Static and cyclic strength properties of brittle adhesives with porosity

Adhesive joints play an important role in structural reliability and durability of assembled load carrying structures. This research is inspired by the application to wind turbine blades, which are built up of composite materials and joined to each other with adhesives. In contrast to traditional guidelines for adhesive joints, wind turbine blade joints have high thickness, in the order of 10 mm. The probability of presence of voids and porosities is high. Still, 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 characterisation and its validation in composite joints, at successively levels. It starts from the characterisation 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 developed 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 an application which is inspired by a wind turbine blade. The predicted resistance of joints is in close agreement with experimental joint tests.