DETERMINATION OF SMALL DILATATIONS AND SURFACE STRESS BY BIREFRINGENCE MEASUREMENTS

Birefringence induced in a suitably oriented transparent optically isotropic body by a zone which has undergone a dilatation is used to determine the local dilatation in that zone from a solution of the photoelastic problem. Dilatations < 1010-6 have been determined, a sensitivity of measurement greater than in conventional interferometry, using specimens of modest optical specification. Local dilatations have been determined in transparent zones 0.2 mm or more in thickness. Mean dilatations have been determined in opaque layers or in zones too thin to observe (some < 1 μm thick) from birefringence in the unaffected zone. Problems which arise in the measurements are discussed. Applications are described. Instrumentation and automation techniques are given. It appears possible to extend sensitivity to measure dilatations < 10-7, which would be the equivalent of determining the effect of inserting a sheet of molecular dimensions. © 1969.