Development of an intermediate strain rate compression testing machine
Master Thesis
2014
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University of Cape Town
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Abstract
The further development of the wedge bar Intermediate Strain rate Tester (IST) presented by Cloete and Oxtoby [1] is presented in this dissertation. The concept uses a wedge mechanism to deform 5 mm diameter, 5 mm long specimens at strain rates in the region of 10 s, up to strains of 30. As impact principles are used to start and stop the experiment, it takes less than 1 of the testing duration to reach testing speed and to stop the experiment. The kinetic energy stored in the wedge bar helps ensure a near constant loading rate over the duration of the experiment. The yield stress is captured at above 80 of the average strain rate for the experiment for most of the materials investigated. The design work focuses primarily around the design of the new load-frame, load-cell, loading platforms and wedge bar displacement sensor. Finite Element Analysis (FEA) was used to investigate the dynamic response of the IST and modify the design of components where necessary. The modifications made the load-frame and load-cell result in a significant improvement in the quality of the measured signals. The wedge bar displacement sensor performed well. The back pressure driven loading concept shows potential but needs more development before it is suitable for routine testing. A range of common engineering materials was tested at quasi static, intermediate and high strain rates and compared to the results found in the literature. The results match well, however, friction effects dominate the large strain response of the metal specimens. In its current configuration the IST is used effective for testing polymers up to large strains as well as yield stress and small strain measurement of metals. For materials for which specimen friction effects are a problem, interrupted tests can be used to extend the maximum strain achievable.
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Includes bibliographical references.
Reference:
Stander, M. 2014. Development of an intermediate strain rate compression testing machine. University of Cape Town.