A Study of the Fundamental Relationships between Deformation-Induced Surface Roughness and Strain Localization in AA5754
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TRODUCTION
THE introduction of new alloys into the automobile body is an expensive and time-consuming process. For this reason, numerical predictions of the formability of metal sheet have become integral components in the automotive design process, generating a strong demand for accurate and reliable predictions of mechanical behavior under a wide range of deformation conditions. The inability to reliably model the evolution of the surface inhomogeneities produced during sheet metal forming creates a significant obstacle that impedes the widespread incorporation of these alloys. One of the most common approaches to assessing the formability, and thus the suitability, of an alloy for a particular application is the forming limit diagram. It is well known that macroscopic deformation in a typical metal stamping occurs through a combination of strain modes, or paths (e.g., biaxial, uniaxial, and plane strain). Considering that a combination of strain modes will likely promote failure at an overall strain that is lower than would be expected if the deformation occurred in a single strain mode, it is essential that the relative influence of each component of the macroscopic strain M.R. STOUDT, M.A. IADICOLA and S.W. BANOVIC, Materials Research Engineers, and J.B. HUBBARD, Research Associate, are with the NIST Center for Metal Forming, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899. Contact e-mail: [email protected] Manuscript submitted October 22, 2008. Article published online June 11, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
combination be properly identified and accurately assessed. Such a characterization is a complicated process requiring numerous high-resolution measurements of the deformation under each strain mode.[1] Consequently, the limiting strains in a typical stamping are most often estimated via numerical simulations specifically designed to predict the onset of macroscopic, or gross, strain localization. These complex simulations are usually based on phenomenological constitutive relations that assume a homogeneous response to an imposed macroscopic strain at the microstructural level until the onset of localization.[2] Accordingly, significant deviations from the homogeneous response are then considered to indicate the onset of a critical localization event (i.e., a direct precursor to failure such as the formation of cracks or splits, necks, etc.).[3] Even though models based on this approach tend to correctly indicate the general trends, they often fail to consistently predict the precise strains at which localization occurs.[4] The addition of revised plasticity and kinematic hardening models and the results from studies of the influence of various material parameters (such as grain size and grain orientation effects, surface roughening effects, and other damage mechanisms) on strain localization have enhanced the reliability of the numerical models.[5–12] Despite the significant improvements that have been made to these models, incon
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