In this paper a new full-field method for the automatic analysis of isochromatic fringes in white light is presented. The method, named RGB photoelasticity, eliminates the typical drawbacks of the classical approach to photoelasticity in white light which requires a subjective analysis of colors and an experienced analyst to acquire and interpret the results.

Towards RGB photoelasticity: Full-field automated photoelasticity in white light

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Experimental analysis and computational modelling of damage and fracture

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

Photoelasticity is one of the most widely used full-field methods for experimental stress analysis. However, the collection of photoelastic parameters can be a long and tedious process. The advent of automated photoelastic systems has allowed the experimentalists to speed up the rate of analysis and to perform more complex investigations.This paper provides a survey of recent methods of automated photoelasticity developed in the last 20 years, i.e. methods of the fringe centres, half-fringe photoelasticity, phase-stepping photoelasticity, methods based on the Fourier transform, spectral content analysis (SCA) and RGB (red, green, blue) photoelasticity.

A review of automated methods for the collection and analysis of photoelastic data

A novel instrument is described for the simultaneous observation and capture of four phase-stepped photoelastic images. A theoretical description of the optics of the instrument is presented for the first time. Three examples are given of the use of the instrument in reflection photoelasticity to generate full-field maps of isochromatic and isoclinic parameters. The results from these experiments show close correlation to results from both theoretical analyses and manual measurements. The instrument can be used in either reflection or transmission mode and it is concluded that the new instrument significantly enhances the potential for real-time studies using reflection photoelasticity.

Simultaneous observation of phase-stepped images for automated photoelasticity

A new method of photoelastic measurement has been developed. The light emerging from a polariscope is spectrally separated and projected on a photodiode array. It is shown that the relative retardation can be retrieved from light intensity measured at several wavelengths. Key parameters affecting the precision of this approach are discussed and evaluated.

Photoelastic measurements by means of computer-assisted spectral-contents analysis

While photoelastic-model analysis is an effective method to measure stresses, its practical use is limited to solving problems under well-defined loading conditions which can be successfully applied on a simulated basis to the model. Further, model analysis does not take into account such conditions as hidden material defects, assembly stresses, residual stresses, inelastic behavior, and other parameters that are contributing factors to the structural integrity of a part or structure.On the other hand, with photoelastic coatings, the stress analysis is conducted on real parts operating under actual service conditions. The coating reveals the true surface strains occurring on a part, since most, if not all, of the contributing stress conditions mentioned above can be taken into account during testing.Photoelastic coatings are easy to apply and, with proper test planning, are very economical to use. Since a visible picture of the stress field is provided over the entire area coated, intelligent application of the technique can save many hours of testing time and provide quick solutions to design or service-failure problems.

Photoelastic coatings: Some practical difficulties, mostly in the area of test planning and handling of the coating application efficiently and economically, are reviewed by the author

Alex S. Redner

Papers

Photoelastic measurements by means of computer-assisted spectral-contents analysis

Photoelastic coatings: Some practical difficulties, mostly in the area of test planning and handling of the coating application efficiently and economically, are reviewed by the author