The generic nature of the absolute-retardation method of photoelasticity

The method of absolute-retardation measurements does not depend upon the use of any specific interferometer. Any one of the many extended field interferometers can be employed, including the Series, Michelson, Mach-Zehnder and Fizeau types. Where adequate monochromatic purity of light source is available, reflections from the two surfaces of the model can form the interferogram, thus dispensing with the necessity of any separate optical interferometer. The two-dimensional stress-optical relationships are derived in their most general form. The relationships are identical for all instruments or interference systems—only the calibration constants change with different systems. Interferograms from four distinctly different systems are shown. Interference-fringe sharpening is introduced and demonstrated. A laser-light source was used with two systems to provide high monochromatic purity. The interferometric method of Nisida and Saito is correlated with the absolute-retardation method. The opportunity to obtain information for a complete stress solution from one photograph makes the method attractive for both static and dynamic problems.