Accuracy of birefringent-coating method for coatings of arbitrary thickness

Rational concepts of the behavior of birefringent coatings are developed for both plane-stress problems and general surface-strain problems. For plane stress in elastic bodies, accuracy of stress or strain determinations along free boundaries is almost entirely independent of coating thickness; however, where Poisson's ratios of coating and structure are unequal, the state of stress in the structure might be influenced by the presence of the coating. In experimental investigations of four geometrical discontinuities with coating thicknesses up to 0.9 in. and materials of unequal Poisson's ratios, no influence of coating thickness on boundary strains was observed. For some three-dimensional problems, local shear tractions developed at the interface introduce a dependence upon coating thickness, but the resultant error in strain determinations was approximately 5% for a case studied experimentally. Strain distributions on singly and multiply connected surfaces in two- or three-dimensional bodies can be studied effectively by the method of birefringent-coating measurements.