Phase analysis techniques of fringe patterns have been widely used for noncontact three-dimensional shape and deformation measurement by the fringe projection method. Recently, we developed two novel accurate phase analysis methods. One is the two-dimensional sampling moiré method to perform robust phase analysis for a single-shot fringe pattern. The other is the two-dimensional spatiotemporal phase-shifting method to analyze phase distribution accurately for multi-step phase-shifted fringe patterns. To perform accurate phase analysis under low signal-to-noise ratio conditions, both the above two methods use the two-dimensional discrete Fourier transform or fast Fourier transform. Therefore, these algorithms are computationally expensive compared with the conventional one-dimensional sampling moiré and phase-shifting methods. In this study, a fast parallelization implementation for two-dimensional phase-shifting methods, including the two-dimensional sampling moiré method and the spatiotemporal phase-shifting method, are presented by utilizing multi-core CPU. Simulation and experimental results demonstrate that phase analysis can reach 7.5 and 5.9 times faster by use of a 12-core CPU compared with a single CPU.
In this study, the influence of the working distance (WD) on strain measurement under a laser scanning microscope and a way to achieve precise focus were investigated by the scanning moiré method. Experimental results showed that the strain measurement has a good repeatability at a fixed WD. Scanning moiré fringes were clearly observable when the WD variation range was within 0.9% of the given WD of the used objective lens. The relationship of the measured strain error and the WD difference was approximately linear, and the greatest strain error was near 700 με. Fortunately, 2D moiré fringes were distinct only in a very narrow range, i.e., the WD difference was less than 0.1% of the given WD, and the greatest strain error was less than 100 με. 1D moiré fringes in the y direction, 2D moiré fringes in the both x and y directions, and 1D moiré fringes in the x direction became distinct alternately along with the WD change. Consequently, we suggest to use 2D moiré fringes for microscale strain measurement in each focusing process to reduce the errors caused by the WD variation. Moreover, a single-shot 2D moiré image is useful to measure the strain distributions in both two directions quickly and simply, and there is no need to rotate the sample or scanning lines and scan twice as in the conventional way.
The deformation distributions of carbon fiber reinforced plastics (CFRP) under a three-point bending load were nondestructively investigated using the phase shifting scanning electron microscope (SEM) moiré method. The complex fast Fourier transform (FFT) and the discrete Fourier transform (DFT) were used to filter the useless moiré fringes in the case of bidirectional moiré fringes. The SEM moiré fringes under different magnifications and the deformation results measured by the direct, complex FFT- and the DFT- phase shifting moiré methods as well as the moiré fringe centering method were compared and analyzed. Experiments demonstrate that the deformation measurement is a bit influenced by the useless moiré fringes in the phase shifting moiré methods and complex FFT processing works better for nondense moiré fringes. The relative strain changes gradually and the specimen grating pitch increases gradually from top to bottom along the loading direction, suggesting that the real compressive strain is greater in the upper side. The micro/nano-scale deformation distribution characteristic is helpful for better understanding of the mechanical properties of the CFRP specimen.