Strain determination of polymeric materials using digital image correlation

Application of digital image correlation (DIC) to polymeric materials has been proven to be a powerful tool for non-contact strain measurement. In this paper the limits of accuracy of this optical strain measurement system under different environmental conditions were investigated, and the technique was applied to the characterization of polypropylene (PP) and PP composites (PP-C) in the pre- and post-yield regimes. As regards accuracy, a fine speckle pattern and a light intensity just below overexposure provided best results. While vibrations related to the operation of the test machine were of minor influence in reducing the strain measurement accuracy, more pronounced effects were found for the operation of the temperature chamber. In characterizing the transverse strain behavior of PP-C, DIC results exhibited smaller values compared to transverse strains determined utilizing a mechanical clip-on extensometer. The latter effect is attributed to viscoelastic creep indentation of the extensometer pins, which mechanically interact with the specimen via the clip-on spring forces of the extensometer, into the surface. For the DIC system, it could be shown that it allows for the proper strain determination both in the pre- and post-yield regimes, and in terms of longitudinal and transverse strains as well as in terms of global average and local strains.