Interferometric Measurement Techniques for Small Diameter Kolsky Bars

Abstract

The use of optical measuring techniques for small diameter Kolsky bar experiments is discussed. The goal is to develop methods that can eliminate the need for more commonly used strain gages which become impractical as bar sizes decrease. The basic approach taken here is to adapt interferometer-based methods, used commonly in pressure-shear plate impact experiments, to high-rate Kolsky bar experiments. A Normal Displacement Interferometer (NDI) is used to measure the motion of the free end of the transmitter bar and provide a measurement of the transmitted pulse. Similarly, the incident and reflected pulses are measured with a Transverse Displacement Interferometer (TDI) utilizing a diffraction grating at the midpoint of the incident bar. Both techniques are applied to 1.59 mm diameter steel pressure bars. In the case of the transmitter bar, measurements are also made with the traditional strain gage instrumentation and comparisons between the two are made. The incident bar measurements made via TDI are validated with a simple bar impact against a single incident bar, i.e., without a specimen or transmitter bar. The possible application of these methods to smaller systems is also discussed.