An ultrasonic based measurement method considering viscoelastic properties to characterize the fibre matrix adhesion of organic sheets and their realistic modelling

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Fibre-reinforced plastics are becoming increasingly important due to the advantageous synergy between lightweight plastics and durable fibres. They are particularly attractive to the automotive industry due to their low weight. However, the understanding of materials is still insufficient, especially for thermoplastics-based fibre composites. The properties of fibre composites are significantly influenced by the fibre matrix adhesion (FMA), implying that the quantification of FMA is of great interest for quality assessment. However, as of now only destructive testing methods on specially manufactured test specimens (e.g. "Single-Fibre Pull-Out Test") are available. A non-destructive characterization and monitoring of the FMA for real components is therefore not possible.The aim of this research project is the development of an ultrasound-based measuring method, which enables the characterization of the fibre matrix adhesion of organic sheets, as well as their realistic modelling. Starting points are the classical laminate theory (CLT), which is an established method for the design of multilayer composites, and an acoustic measurement method based on Lamb waves in plate waveguides, which was developed at the Measurement Engineering Group. The classical laminate theory assumes an ideal adhesion between the individual elements of the multilayer composite, which is why an extension to a non-ideal FMA must first be made in order to achieve the objective. In addition, high-frequency acoustic waves in fibre composites are influenced by the viscoelastic properties of the material. As could be shown in preliminary work, these lead to a deviation between mechanically and acoustically determined material parameters (e.g. Young's modulus). Therefore, the acoustic measurement method has to be extended or adapted to take these into account. By subsequently linking the two methods, a novel measurement procedure will be developed which allows the identification of the component-related parameters of the extended CLT model on the basis of the macroscopic overall material behaviour. Since this method works non-destructively, the feasibility and suitability in principle of the measuring method developed here for a later application for preventive maintenance, long-term material monitoring and 100% testing will be demonstrated. With these tools, the influence of FMA is investigated on the basis of selected material combinations, i.e. by varying the FMA, in order to gain a deeper understanding of material behaviour that can be taken into account in future component design processes.