Shear Localization and Damage in AA5754 Aluminum Alloy Sheets Jidong Kang, David S. Wilkinson, Mike Bruhis, Mukesh Jain, Pei Dong Wu, J. David Embury, Raja K. Mishra, and Anil K. Sachdev (Submitted January 28, 2008; in revised form February 26, 2008) In this article, we study the Portevin-Le Chatelier (PLC) bands and their influences on strain localization and fracture in continuous cast (CC) AA5754 aluminum sheets. Three types of tensile tests are conducted: (1) tensile samples are pulled directly to fracture at 223 K, (2) tensile samples are pulled at 223 K to initiate diffuse necking followed by unloading and reloading to fracture at room temperature, and (3) tensile samples are pulled at 223 K to localized necking and unloaded followed by reloading to fracture at room temperature. Furthermore, in situ V-bending test coupled with deformation mapping using digital image correlation is used to study damage at large strains. The results show that PLC bands detect favorable geometrical sites for shear band initiation. The formation of shear bands precedes damage and damage is a consequence of shear band formation. Keywords aluminum, automotive, dynamic strain aging, mechanical testing, shear localization 1. Introduction Aluminum alloys are finding increasing usage in the transportation industry because of their combination of low density, high strength, and good formability. Continuous cast (CC) AA5754 sheets are being produced to reduce cost of the final products in automotive applications. The onset of failure in aluminum sheets, especially CC sheets, remains a challenge for increased applications involving complex shapes as in body panel applications. There are two types of shear localizations in solid solution alloys such as the Al-Mg system, namely Portevin-Le Chatelier (PLC) effect and shear bands. An evidence of PLC effect is the serrated flow in the tensile curves which occurs over a wide range of temperatures and strain rates. It is also well known that Al-Mg alloys fail by localization of flow into bands of intense shear (Ref 1-6). However, the mechanism governing the formation of shear bands is still controversial in the literature and current models cannot account for many experimental observations (Ref 7). For example, very little damage is seen just underneath the fracture surface that is dominated by void sheeting (Ref 8). The relationship between shear localization and damage in aluminum, for example, is different from that in steel. Furthermore, the relationship between PLC bands and shear bands needs further elucidation (Ref 9). As the progress from necking to fracture occurs over a very small strain range during tensile deformation of aluminum alloys compared to steels, new experimental test methods are needed to examine the processes at work leading to failure. Failure in bending is controlled by fracture, while necking is the precursor to failure in tensile tests. Therefore, the bend testing provides an alternative way to observe damage in the absence of shear bands at large strains. A new instrumentally controlled bending fixture in which load-displacement curves can be recorded during the bending process (Ref 10) can be used to follow processes at large strains. Similarly, PLC bands can be suppressed at lower temperatures and/or higher strain rates, and the relationship between PLC bands and shear bands can be investigated by combining low temperature testing with room temperature tests. In the present study, tensile tests were first carried out at 223 K to isolate PLC bands from shear bands, providing an opportunity to follow the deformation pattern that leads to the shear banding process. Tensile samples were then pulled to diffuse and localized necking at 223 K, and the deformation was continued at room temperature to delineate the relationship between PLC bands and shear bands. In situ V-bending test coupled with deformation mapping using digital image correlation (DIC) was used to reveal damage mechanisms at large strains. 1.1 Experiment The material used for the present study was a commercially produced 2-mm-thick continuous cast AA5754-O sheet from Novelis. The chemical composition of the material was Al-3.1Mg-0.25Mn-0.24Fe-<0.1Si-0.02Cu-<0.01Cr (wt.%). Uniaxial tensile tests were carried out at a cross-head speed of 0.9 mm/min (equivalent to a nominal strain rate of This article was presented at Materials Science & Technology 2007, Automotive and Ground Vehicles symposium held September 16-20, 2007, in Detroit, MI. Jidong Kang, School of Welding Engineering Technology, Northern College, 140 Government Road East, Kirkland Lake, ON, Canada P2N 3L8; Jidong Kang, David S. Wilkinson, and J. David Embury, Department of Materials Science and Engineering, McMaster University, 1280 Main street West, Hamilton, ON, Canada L8S 4L7; Mike Bruhis, Mukesh Jain, and Pei Dong Wu, Department of Mechanical Engineering, McMaster University, 1280 Main street West, Hamilton, ON, Canada L8S 4L7; and Raja K. Mishra and Anil K. Sachdev, Materials and Processes Lab, GM R&D Center, 30500 Mound Road, Warren, MI 48090-9055. Contact e-mail: kangj@ northern.on.ca. JMEPEG (2008) 17:395–401 ÓASM International DOI: 10.1007/s11665-008-9224-6 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 17(3) June 2008—395