Measurement of the time-dependent principal-strain directions in birefringent materials

In this paper a simple method is presented to analyze the time variation of the principal-strain directions. The experimental arrangement to be used consists of a dark-field linear polariscope, a beam of monochromatic parallel light with a small diameter penetrating the measuring point with normal incidence. The variation of the light intensity behind the analyzer was measured by a photocell and a CRO. According to the method introduced here, two variations of light intensity were measured by choosing two orientations of the crossed system of polarizer and analyzer. The difference between the orientations is 45 deg. This method was applied to the analysis of the resulting principal directions in a region in which a superposition of a compression and a shear wave occurred. These waves were produced in a photoelastic foil, which was cemented on a brass rod impacted longitudinally. The compressional main pulse generated in the rod has a duration of about 25 μs. This experimental arrangement will be used later to investigate the behavior of the photoelastic material under short-time loading.