Structural health monitoring (SHM) helps engineers maintain structures, enhance their safety, and operate them cost-effectively. Digital image correlation (DIC) techniques applied to SHM are noncontact techniques; they are also fast, accurate, and simple to implement. The goal of this investigation was to determine the smallest strain accurately measurable by a standard opensource DIC-based tool, in a specimen subjected to out-of-plane movement (toward or away from sensor). The investigation builds upon an initial study which determined the smallest accurately measurable strains using the same DIC-based tool in a specimen that was not subjected to controlled out-of-plane specimen movement. This initial study was motivated by initially undetected damage at low strains in connections of the Storstrøm Bridge in Denmark. Timely identifi cation of this damage would have prevented its spread, resulting in lower repair costs. It was observed in this initial study that the minute out-of-plane specimen movements that inadvertently occurred even in the controlled conditions of the laboratory created noise in the DIC-based strain measurements. Hence, before implementing this technique in a real-world structure, it is desirable to determine the bounds of out-of-plane displacement of the system within which it is practical to use this technique to measure strain. A cantilevered plate specimen was transversely loaded in the laboratory, and the longitudinal strain was measured at a selected location at different values of out-of-plane displacement of the specimen toward the sensor, using the open-source DIC-based tool. DIC-based strains were compared against those measured by conventional strain gauges. The smallest strains accurately measurable using DIC techniques, over a range of specimen out-of-plane displacement amplitudes, were determined.
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