SCANNING MOIRE AT HIGH MAGNIFICATION USING OPTICAL METHODS

Methods of employing scanning moire at high magnification are developed and demonstrated. Modern lithographic techniques for producing custom moire gratings with a frequency up to 250 l/mm are described. On a probing station equipped with a video system, pseudo-color moire fringes are produced using the scanning lines of the color charge-coupled-device (CCD) camera. Fringe multiplication from 1 to 5 is possible with correct combinations of magnification and grating pitch. An analysis is given to show that strain sensitivity depends only on the number of scanning lines used to record the image. The grating pitch and the magnification are important because they reduce the gage length of the strain measurement. The high-magnification scanning moire was used to study plastic- strain fields in an aluminum tensile specimen. Local disturbances in the strain field were observed at 2 to 2.5 percent applied strain. These discontinuities became more significant at higher levels of applied strain.