Investigation of various necking criteria for sheet metal formability analysis using digital image strain data
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ORIGINAL RESEARCH
Investigation of various necking criteria for sheet metal formability analysis using digital image strain data Yang Song 1
&
Daniel E. Green 1 & Alexandra Rose 1
Received: 30 January 2019 / Accepted: 17 November 2019 # Springer-Verlag France SAS, part of Springer Nature 2019
Abstract The precise determination of the forming limit strains of a sheet metal is necessary in order to accurately predict the onset of failure in sheet metal forming processes. The method used to detect the onset of necking (i.e. the necking criterion) is a most important factor in formability analysis as it significantly affects the forming limit strains. Many different necking criteria have been developed, which take advantage of the flexibility and high resolution of digital image correlation (DIC) strain measurements. Several time-dependent and time/geometry-dependent necking criteria were carefully investigated in order to evaluate their ability to reliably and consistently detect the onset of necking of TRIP780 sheet specimens that were stretch-formed over a hemispherical punch. It was found that the geometry-dependent, surface slope criterion was the most robust and consistent criterion of those evaluated. Keywords Forming limit . Nakazima test . TRIP steel . DIC . Necking criterion
Introduction Automotive and aerospace manufacturers use a wide range of sheet metal alloys to form parts with complex shapes and features. During a forming process, a metal sheet is stretched or drawn into shape between a pair of mating dies and the sheet safely deforms until it reaches its limit of formability. When the formability of the sheet metal is exhausted, the strains tend to localize in a narrow band and a throughthickness neck develops, thus compromising the structural integrity of the part. Determining the forming limit of a particular sheet metal with accuracy is essential in order to reliably predict the outcome of a forming process. For several decades, forming limit curves (FLC) have been effectively used to assess the robustness of forming operations. An FLC is a curve that represents the lowest threshold strain states in a major strain versus minor strain coordinate system. This threshold is defined as the condition where the material is at risk of developing a neck. The sheet metal forming industry has adopted this model of representing the forming limit of a given sheet metal for * Yang Song [email protected] 1
Mechanical, Automotive and Materials Engineering, University of Windsor, Windsor, ON N9B 3P4, Canada
numerical modeling and process design as it represents the strain state of maximum plastic deformation for the material based on experimental data. The Keeler method is traditionally used for the experimental determination of FLCs in North America [1]. It uses electro-etched grids to mark un-deformed specimens and requires that the forming process be stopped just at the onset of necking. The spatial resolution of the strains measured from electro-etched grids is generally 2.5 mm (0.1 in.). Moreover, the s
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