Photoelasticity - definition

Photoelasticity is the physical phenomenon whereby certain dielectric materials - especially transparent solids such as glass, polycarbonates or epoxies - exhibit a variation in refractive index in response to applied stresses mechanical stresses. When subjected to internal stresses, the material becomes birefringent, meaning that it splits polarised light into two rays with different propagation velocities, depending on the direction and value of the stresses.

This phenomenon is used in an experimental technique called photoelasticity (or photoelastic method), which allows the visualisation and analysis of stress distributions in structural models. It is measured by observing the interference of polarised light passing through a stressed model placed between crossed polarisation filters. The observed isochromes (lines of equal difference in principal stresses) and isopachs (lines of equal sum of stresses) form an interference pattern that can be quantitatively analysed to determine the intensity and orientation of the stresses.

Photoelasticity is used in solid mechanics, materials engineering, biomechanics and in the design of high-reliability components where it is important to know the local stress concentrations. It is non-destructive and highly sensitive, making it useful for both basic research, quality control or verification of numerical models FEA (finite element method). The characteristic parameter for a given photoelastic material is the photoelastic constant, expressing the relationship between the difference in principal stresses and the observed phase difference of light rays.

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