The enormously enhanced power of photoelasticity resulting from adoption of digital technologies is highlighted and discussed. An overview of the principal techniques of digital fringe processing is provided within a single theoretical framework. The practical application of the new technologies using both conventional instruments and novel optical devices is discussed. Experiments involving more I x 10 6 quantitative fringe order measurements are possible and practical on a routine basis using the current technology. Products based on this research are beginning to appear on the market so that many new application areas are opening up for photoelasticity, such as dynamic events, real-time fatigue crack analysis, monitoring polarisation changes at a microscopic level in materials; detailed validation of numerical simulations, particularly of complex geometry and loading; and in-service monitoring using reflection photoelasticity of damage in both homogeneous and heterogeneous materials, such as composites.

Digital photoelasticity: Principles, practice and potential

Review of RGB photoelasticity

The application of phase-shifting photoelasticity to a real-time dynamic event involves simultaneous recording of the four phase-shifted images. Here an instrument, believed to be novel, is developed and described for this purpose. Use of a Multispec Imager is introduced into digital photoelasticity for the first time to our knowledge. This device enables splitting the optical energy of an object into four identical paths, thus permitting recording of the required four phase-shifted images. Experimental demonstration is provided for validation.

Dynamic phase-shifting photoelasticity.

Ekman and Nurse's load-stepping approach to phase-shifting photoelasticity (1998) is able to determine automatically the isoclinic and isochromatic parameters. A problem associated with this method is that, because of the small load increments used, it is difficult for the user to obtain three accurately equal-stepped loads even with a rather precise loading rig. This deviation between two load increments then has a direct effect on the accuracy of the results. Also, experiment shows that this method does not work well for low stressed regions because the phase differences between loads are too small. In view of this, a modified approach to load stepping is proposed and is called the three-load method. This new method uses the relationships of phase retardations at each point for three different loads to determine the isoclinic angle θ in the range -π/2 < θ < π/2. Then the isochromatic parameter can be determined unambiguously. This method is easy to carry out and basically retains the accuracy of the phase shifting technique except for the change in sign of the parameters in the ambiguous regions.

http://iopscience.iop.org/article/10.1088/0957-0233/11/5/313/pdf

Determination of isoclinic and isochromatic parameters using the three-load method

A new two-load-step approach to the phase-shifting method is presented for the full field determination of both isoclinic and isochro- matic parameters in photoelasticity. The ambiguity that exists in the phase-shifting photoelasticity is overcome by investigating the relation- ships between the phase retardation introduced by two different load steps. The isoclinic angle is automatically determined in the correct range (2p/2, p/2) and the isochromatic parameter is therefore obtained unambiguously. The results obtained from a bending test and an irregu- lar specimen subjected to multiple forces demonstrate the effectiveness of this method. Compared with the load-stepped method (Ekman and Nurse, 1998) or the three-load method (Asundi et al., 2000), this new algorithm is a simpler approach and is easier to execute. © 2001 Society of

Full field automated photoelasticity using two- load-step method

A new approach to full-field automated photoelasticity is presented in which a circular polariscope is used to enable the isochromatic phase value (α) to be determined unambiguously and without input of a known isochromatic value obtained using an auxiliary technique. Values of cosα are obtained from light-field and dark-field images for three loads of small incremental steps. Using a relatively straight-forward procedure, ramped phase maps for α are produced which can be unwrapped using conventional techniques. The resulting distribution of α is then found absolutely using information provided by which δα is the incremental change in the isochromatic phase value between the load 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.

https://link.springer.com/content/pdf/10.1007%2FBF02410394.pdf

M. J. Ekman

Papers

Absolute determination of the isochromatic parameter by loadstepping photoelasticity