Page 46 - MetalForming magazine • December 2022

deformation concentrates in the necked region, becoming the site of eventual fracture.
Accurate metal forming simulation requires capturing the stress-strain relationship at all strains, but a tensile test alone provides valid data only through uniform elongation. Beyond this value, simulation engineers assume either a certain behavior, or more accurately rely on the results from bulge testing or other methods to cap- ture the correct response.
Theory and experiments show that uniform elongation expressed in true strain units is numerically equivalent to the instantaneous n-value.
Uniform Elongation: Not That Uniform
Load increases during the tensile test, and until recently, the deformation was thought to be distributed uniformly over the gauge length prior to reaching uniform elongation. Recently, however, researchers have gathered evidence confirming that a nonuniform strain distribution exists within the gauge region prior to reaching uniform elon- gation. Acquiring this new knowledge required moving from traditional exten- someters to the use of a refined non- contact imaging approach called digital image correlation (DIC), discussed in the Cutting Edge column in the Feb- ruary 2020 issue of MetalForming mag- azine (www.metalformingmagazine.com/ article/?/quality-control/material-test- ing/digital-image-correlation-how-it- changed-the-tensile-test).
Traditional extensometers calibrated for 50- or 80-mm gauge lengths deter- mine elongation from deformation measured relative to this initial length. This approach averages results over these lengths and leads to greater peak- strain measurements over the smaller reference length.
With the aid of DIC, researchers now can use a much smaller reference length by setting a virtual gauge length generated through application of a fine, random speckle pattern on the sample before testing. A camera cap- tures the motion of the speckle pattern
during the test, calculating elongation relative to gauge lengths as small as 0.5 mm.
While conventional wisdom held that uniform strains exist prior to strains reaching uniform elongation, researcher Thomas Stoughton, in 2021, monitored the movement of more than 200 points in the traditional 50-mm span and showed that each one expe- rienced a unique strain evolution, with
differences starting before reaching uniform elongation (Fig. 2). In addition, he showed that for DP980 steel, this approach obtains useful data at as much as 61-percent strain rather than only to the uniform elongation of 8- percent strain. This study suggests that DIC-aided measurements potentially can reduce or eliminate the need for many of the tests required for accurate metal forming simulations. MF
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