The failure of adhesive bondlines has been studied at the microscopic level via tensile tests. Stable crack propagation could be generated by means of samples with improved geometry, which made in-situ observations possible. The interaction of cracks with adhesive bondlines under various angles to the crack propagation was the focus of this study as well as the respective loading situations for the adhesives UF, PUR, and PVAc, which have distinctly different mechanical behaviors. It is shown how adhesive properties influence the occurrence of certain failure mechanisms and determine their appearance and order of magnitude. With the observed failure mechanisms, it becomes possible to predict the propagation path of a crack through the specimen.
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