The return of photoelastic stress measurements: utilizing birefringence to monitor damage and repair in healable materials

Abstract

Photoelastic stress measurements have proven quite useful in the analysis of residual and induced stress within transparent polymer materials. We show the utility of polymer birefringence as an avenue for assessing location and type of damage, as well as the extent of repair, in healable polymers based on the thermally reversible Diels–Alder cycloaddition reaction of dicyclopentadiene. These healable polymers were subjected to compressive loading in a controlled fashion to create an isochromatic–stress calibration curve. The correlation between birefringence and stress was then utilized to analyze samples visibly deformed in compressive loading; following a thermal healing treatment, these samples not only regained their initial mechanical properties and recovered their initial specimen dimensions, but also showed a complete release of the residual stress induced during the damage event. The photoelastic properties of these and similar materials can be utilized as a non-contact and non-destructive method for real-time analysis of damage and repair in healable materials.