In-situ cure monitoring of structural composite by embedment of vertically aligned carbon nanotube forests

The next generation of composite structures within aerospace is envisioned to evolve from a strictly mechanical to a multifunctional structure, adding functionalities to the structure by embedment of functional filler material or incorporation of foreign structures. The introduction of carbon nanotubes (CNTs) into the composite structure to achieve sensing capabilities is one example. In this paper, online cure monitoring of aerospace-grade glass fibre/epoxy prepreg laminates is performed by in-situ resistive measurements on embedded vertically aligned carbon nanotube (VACNT) forests. The measured resistance over the course of the cure cycle has a reproducibility in its shape, itself a reflection of the state of the embedded CNTs. The measured resistance is interpreted after studying the morphology of the VACNT forest, cure kinetics and viscosity of the resin, and volumetric changes of both resin and laminate during the cure cycle. The resistive signal is determined to detect the transition between the air-evacuation and consolidation regimes of the laminate compaction and the gel point of the epoxy. Unique observations after the gel point are recorded, theorised to be caused by the build-up of residual stresses in the laminate. The proposed cure monitoring sensor system offers great flexibility, being able to monitor the curing process locally anywhere in the laminate. Additionally, the proposed sensor offers a life-span multifunctionality to the produced component, possessing strain and temperature sensing capabilities in the cured state ideal for structural health monitoring.