Method development for the characterisation and monitoring of local conditions in adhesive layers of structural adhesively bonded joints

• Applicant: Professor Dr.-Ing. Gerson Meschut
• Subject Area: Joining and Separation Technology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 521413543

For dimensioning and simulation of adhesively bonded structures, destructive tested specimens or components are analyzed by methods whose principle is based only on the measurement of deformation on freely accessible surfaces. However, the external high-resolution strain measurement methods prevent the determination and observation of local states in the adhesive layer, which in turn prevents a gain in knowledge about fundamental phenomena. Therefore this project aims to contribute to a local determination of the inner adhesive layer condition of structural bonded joints by a systematically and detailed investigation of the causes of the measurement signal of a quasi-continuous glass fibre sensor (FOS) integrated into the adhesive layer and demonstrating an approach for exploiting it. For this purpose, a combination of experimental and numerical approaches is provided, which is used to develop a methodology that can be used to monitor and evaluate the internal local state of adhesive joints under different loads. To achieve this, the elementary influence of adhesive-specific factors, such as type of stress, adhesive layer geometry and fibre position, on the measurement signal of the integrated FOS is investigated and quantified. Based on the investigations, the cause-effect mechanisms of the resulting measurement signal are revealed, creating a fundamental understanding of the metrological evaluation. From the conclusions, strategies for the metrologically adequate arrangement and insertion of the fibre-optic sensor are derived, as well as the effects concerning the joint strength. Based on the profound knowledge gained, the method is developed with which the inner adhesive layer state can be recorded with high sensitivity and local resolution while simultaneously taking into account production and operation-related influences. This also includes the detection of local adhesive bond defects and their effects on the behaviour of the joint. The developed methodology is evaluated through technological and component-like specimens. Target-oriented practices are combined for the utilisation of the strain signals of the FOS, so that the formulated adhesive behaviour by analytical dimensioning methods or FE models can no longer only be adapted and validated on global measurement data, but on in-situ measured local adhesive layer data. Furthermore, the determined local internal and external stresses are used to classify existing material models or dimensioning methods and to derive critical strain values for taking into account adhesive layer damage and to estimate reductions in load-bearing capacity for a reliable prediction of the mechanical adhesive behaviour.

• DFG Programme: Research Grants