Helmut Schmidt University - PD Dr.-Ing. Natalie Rauter
Helmut Schmidt University - Prof. Dr.-Ing. habil. Wolfgang Weber
Funding German Research Foundation (DFG) - Project number: 418311604
Subproject of FOR 3022:Ultrasonic Monitoring of Fibre Metal Laminates Using Integrated Sensors
Website of the Research Unit: FOR 3022
Due to the layered structure and multiple material interfaces in fibre metal laminates (FML), the risk of delaminations hidden within the structure is significantly increased. One technique to detect such hidden damage is guided ultrasonic wave (GUW) based structural health monitoring (SHM).
Based on the analytical framework, only little attention was paid to the wave propagation characteristics in FML prior to the work of the research unit covering FML and dealing with bonded layer interfaces. The main difference to the profoundly investigated GUW propagation in pure fibre reinforced polymer (FRP) laminates and isotropic waveguides are the impedance changes over the thickness and both production and design induced inhomogeneities.
The main wave propagation characteristics have been addressed outside the research unit only by a few publications focusing on a combination of glass fibre reinforced polymer (GFRP) and aluminium. Beside this, additional work focuses on metal and FRP interfaces. These publications and the results obtained within the research unit during the first funding period indicate that the analytical framework of GUW also covers the wave propagation in FML. This finding is not limited to GFRP-aluminium but also valid for carbon fibre-reinforced polymer (CFRP) steel laminates.
For the analysis of the influence of stress states on the wave propagation, initial investigations outside the research unit are limited to certain frequencies. Besides the material inherent complexities, covering different state characteristics, the large-scale production process causes further inhomogeneities that significantly influence the wave propagation and hence, the damage detection. Important aspects are local hybridizations and splices. The integration of such phenomena requires an increased complexity in the numerical modelling approach including, e.g., interphase modelling.
Furthermore, a comprehensive validation of numerical models with experimental data is necessary to allow a profound understanding of the GUW propagation in FML and the consequences of structural complexity on damage detection and design of SHM systems for FML.
In conclusion, this subproject investigates the effect of design and manufacturing induced inhomogeneities in FML structures and their consequences for a SHM system on an experimental and numerical level leading to the following research hypothesis: Based on the findings of the first funding period, GUW are a proper means to detect damage in FML. This also holds if production and design induced transition zones occur. Thus, evaluating measured signals necessitates advanced numerical modelling. This modelling incorporates both geometric and material uncertainties as well as multi-scale approaches. The developed numerical models can be validated by experimental investigations leading to a reliable model of real-world applications.