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Cruciform Multiaxial Mechanical Testing

Summary:

The cruciform mechanical test is widely used to measure the multiaxial deformation behavior of materials. Standardization of this technique would have widespread technical benefits, and this project is aimed toward that goal. A new test facility, funded by ARRA, is being established within the NIST Center for Automotive Lightweighting, along with associated metrology for measuring the complete multiaxial stress matrix, 3D multiaxial strains over two fields of view and high resolution temperature changes. The new facility will be online at the end of March 2013.

Description:

A new ARRA funded cruciform test machine is being installed in the NIST Center for Automotive Lightweighting. Its capabilities include 500kN load per axis, two DIC strain mapping systems (one for large field mapping and one for zooming in to study localizations or features), a high resolution, high speed IR camera to measure adiabatic heating, xray diffraction to measure stresses in situ, and an optical DIC-based biaxial extensometer for control of the strain path during the test.

Most cruciform sample geometries are only able to reach a maximum of 4-5% plastic strain due to premature failure at stress concentrations. In addition, the stresses within the deforming gauge section can only be inferred indirectly, either by FEA analysis or simply determining load divided by area of each arm. If the test is being run to measure constitutive behavior, the first method fails as a constitutive law must be assumed a priori. In the second case, the details of the stress tensor are being oversimplified. To eliminate all of these problems, sample geometries are being developed whereby the test specimen is sandwiched between two additional sheets containing center holes, forcing the deformation to the center of the sample and reinforcing the stress raisers. Additionally, the stresses are determined directly using xray diffraction, a technique developed for the Marciniak system. Perhaps the most innovative aspect to this machine is the biaxial extensometer, which allows the path in strain space to be changed while under load, allowing variable non-linear path behavior to be probed.