, r-value, fracture 1. Introduction In response to the more stringent regulations on fuel consumption in vehicles, lighweighting via the utilization of aluminum and magnesium alloys are being seen to replace steels for automotive body structural applications. Aluminum can reduce the vehicle weight by 20-30% while magnesium 40-50% compared to a full steel vehicle. A 10% weight reduction will save 6-8% in fuel and related GHG emissions. However, formability of aluminium and magnesium sheets is inferior to that of conventional interstitial free (IF) steel. The formability of alloy sheet can be limited either by instability or fracture depending on the operation [1]. The forming limit is usually defined as the locus in uniform strain space required for the onset of localized necking while the fracture limit is defined as that required for material separation. In uniaxial tension strain path, tensile instability or diffuse necking, as characterized by the Considère law, is one of the factors governing formability of metallic sheets. Further, material thins in a narrow band due to shear localization, prior to final fracture. Strain rate value within the localized necking band tends to be higher than outside it and final fracture is governed both by the nature of the shear localization as well as the strain rate differential between the neck and the material outside the neck. This forms the foundation of the so-called Marciniak-Kuczynski (M-K) approach [2] for forming limit diagram (FLD) analysis. While M-K approach has been successfully applied to prediction of FLD of numerous sheet materials including steels and aluminum alloys, it is commonly recognized that the success of such approach for aluminum alloys depends significantly on the selection of yield functions that represent the effect of plastic anisotropy and texture. On the other hand, whether or not thinning occurs in magnesium alloys is challenged by many experimental observations. For example, for AZ31 sheets, it has been reported that very little thinning occurs during uniaxial tension at room temperature [3] and the deformation process becomes more complicated as a variety of deformation mechanisms become activated at
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