of both damage initiation and growth. Such solutions are closer to the actual physics of observed failure processes but are more challenging for analysis. Cohesive element based (CE) FEA modeling approaches [3, 9–13] seem to be among the most promising ways to mitigate these challenges. In CE-based FEA of repaired parts, simulation of damage along a predefined crack path is considered. In this case, a traction-separation response is specified between initially coincidental nodes on either side of the path. Such modeling considers both damage initiation and its potential growth through understanding of the strength and toughness response, respectively. Although corresponding computational CE-based solutions are rapidly developed [3, 9–14], their experimental validation is still a much less explored area. In addition, reliable characterization of input data for such analysis is an equally challenging issue requiring development of test methods specifically for repaired zones, where more traditional methods of fracture mechanics characterization have only marginal applicability. Thus, the main motivation of this study is the development of test implementations to understand and quantify processes of both damage initiation and growth at repaired surfaces. Digital Image Correlation (DIC) technique provides invaluable opportunity for non-contact 2D assessment of strain fields and seems to be especially helpful in analysis of progressive damage networks. Thus, the objective is the development of a DIC-based experimental approach for analysis of repaired zones in composite structures including its demonstration for typical composite materials and parameters of repair design. 12.2 Approach Major interest is focused on the understanding of processes controlling damage initiation and growth and, especially, behavior at load levels beforeany visible damage occurs. It can help to understand redistribution of local strains as functions of applied load, assess inevitable local statistical variability, and clearly identify the weakest points or areas. In addition to the damage process itself, characterization of pre-damage behavior can also be used for understanding of the actual interaction between bonded composite materials for purposes of potential optimization. For these purposes, direct application of in-planeDIChas some limitations (here, “in plane” is defined as at either outer or inner surfaces of the composite zone). The key disadvantage of in-plane DIC is a relatively small area to be observed, i.e., relatively low sensitivity to any changes in strain fields. It is suggested, therefore, in this study to consider through-thickness DIC-based characterization of repaired zones. This approach is schematically illustrated in Fig. 12.1, where cross-sectional slices of repaired zones are the surfaces to be analyzed by DIC (let’s emphasize that this approach is suggested for experimental characterization in laboratory conditions Scheme oftesting a b c d Coupon before testing Coupon after testing Damage process using Digital Image Correlation(DIC) 10000 [μm/m] 8000 6000 4000 2000 0 -2000 -5000 Fig. 12.1 Scheme of testing (a), typical coupon (b), its post-damage view (c), results of representative test (d) 92 M.R. Gurvich et al.
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