Optical EngineeringHybrid grid-moiré method for measuring strain and displacement
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A hybrid grid-moiré method for measuring strain is presented. The method employs a line grating attached to a plane surface of the specimen. Images of undeformed and deformed gratings are acquired and stored using a video system and a frame digitizer. The magnification is adjusted to record 10 to 30 lines of a grating with a pitch of 4 μm over the frame width. A fast Fourier transform (FFT) of the intensity data gives a close estimate of the spatial line frequency. A moiré function is formed as the product of a reference sine function and the image intensity-position function. FFTs of the moiré function yield an improved estimate of the line frequency. The line frequency is related to an average strain over the recorded image. The method was demonstrated at a magnification of 1500 x with a grating pitch ps = 4 μm. The accuracy achieved depended on strain. For strains in excess of 0.1%, the errors were usually less than 5% when compared to strain gauge results. For plastic strains, in excess of 1%, the differences between two comparable measurements were less than 3%.