A novel instrument for automated principal strain separation in reflection photoelasticity

A novel instrument is presented capable of measuring the principal strains at a point on a photoelastic coating. The oblique incidence method of principal strain separation is employed, using three fringe order measurements, which are made by the automated method known as spectral contents analysis. The oblique incidence measurements are made after rotation of the polariscope about any two orthogonal axes. Three polariscopes are built into the instrument, as is a CCD camera, to facilitate accurate positioning of the instrument. The instrument was used to analyze the principal strain distributions in a disk under diametrical compression, a plate with a central hole under tension, and a stepped plate in tension. The strains in the disk and the plate with a hole were compared with theoretical results, and the strains in the stepped plate were compared with a finite element analysis of the problem. The correlation between the experimental and theoretical or numerical strains was better than 0.95 in most cases.