The method of birefringent coatings for the determination of elastic and plastic surface strains of opaque bodies like metals assumes perfect elasticity of the coating material. In reality the coating, assumed much softer than the metal, presents the problem of a general viscoelastic layer under prescribed boundary displacements (at the metal-plastic interface). As already shown, this problem is greatly simplified for isotropic linear viscoelastic coatings, for small strains, for a linear law of strain birefringence, and for interface displacements expressed as a product of a function of space co-ordinates by a time function. The obviously advantageous stress-strain-optical linearity was experimentally verified in pure and in plasticized epoxy resins which make the best coatings. Tests were carried out in uniaxial loading and in shear, in creep, as well as in relaxation. The main conclusion is that the pure epoxy resins show negligible inelasticity, and the plasticized have a linear photo-viscoelastic behavior. Explicit laws were fitted to the creep and relaxation curves. Tests were also carried out with deeply notched coated steel bars deformed in the plastic range. The variation of fringe pattern with time was found to be negligible for the pure epoxy resins and to diminish slightly and proportionally with time throughout the model for the plasticized resins.

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