Ann-Kathrin Klein

Rivetless structural bonding is a key technology for increasing the structural efficiency of composite aircraft. Structural bonding provides a more uniform load distribution than mechanical fastening, resulting in higher composite joint strength at a reduced weight. However, the certification of rivetless structural bonds is challenging due to their more complex design, their inability to retard damage within the adhesive layer, and concerns regarding their long-term durability. As a result, it is state-of-the-art to certify safety-critical composite joints using well-established mechanical fastening processes, despite their weight penalty. Therefore, there is a need for a rivetless structural bond design that is lightweight and complies with the strict certification requirements. Increasing damage tolerance through design features and monitoring the bondline integrity are two options defined by aviation authorities to obtain certification. The multifunctional bondline combines both approaches because they complement each other well. In particular, monitoring the structural health of the bondline can be simplified due to the damage tolerance feature. Since the damage tolerance feature arrests damage at defined positions and maintains limit load capacity after arrest, the idea is to reduce the requirements for the structural health monitoring (SHM) system. Cracked-lap shear specimens equipped with multifunctional disbond arrest features (MDAFs) illustrate the feasibility of the multifunctional bondline approach. The MDAFs consist of fibre Bragg grating sensors or miniaturised strain gauges for damage monitoring and thermoplastic surface-toughening elements for damage arrest. As damage alters the load transfer across the bondline, comparing strains measured near the damage front to those in an undamaged region enables detection. The minimal distance between the two measurement points is the minimum load-transfer length---the distance required to complete 95 % of the load redistribution in structural bonds. Therefore, a two-staged MDAF design consisting of two surface toughening elements with integrated sensors, positioned slightly further apart than the estimated minimum load-transfer length, was tested. The results demonstrate that the two-stage MDAFs reliably arrest damage and enable damage detection by comparing the strain values obtained at both stages. For the given setup, comparing the strain differences to a threshold suffices as a damage criterion. Even though this criterion requires adjustments for the application in more complex systems, the results confirm that it is feasible to reduce the requirements of the SHM system by combining it with a disbond arrest function. Instead of tracking damage initiation and providing precise damage localisation and residual strength estimates, the SHM system is only required to identify the event of arrested damage by the MDAF. The residual strength can then be determined from the remaining undamaged bondline regions. Therefore, the complexity of the SHM system can be reduced in terms of both the number of installed sensors and the damage-detection algorithms.

Continuous and long‐term pressure monitoring by devices implanted in the human body promises substantial improvements for diagnosis, health monitoring, surveillance of disease progression, or surveillance of the response to a treatment but also for precision during surgeries. Here, we report on a piezoresistive sensor made entirely of parylene‐C eliminating the need for heterogeneous encapsulation. Material characterization demonstrated that parylene‐C is excellently suited as a construction material. The pressure sensor comprises a parylene‐C membrane, which is functionalized with gold strain sensors for pressure sensitivity. It was bonded to the reference pressure cavity, thereby establishing a long‐time stable hermetic sealing. The presented design and fabrication enable for the first time a monolithic, highly soft and flexible pressure sensor entirely made of biocompatible material. It exhibited high pressure sensitivity of up to 6.2 µV·mmHg ⁻¹ with very good repeatability and negligible hysteresis, made possible by thin‐film strain sensors on a parylene‐C membrane with a gauge factor of 7.5. Simulations show how the design can be easily even further miniaturized. Initial in vitro assay confirmed that parylene‐C is biocompatible on cell culture level. Such biocompatible and chronically implantable pressure sensors are opening new possibilities in medical treatments and long‐term supervision.