Screen-printed polyetherimide smart inlays for surveillance of bonded joints in composite structures

Abstract

Adhesive bonding is ideal for joining lightweight components made of composite materials. However, the reliability of these structural bonds, particularly in terms of fatigue crack growth, is a major concern. To mitigate this risk, structural health monitoring of the bonding is desired. This can be achieved by measuring strains at different locations in the adhesive bondline. Here, we present screen-printing on polyetherimide to realize a smart inlay that can be placed into the adhesive bondline during CFRP integration and can replace cleanroom-fabricated crack propagation sensors demonstrated earlier. The sensor layout is designed to enable crack monitoring by evaluating strain differences between two sensor rows. For the first time, smart inlays were fabricated and encapsulated entirely on polyetherimide, which has excellent adhesion strength to the epoxy matrix of CFRP. For strain detection, we used screen-printed carbon sensors. We selected screen-printing inks that could withstand the harsh conditions during CFRP fabrication. For the characterization of our smart inlay, we analyzed the screen-printed carbon sensors before and after integration into composites. After integration, the sensors were fully functional. As a striking result, after the entire fabrication and integration, the sensitivity of the screen-printed carbon sensors reached a gauge factor of up to 49.