48 



States in 1956. The plastic which Zandman discovered is now referred 

 to as Photostress and is commercially available from the Budd Company. 

 .2% Principles of the Birefringent Coating Analysis 



Birefringent coatings combine the functions of photoelasticity, 

 brittle lacquer, and electric resistance strain gages. The birefringent 

 coating is analogous to an infinite number of strain gages, since the strain 

 is identified by the contrast of colors. 



Basically, birefringent coating principles are the same as in photo- 

 elasticity. The mathematical formulas expressing the fundamental relation- 

 ships used in photoelasticity analysis have been thoroughly developed in the 



(12 13 14) 

 literature* ' ' ' and will not be reiterated here. 



The primary difference between photoelasticity and the birefringent 



coating analysis is that a transparent model, such as glass oy plastic, is 



utilized in photoelasticity, whereas the actual structure is tested in a 



birefringent coating analysis. When a transparent coating is applied to a 



structure, the coating follows the surface strains of the structure. With 



applied load, the coating exhibits a relative retardation of polarized light 



proportional to the strain because of the doubly refractive, or birefringent, 



characteristic of the coating. Since the light waves are polarized at 90® 



to each other, corresponding to the principal stress directions, only the 



difference of principal stresses is obtained with normal incidence. Fortunately, 



there are several means of determining the individual principal stresses. 



