Characterization of a Laser Extensometer for Split Hopkinson Pressure bar Experiments

This paper examines a laser extensometer to calculate the strain in a sample during split Hopkinson pressure bar (SHPB) experiments. This method offers a non-contact, direct method for measuring sample strain which does not rely on one-dimensional wave propagation assumptions. First a single bar experiment is used to compare the extensometer's accuracy and frequency response against a laser vibrometer and an accelerometer. The extensometer showed a close match with the vibrometer up to a bandwidth of 10 kHz. With the performance validated, the extensometer is applied to a SHPB experiment with silicone samples. For low strains, the extensometer shows a good match to the strain determined by strain gauges on the bars. At higher strains, the radial expansion of the sample can interfere with the measurement beam from the extensometer and produce inaccurate results.