Simultaneous observation of phase-stepped photoelastic images using diffraction gratings

Phase-stepped photoelasticity is a powerful method for full-field stress analysis, but sequential collection of the multiple required images limits the technique to static loading applications. In an effort to collect four phase-stepped images simultaneously with a single camera for transient loading applications, we have developed a system that utilizes diffraction gratings, which are for separation of coherent monochromatic laser light. Two adjacent, perpendicularly oriented, 1D Ronchi rulings that are placed after a transparent sample split the light into equal intensity beams for each diffraction order. The four beams that are diffracted once in the x direction and once in the y direction transmit through arrays of analyzing polariscope elements, with different combinations of fast-axis orientations for four phased-stepped images. The mirrors and imaging lenses in the system work in concert to focus each beam onto separate quadrants of the same CCD. We demonstrate the system for stress analysis of compressive loading of a Homalite-100 disk. We can utilize this system for photoelastic analysis of time-dependent materials and of dynamic events, when equipped with a high-speed camera.