The study of combining hive-grid target with sub-pixel analysis for measurement of structural experiment

This study presents a digital image processing technique, by integrating sub-pixel analysis using digital image correlation method with a novel hive-grid target, for the measurement of structural experiment data. First, a numerical simulation of photography experiment at short range is employed to validate the feasibility of proposed approach. Herein, the simulation image caught from digital speckle and hive-grid target are compared, and the measured time-history displacement of simulation image is computed by the digital image correlation and edge detection methods. The results reveal that, via the digital image correlation method with hive-grid target, the measured time-history displacement at specific position can be analyzed accurately. Following, the experimental data gained from LVDT and digital camera for a six-storey steel frame at National Center for Research on Earthquake Engineering (NCREE) in Taiwan is used to verify the performance of the proposed approach. The measured time-history displacements of the steel frame are converted into frequency domain via Fourier Transform and Wavelet Transform schemes. The results revealed that the relative error between data from LVDT and analyzed data from digital image correlation is below 1% on frequency domain. Herein, the sampling rate of digital camera is lower than LVDT, because high sampling rate led to underexposure and large data storage. The frame rate of common digital camera is about 30 frames/sec nowadays, however the high speed digital camera can overcome this restriction. The limitation of high sampling rate is exposure time and data storage. The experimental results of numerical simulation and shaking table test revealed that digital image correlation method with hive-grid target is accurate in high resolution images.