Composite materials are widely used for lightweight design applications due to their beneficial mechanical properties. A crucial aspect to exploit their full potential is to join them efficiently without adding too much additional weight. Especially in aeronautic applications, glued inserts are a common approach to establish contact points with threads. This research is concerned with an advanced design and joining approach for inserts and focuses on static out-of-plane loading as a critical load case for such components. In comparison to classical inserts, no additional adhesive is needed and the weight of the joint is reduced. Two different designs following this approach are developed and tested. The first design is a disc shape, the second one features roots based on a biomimetic approach and both designs are circular symmetric. Damage initiation for both designs starts at a similar load, but the total failure load of the second design is significantly higher than of the first design. For further improvements regarding safety aspects and maintenance time intervals of the joint, a Structural Health Monitoring (SHM) system is implemented. During damage initiation and propagation, strain energy is released and resulting propagating waves can be picked-up on the surface of the laminate using piezoelectric elements. The first Acoustic Emission (AE) events indicate damage initiation. While the first design fails concurrent with the first big AE event, the second design is more suitable for monitoring due to many high peak AE events before its total failure. The results are validated by optical measurements using a Digital Image Correlation (DIC) system.

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Thomas Wolfsgruber, Martin Schagerl, and Stefan Sieberer, "Improvements for out-of-plane loaded hybrid joints regarding mechanical strength and SHM applicability," Proc. SPIE 12488, Health Monitoring of Structural and Biological Systems XVII, 124880U (Presented at SPIE Smart Structures + Nondestructive Evaluation: March 14, 2023; Published: 25 April 2023); https://doi.org/10.1117/12.2657684.