Application of optical measurement techniques to high strain rate deformations in composite materials

Traditionally high strain rate material characterisations have been conducted using strain gauges and/or cross head displacements in servo-hydraulic test machines, and force transducers in split Hopkinson bar experiments. Non-contact full-field techniques for experimental stress/strain analysis have been available for many years and used extensively for structural analysis under static or quasi static loading. These techniques have the advantage that they are non-contact and high resolution, so damage initiation can be captured within the field of view and the material behaviour is not modified by the sensor. In the paper, one such technique known as Digital Image Correlation (DIC) is used to assess the material behaviour by using high-speed digital cameras to capture images from material subject to high strain rate events. The high strain rate loading is achieved using an Instron VHS high speed tensile test machine that allows the applied strain rates to vary from 12.5 s-1 to 125 s -1. Although the strain rates that can be achieved are low in comparison to those achieved with the Hopkinson bar, the test machine provides better optical access and opportunities for illumination of the specimen necessary for the DIC. In the paper, a review of the literature associated with high strain rate testing using servo-hydraulic machines is first provided. Then, an experimental study of the high strain rate behaviour of the both composite material and the resin alone is described. The results from both the DIC and strain gauges are compared and discussed.