Digital Image Correlation

In order to detect the occurrence of failure in a material, visual observation is a very intuitive approach. Although global failure is many times easier to observe visually, more dedicated analysis is required to detect small changes indicating the occurrence of first failure. One specific type of visual observation follows the idea to track and measure changes between subsequently acquired images of an object. The concept used for that purpose is based on digital image acquisition of a moving and deforming object and is known as digital image correlation. This chapter starts with a description of the mathematical and physical principles used by 2D and 3D digital image correlation. Focus is given on the parameters that limit detectability for state-of-the-art equipment and comparison is made between theoretical predictions and measurements under laboratory conditions. The second part of the chapter has its focus on the theoretical description of strain concentration due to internal flaws, especially for those types found in fiber reinforced composites. Measurements with a variety of defect types are compared to calculations using finite element modeling. Detectability of failure mechanisms in fiber reinforced composites is discussed as function of failure type, failure size and depth position below surface. The last section elaborates some typical applications of digital image correlation measurements in conjunction with testing of fiber reinforced composites.

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