Optical Full-Field Displacement Measurement Method Based on Phase Only Correlation

Abstract eng:
Measuring the deformation of structural members is fundamental to evaluating the performance of structures. As an effective and practical method for measuring the deformation of structures, digital image analysis techniques are gradually becoming accepted in the field of experimental mechanics. In the present study, an optical measurement method for reinforced concrete frame structures based on the phase-only correlation method is proposed. The proposed method directly provides displacements and strains on the surface of a reinforced concrete frame by analyzing digital images. The proposed non-contact measurement method can be used to measure deformations of a measurement object and has a sub-pixel-level measurement accuracy when using an analytical algorithm. Moreover, the proposed method can be used to measure the fullfield deformation and to visualize deformations. The main objective of the present study is to evaluate the validity of the proposed measurement method by conducting an experimental test on a reinforced concrete frame specimen. The proposed measurement method consists of optical imaging devices and computers for image analysis. After capturing digital images of the surface of the measurement object, the computer calculates the motion of each measurement point on the images by comparing the phase correlation of a subset field in the reference image and the deformed image. The variation of the center of the subset field is calculated as a displacement, by tracking the highest correlation subset. The measurement accuracy depends on the pixel size because the phase correlation of pixel values of the image data is used. However, the deformation of the structural members is smaller than the image pixel size. In order to measure the deformation with high accuracy using an optical method, measurement of the sub-pixel level displacement is required. The proposed method can be used to measure sub-pixel level displacement by using the phase-only correlation method. Verification is performed based on the measurement results of a test involving a reinforced concrete frame specimen. Comparison of the measurement results reveals that the accuracy of the proposed measurement method is approximately the same as that of the results obtained using a displacement meter. Moreover, the proposed method can track the behavior of a concrete specimen after it has cracked. The agreement between the concrete cracking position and the local strain position in the strain distribution confirms that the proposed method can be used to visualize the full-field deformation of an object surface.

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