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Normal and Transverse Displacement Interferometers Applied to Small Diameter Kolsky Bars

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Abstract

For Kolsky bar testing beyond strain-rates of 10,000/s, it is useful to employ bars with diameters of only a few millimeters or less. Furthermore, very small (sub-millimeter) systems are compatible with micron-sized specimens, to be used, for example, for the determination of mesoscale properties. However, at these sizes, traditional strain-gage measurements of the longitudinal waves within the bars become impractical. In this paper we describe the application of optical measurement techniques to two Kolsky bars, with 3.2 and 1.6 mm diameters. A transverse displacement interferometer is used to measure the displacement of the mid-point of the incident bar and provide measurements of the incident and reflected pulses. Similarly, a normal displacement interferometer is used to measure the displacement of the free-end of the transmitter bar and provide a measurement of the transmitted pulse. The new methods are used to characterize the behavior of 6061-T6 aluminum at rates greater than 100,000/s. The feasibility of application to smaller bars is also discussed.

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Notes

  1. Stress and strain, whether referring to the specimen or to the bars, are assumed positive in compression.

  2. A bar with a higher wave speed, and/or lower Poisson’s ratio can also be used.

  3. If strain and particle velocity are measured simultaneously at the same location, a simple wave separation method is available, see for example [18].

  4. An expanded beam is used to simplify alignment.

  5. The grating also deforms slightly; this is assumed negligible.

  6. There is also a reflected beam (0th order diffracted beam) that is not used, although it is used as one leg of an NDI in the PSPI application.

  7. Subscripts i, r, and t denote the incident, reflected, and transmitted pulses. Subscripts I and T denote the incident and transmitter bars. Subscripts 1 and 2 denote the I 1 and I 2 interfaces, as labeled in Figs. 1, 2 and 3.

  8. It is possible to use a momentum trap and measure its free surface displacement to measure the transmitted pulse, but care must be taken that the trap and transmitter bar do not separate until after the required time window.

  9. [19] has used the fact that the free-end is stress-free for wave separation in a strain-gage instrumented bar.

  10. In general, the strain-rate of the specimen is not constant during a Kolsky bar experiment. The reported rates are the true strain-rates at 10% true strain.

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Acknowledgement

This research was supported in part by an appointment to the Postgraduate Research Participation Program at the U.S. Army Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USARL.

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Authors and Affiliations

  1. US Army Research Laboratory, RDRL-WMP-B, Aberdeen Proving Ground, Aberdeen, MD, 21005-5069, USA

    D.T. Casem (SEM member)

  2. Oak Ridge Institute for Science and Education, Oak Ridge, TN, USA

    S.E. Grunschel

  3. US Army Research Laboratory, RDRL-WML-H, Aberdeen Proving Ground, Aberdeen, MD, 21005-5069, USA

    B.E. Schuster

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  1. D.T. Casem
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  2. S.E. Grunschel
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  3. B.E. Schuster
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Correspondence to D.T. Casem.

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Casem, D., Grunschel, S. & Schuster, B. Normal and Transverse Displacement Interferometers Applied to Small Diameter Kolsky Bars. Exp Mech 52, 173–184 (2012). https://doi.org/10.1007/s11340-011-9524-x

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