Carbon Nanotube Yarn Sensors for Precise Monitoring of Damage Evolution in Laminated Composite Materials: Latest Experimental Results in In-Situ and Post-Testing Validation

Abstract

The increasing demand for materials with high mechanical strength, stiffness, without the weight of traditional metals like steels in industrial applications have led to the need for an improved understanding of the lifecycle and failure mechanisms of composite materials. Composite materials and structures are known to be tailorable to specific industrial needs due to their ability to combine properties from different materials to achieve high strength-to-weight ratio, high stiffness, creep-, fatigue- and corrosion-resistance, without losing its weight advantage. However, laminated composites like carbon fiber- reinforced plastics (CFRPs) and glass fiber-reinforced plastics (GFRPs) with their excellent in-plane properties are weakened by the poor outof- plane properties, which may lead to early failure. The failure mechanisms in laminated composites are dissimilar to the well-understood failure in conventional materials like metals or ceramics. They are dominated by fiber breaking, delamination and matrix cracking. Of concern is that delamination and matrix cracking can occur at any time in the composite structure unlike fiber failure that typically occurs at the end of a lifecycle of a fiber-reinforced plastics (FRPs). They occur mostly within the composite structure often times undetected leading to catastrophic failure. The results of a laminated composite sample implemented with integrated carbon nanotube yarn sensors demonstrate the ability of the sensors to detect the damage and provide their locations as a function of the applied load and in real-time. The previous results obtained with the CNT yarn sensors were validated using two experimental techniques: integrated optical fibers monitored through time-domain reflectometry, and x-ray tomography of the entire laminated composite samples post-testing. High performance plastic multimode optical fibers were integrated in the laminated composite plates. Furthermore, a three-dimensional computerized tomography (CT) imaging technique was used to corroborate the damage in the laminated composite samples.