Photoelastic Effect

(also photoelasticity), the occurrence of optical anisotropy in initially isotropic solids, including polymers, when the solids are subjected to mechanical stresses. The photoelastic effect was discovered by T. J. Seebeck in 1813 and by D. Brewster in 1816.

The photoelastic effect is a consequence of the strain dependence of the dielectric constant of a substance and is manifested as double refraction, or birefringence, and dichroism, which occur when a substance is mechanically loaded. Under uniaxial tension or compression, an isotropic solid takes on the properties of a uniaxial crystal with the optical axis parallel to the axis of tension or compression. Under more complex strain—for example, under bilateral tension—a specimen, or model, becomes biaxial (seeCRYSTAL OPTICS).

Photoelasticity is caused by the deformation of the electron shells of atoms and molecules and by the orientation of optically anisotropic molecules or components of such molecules; in polymers, it is caused by the uncoiling and orientation of polymer chains. For a small uniaxial tension or compression, Brewster’s law is satisfied: Δn = kP, where Δn is the magnitude of the birefringence (that is, the difference between the refractive indexes for the ordinary and extraordinary waves), P is the stress, and k is the engineering stress-optical coefficient. For glasses, k = 10^–1310^–12 cm²/dyne; for celluloid, k = –10^–12–10^–11 cm²/dyne.

The photoelastic effect is used in the study of stresses in mechanical structures for which calculation of the stresses is too complicated. The investigation of the birefringence due to loading in a transparent model—usually a small-scale model—of a structure being studied makes it possible to determine the nature and distribution of the stresses in the structure (seePHOTOELASTIC TESTING). The photoelastic effect underlies the interaction of light and ultrasound in solids.