SCANNING MOIRE AT HIGH MAGNIFICATION USING OPTICAL METHODS

Methods of employing scanning moire at high magnification are develope d and demonstrated. Modern lithographic techniques for producing custo m moire gratings with a frequency up to 250 l/mm are described. On a p robing station equipped with a video system, pseudo-color moire fringe s are produced using the scanning lines of the color charge-coupled-de vice (CCD) camera. Fringe multiplication from 1 to 5 is possible with correct combinations of magnification and grating pitch. An analysis i s given to show that strain sensitivity depends only on the number of scanning lines used to record the image. The grating pitch and the mag nification are important because they reduce the gage length of the st rain measurement. The high-magnification scanning moire was used to st udy plastic- strain fields in an aluminum tensile specimen. Local dist urbances in the strain field were observed at 2 to 2.5 percent applied strain. These discontinuities became more significant at higher level s of applied strain.