A new video extensometer system for testing materials undergoing severe plastic deformation

[+] Author and Article Information
Andrea J Felling

Dalhousie University, P.O. Box 15000, Halifax, NS B3H 4R2

Darrel A. Doman

Dalhousie University, P.O. Box 15000, Halifax, NS B3H 4R2

1Corresponding author.

ASME doi:10.1115/1.4039291 History: Received May 12, 2017; Revised January 16, 2018


Characterization of materials undergoing severe plastic deformation requires the careful measurement of instantaneous sample dimensions throughout testing. For compressive testing, it is insufficient to simply estimate sample diameter from an easily measured height and volume. Not all materials exhibit incompressibility, and friction during testing can lead to a barreled sample with diameter that varies with height. Video extensometry has the potential to greatly improve testing by capturing the full profile of a sample, allowing researchers to account for such effects. Common 2D video extensometry algorithms require thin, planar samples, as they are unable to account for out-of-plane deformation. They are therefore inappropriate for standard compressive tests which use cylindrical samples that exhibit large degrees of out-of-plane deformation. In this paper, a new approach to 2D video extensometry is proposed. By using background subtraction, the profile of a cylindrical sample can be isolated and measured. Calibration experiments show the proposed system has a 3.1% error on calculating true yield stress - similar to ASTM standard methods for compressive testing. The system is tested against Aluminum 2024-T351 in a series of cold upsetting tests. The results of these tests match very closely with similar tests from the literature. A preliminary finite element model constructed using data from these tests successfully reproduced experimental results. Diameter data from the finite element model undershot, but otherwise closely matched experimental data.

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