High-resolution large-strain measurement of plastically deformed specimen by Fourier phase correlation

The Fourier phase correlation method is applied to position circular marks one by one printed on the specimen before and after deformation in order to measure the large plastic strain. This method is extremely sensitive to the differences between the profile of a mark and noise. Therefore, it detects marks easily even under non-uniform illumination without any processing such as flattening illumination, noise exclusion and boundary enhancement in image transformation into binary codes. In addition, a new method of moving reference images (MRI) to position the observed marks in the sub-pixel range is proposed. The MRI technique introduces the theoretical resolution 1/n (n: number of divisions of one pixel). The MRI method enables the positions of the deformed marks to be determined with a resolution of ±0.1 pixel (standard deviation σp is 0.042 pixel). Strain of the tensile specimen can be measured within an error of ±0.0015 (standard deviation σε=0.00055).