Boundary extraction and polarimetry in translucent specimens for photoelastic stress analysis

We address the problem of providing input to a novel method of interpreting photoelastic data for performing experimental stress analysis of models of engineering components. This method employs conventional photoelastic data relating to the directions of the principal stresses in the specimen (isoclinic data), along with the difference in principal stresses (isochromatic data). Both are used within an inverse boundary element model to reconstruct the load conditions at the model boundary and hence to recover the principal stresses in the specimen without recourse to numerical integration of shear stress gradient. We describe methods of obtaining unwrapped isoclinic and isochromatic phase maps from sequences of images captured within a computer-controlled polariscope. A boundary element model of the specimen, congruent with the isoclinic and isochromatic phase maps, is obtained from an image captured within the polariscope under either traditional lighting conditions or by configuring the polariscope to provide a light field background. Image segmentation reveals the boundary of the specimen, which is then described in terms of simple geometric primitives. Boundary points and geometric descriptions are both used to produce the final boundary element model. The techniques described have been applied to a number of contact specimens; results are presented and discussed.