Digital Image Correlation for Evaluating Structural Engineering Materials
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In the structural engineering community, a need exists for a non-contact two-dimensional measurement system which could provide information for field monitoring and greatly enhance the accuracy of numerical structural models. Recent advances have enabled the use of digital image correlation (DIC) to calculate the surface displacements of chosen targets in a series of digital images with a high degree of accuracy. Images are recorded during an experiment and are afterwards post-processed to find relevant information including, but not limited to, a) global displacement, b) relative displacement and c) changes in strain. In this research, a series of experiments were conducted to create measurement techniques for monitoring steel and reinforced concrete (RC) structures utilizing DIC. However, to ensure accurate DIC measurements, the addition of artificial texture from lightly applied spray paint on finished concrete was investigated and was determined to noticeably improve results. Furthermore, the placement of the digital camera relative to the structure being monitored was shown to control not only the desired field of view in the region of interest, but also the resulting image texture and DIC measurement accuracy. The DIC technique was applied to monitor and understand two important aspects of structural evaluation: a) the movement along shear planes and b) the evaluation of changes in strain due to curvature in beam elements. To monitor the change in crack width and slip, a method was created and validated on a series of artificial and reinforced concrete images for the cases of pure shear, pure flexure and combine flexure and shear. Curvature was found to impact the crack slip measurement, but its effect can be removed by using an innovative averaging technique. The curvature of a steel HSS and RC beams was found by using virtual DIC strain gauges and the horizontal strain profile. Results matched well with the curvature from electrical foil gauges and numerical models when the gauge length was maximized and selected so that the effects of cracking were accounted for.