Adhesive joints play an important role in structural reliability and durability of assembled load carrying structures. In the application of wind turbine blades the wing box is built up of shear webs and shear caps which are joined to each other with brittle adhesives. With blade dimensions in the order of 40 to 50 m and with current manufacturing tolerances and assembly procedures, adhesive joint thickness may be up to 10 mm, with a high probability on the presence of voids and cavities. As the blade is subjected to simultaneous bending, torsion and shear force, the stress state in the adhesive layers is multi-axial and stress components are non-proportional. The machine has an economical life of 20 years and fatigue may be a critical phenomenon. This research focuses on a bottom-up adhesive properties characterization and its validation in composite joints. It starts from the characterization of bulk adhesive going through bonded joint specimens and subcomponents. This paper focusses on the levels of the adhesive material itself and of the joint. After an extensive experimental campaign with particular attention to porosity in the adhesive a probabilistic approach is used to identify the most appropriate failure criterion. The strength prediction method considers a statistical size effect in the strength of the material by considering not only the magnitude of the stress distributions, but also the volume over which they act. This approach is subsequently used for the numerical prediction of the strength of joints in simple joints and in spar-cap-shear web subcomponent. The predicted resistance of joints are in agreement with experimental joint tests.
Fernandez, G., Vandepitte, D., Usabiaga, H., & Debruyne, S. (2017). Static and cyclic strength properties of brittle adhesives with porosity. In Procedia Structural Integrity (Vol. 7, pp. 291–298). Elsevier B.V. https://doi.org/10.1016/j.prostr.2017.11.091