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Analyzing Rapid Surface Motions With Moire Fringes

High speed photography of moire fringes was used to analyze the motion of surfaces subjected to dynamic loadings. The moire fringe device used a high power light source, a pair of Ronchi rulings, and commonly available lenses. One Ronchi ruling was projected at an angle onto the surface to be studied. The projected ruling was then imaged onto the second Ronchi ruling where a fringe pattern was produced which was characteristic of the stationary wall before impact. Any outward displacement of the surface caused a corresponding change in the original fringe pattern. Calculations of wall motion from fringe patterns were greatly simplified by the observation that each fringe can be represented as an ellipse which intersects the target (usually in two places) and both the projection and taking lenses.