We analyze experimental setup requirements and data post-processing methods used in these studies. In this work, we review the existing studies on the DIC application in large-scale structural testing with a particular focus on wind energy and aerospace sectors where fiber-reinforced composite materials are commonly used. Recent years have witnessed more and more applications of DIC in large-scale structural testing with many identified common challenges and possible solutions. The solutions to these challenges are case-dependent and no established protocol exists. Illuminating large structures requires significantly more power and limitations of camera memory and speed introduce inaccuracies in higher-order operational deflection shape measurement. Moreover, camera field of view (FoV) has to be carefully adjusted not to compromise image resolution and multiple camera system has to be properly synchronized and calibrated both individually and globally – all these steps make the experimental setup more cumbersome. This is an additional processing step not present on smaller structural scales. For example, it is not possible to fit a structure into a single image, hence cameras have to be moved to capture all structural parts of interest and images have to be properly stitched to obtain a uniform displacement/strain field. Simple up-scaling of DIC equipment and setup is not feasible. Strain is calculated from the measured displacements and images from different camera positions are stitched together to obtain a final image of geometry, displacement, and strain.Ĭompared to DIC measurements at a material level, the application in large-scale structures faces significant challenges. The camera pair is moved along the structure and images are taken during structural loading. A random speckle pattern of appropriate shape and speckle size (determined through calibration) is applied on the surface of measured objects (either on a structural part or a whole structure) and illuminated with a lighting system for high contrast. The general workflow of DIC measurements of surface geometry, strain, and displacement monitoring is shown in Fig.
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Typical equipment for 3D DIC measurements is as follows – charge-coupled device (CCD) or complementary metal–oxidesemiconductor (CMOS) cameras of high spatial resolution, optical lenses, stable tripod supports for cameras, camera synchronization unit, lighting system, speckle pattern, and computer with appropriate software for visualizing, post-processing and storing data. The true spatial coordinates of points before and after deformation are reconstructed from the coordinates of acquired images and the camera calibration process.
Three-dimensional (3D) DIC operates through correlating two gray-scale images at different time instants before and after deformation by computing a correlation function. It is worthwhile to briefly introduce DIC techniques. Due to differences in test objectives and various practical constraints, there are so far no well-established experimental setups and test procedures for the application of DIC to large-scale structures as they vary on a case-by-case basis. The need of DIC in large-scale structural testing becomes increasingly urgent particularly with the rapid development of wind energy and aerospace industry where the size of composite structures such as rotor blades and wings increase, e.g., the largest wind turbine blade (WTB) nowadays exceeds 100 m. In recent years, the DIC gains more popularity in large-scale structural testing due to its advantage over point-wise measurement techniques – allowing a large area of structures to be measured efficiently from a distance. The DIC has been extensively applied to material testing where the size of specimens is small and the experimental setup is well-established. Digital Image Correlation (DIC) technique was invented in the early 1980s, and it is nowadays a common non-contact full-field technique to measure geometry, displacement and strain of materials and structures.
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