Multifunctional adhesive layers with crack-stop function and gradient sensing technology for structural health monitoring to be used as joining technology in CFRP light-weight structures

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The initial application pursued the goal of integrating a thin-film sensor system directly into highly stressed bondlines of composites and combining it with a crack-stop technology in order to improve the operational reliability of bonded composite structural components. In this context, function compliant sensor integration is of great importance, because the sensor technology must neither impair the adhesive bond nor the crack-stopping functionality. A lithographically produced smart inlay, made of a polyetherimide (PEI) substrate layer and a poly(vinylidene fluoride) PVDF top layer has been developed. This enabled to visualize a growing damage within the bondline by a strain gradient measurement. The PVDF simultaneously functions as crack stopper. Hence, integrated into a bondline, the smart inlay creates a multifunctional disbond arrest feature (MDAF). Regarding the MDAFs, core challenges could be solved. E.g., the surface quality of the new substrate could be adjusted to meet the requirements of the micro fabrication of thin-film sensors. Despite having confirmed the working principals of the Mulifunctional Bondline, there is further need for fundamental research, especially regarding the robustness of the sensors. The goals of the second funding period are developing a design methodology based on characteristic values to be established for functionally compliant multifunctional bondlines, improving the dynamic strength of the sensors and implementing scaling methods for large-area sensor integration. Complementing the first funding period, further investigations are carried out to generate experimental characteristic values. These characteristic values can then be used to evaluate and compare different variants of the Multifunctional Bondline. Alternative substrate materials and pre-treatment methods will be tested to improve the dynamic strength of the smart inlays. Furthermore, with screen printing, a new manufacturing process is investigated that allows the usage of stretchable conductive pastes to manufacture the required sensor structures. Due to this stretchability and the presumably more homogeneous stress distribution compared to the metallic sensor structures, an improved sensor dynamic strength can be expected. With the improved smart inlays, it is the aim to improve the damage detection algorithm to achieve robust damage detection and crack length prediction. To conclude the project, scaling methods for larger area components will be investigated. This part will look at how sensor structures can be interconnected such that large areas can be covered. In addition, piezoelectric energy harvesting concept are investigated to enable self-sustaining smart inlays. The scalability of the multifunctional crack-arrest technology is tested by an omega-stringer to skin adhesive bond that is typical for light-weight design.