Micro/Nano-scale Strain Distribution Measurement from Sampling Moire Fringes

This work describes the measurement procedure and principles of a sampling moire technique for full-field micro/nano-scale deformation measurements. The developed technique can be performed in two ways: using the reconstructed multiplication moire method or the spatial phase-shifting sampling moire method. When the specimen grid pitch is around 2 pixels, 2-pixel sampling moire fringes are generated to reconstruct a multiplication moire pattern for a deformation measurement. Both the displacement and strain sensitivities are twice as high as in the traditional scanning moire method in the same wide field of view. When the specimen grid pitch is around or greater than 3 pixels, multipixel sampling moire fringes are generated, and a spatial phase-shifting technique is combined for a full-field deformation measurement. The strain measurement accuracy is significantly improved, and automatic batch measurement is easily achievable. Both methods can measure the two-dimensional (2D) strain distributions from a single-shot grid image without rotating the specimen or scanning lines, as in traditional moire techniques. As examples, the 2D displacement and strain distributions, including the shear strains of two carbon fiber-reinforced plastic specimens, were measured in three-point bending tests. The proposed technique is expected to play an important role in the non-destructive quantitative evaluations of mechanical properties, crack occurrences, and residual stresses of a variety of materials.