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

Simultaneous measurements of mechanical response, optical birefringence, and fl uorescence signal are acquired in situ during tensile testing of a mechanophore-linked elastomeric polymer. Mechanical stress, deformation, and polymer chain alignment are correlated with force-induced chemical reaction of the mechanophore. The mechanochemically responsive polymer under investigation is spiropyran- (SP-) linked poly(methyl acrylate) (PMA). Forcedriven conversion (activation) of SP to its merocyanine (MC) form is indicated by the emergence of a fl uorescence signal with 532 nm light incident on the sample. Increasing rate of tensile deformation leads to an increase in both stress and SP-to-MC conversion, indicating a positive correlation between macroscopic stress and activation. Simultaneously collected birefringence measurements reveal that rapid mechanophore activation occurs when maximum polymer chain alignment is reached. It is found that SP-to-MC conversion in PMA requires both a suffi cient level of stress and adequate orientation of the polymer chains in the direction of applied force.

The Effect of Polymer Chain Alignment and Relaxation on Force‐Induced Chemical Reactions in an Elastomer

Digital photoelasticity has rapidly progressed in the last few years and has matured into an industry-friendly technique. This review thematically classifies all the developments in digital photoelasticity and highlights the relative merits and drawbacks of the various techniques. The overall objective is to provide enough information and guidance to allow an end-user to make an informed choice on the type of technique to be used in a particular situation.

Digital photoelasticity – A comprehensive review

Review of RGB photoelasticity

Digital photoelasticity of glass: A comprehensive review

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

The theoretical description of spectral contents analysis is extended to red-green-blue (RGB) photoelasticity. The integration of the resultant methodology for spectral analysis with phase stepping is used to achieve automatic analysis of photoelastic fringe patterns containing a minimum fringe order of no greater than four, and without the requirement for a zero-order fringe order to be present in the pattern.

Z F Wang

Papers

Simultaneous observation of phase-stepped images for automated photoelasticity

Integration of spectral and phase-stepping methods in photoelasticity