Monday, 11 October 2004 - 1:25 PM

This presentation is part of : Metal Forming II

Stress measurement during biaxial straining

Mark A. Iadicola and Timothy J. Foecke. National Institute of Standards and Technology, 100 Bureau Dr., Stop 8553, Gaithersburg, MD 20899-8553

Determination of the formability of materials requires experiments performed in multiple arrangements to determine the mechanical behavior for various strain states (drawing, uniaxial, plane strain, balanced biaxial, etc.). Additionally, many of the biaxial experimental methods suffer from assumptions based upon results from numerical models (e.g. extrapolation of end-tab loads into gage section stresses for biaxial cruciform geometries).

Experiments are presented that use the Raghavan variation of the typical Marciniak test. This method is used to determine the formability of materials for a range of strain states (biaxial strain ratios of -1/2 to 1 are achieved through simple changes in specimen shape). In situ diffraction measurements are made, using a one of a kind X-ray system, to determine the stress in the sheet for the various strain states. Although this method requires substantial calibration, the experimental arrangement overcomes some long standing problems in biaxial testing of sheet metals (measurement of stress in biaxial sheet testing, compensation for boundary conditions and no out of plane bending, and observation of stresses at specific spots in the region of interest). Potential applications for this experiment include corroboration of other testing methods (in-plane biaxial specimen geometries and bulge testing), verifying stress predictions from finite element analyses (i.e. benchmarking experiments), analysis of stress states in localized deformation (yield point effects), and tracking of the effect of prestraining on material formability through the process of multistage forming.

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