Optical Methods in Experimental Mechanics

The reflection photoelasticity technique that directly yields reasonably accurate strain measurement over the entire surface of the structure has been discussed. The techniques are quite easy to implement, and it is an efficient way to obtain reasonably accurate whole-field readings of surface strains from a structure of any size or geometry. The fundamental idea of the reflection photoelasticity method is that a birefringent material is bonded to the structure so that it undergoes the same strains as are imposed on the structure. Birefringent coatings having a wide span of properties are available or may be made from ordinary photoelastic materials ranging from low-modulus eurethane rubbers that are applied to specimens that might exhibit large strains to stiff resins used to investigate small strains in stiff structures. The sensitivity of photoelastic coatings is limited in that only 3 or so isochromatic orders are ordinarily observed on specimens that are made of metal and are in the elastic range.