Absolute determination of the Isochromatic parameter by load-stepping photoelasticity

A new approach to full-field automated photoelasticity is presented in which a circular polariscope is used to enable the isochromatic phase value (alpha) to be determined unambiguously and without input of a known isochromatic value obtained using an auxiliary technique. Values of cosa are obtained from light-field and dark-field images for three loads of small incremental steps. Using a relatively straightforward procedure, ramped phase maps for alpha are produced which can be unwrapped using conventional techniques. The resulting distribution of alpha is then found absolutely using information provided by which partial derivative alpha is the incremental change in the isochromatic phase value between the toad steps. The results obtained for disk-in-compression tests presented here in comparison with theoretical solutions demonstrate that the technique is both simple to use and very accurate. A similar approach may be adopted using three wavelengths instead of three load steps.