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Characterization of Cohesive Zone Laws Using Digital Image Correlation



Delamination is a major failure mode in composite structures. This work addresses lack of understanding of delamination cohesive laws which is a fundamental challenge of strength and fatigue progressive damage and failure analysis (PDFA) of composite structures. Typically such laws are based on mathematical assumptions which are fit to simplify convergence of the analysis tools and are not based on physics or measurement. The simplifying assumptions may cause wrong cohesive material properties as they fit to a specified simulation task. This work presents the results of a feasibility study focused on developing experimental methods and analysis techniques for capturing material cohesive properties and increasing confidence in material input data for PDFA. A direct approach is proposed to measure cohesive laws in static Double Cantilever Beam (DCB) and Thick Adherend (TA) shear specimens that have been pre-cracked under fatigue load. High resolution Digital Image Correlation techniques are used to measure crack tip separation in these specimens, supported by X-ray Computed Tomography (CT) reconstructions to locate the crack front. Additionally, by J-integral and VCCT calculations, the precise shapes of the cohesive damage laws in mode I and mode II are characterized. These cohesive laws were verified by direct comparison of load-displacement curves between numerical Finite Element models and experimental data of the DCB and TA test. The specimen compliance is accurately reproduced, and maximum strength prediction is within 4%. It is noteworthy that accurate crack location has been essential for consistent results.


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