K. Aly, A. Li, P. Bradford
North Carolina State University,
Keywords: fiber reinforced composites, carbon nanotubes, structural health monitoring, mechanical loading, electrical resistance change
Summary:The widespread adoption of fiber reinforced polymer (FRP) composites in different sectors such as, automotive, aerospace and energy accounts for a large percentage of the regular increase in the world demand for this class of material. The Outstanding fatigue performance, high specific stiffness and strength, and low density are among the most important properties that FRPs offer. Nevertheless, this class of material is composed of multiple layers of inhomogeneous and anisotropic constituents. Therefore, this laminated nature makes composite materials prone to intrinsic damage including interfacial debonding and delamination. Thus, structural health monitoring (SHM) in composites has become more important than ever. New and improved methods for early damage detection and structural health monitoring of composite materials may allow for enhanced reliability, lifetime and performance in addition to minimizing maintenance time. Owing to their superior mechanical and electrical properties, carbon nanotubes (CNTs) are used extensively to impart multifunctional capabilities into composite structures. In this context, CNTs have been used in the recent years to enable sensing capabilities. This is achieved through employing different CNT architectures to create electrically conductive composite structures that are afterwards subjected to mechanical loading. In this paper, the utilization of CNT sheets as strain and damage sensing material in composites is assessed. This is enabled by embedding aligned sheets of two millimeters long, interconnected CNTs into laminated structures that are then subjected to different forms of mechanical loading. Unlike other CNT morphologies, the sheets are a good candidate that can be localized in specific locations such as ply interface and sense for delamination at this location. Additionally, the CNT sheets embedment inside the host structure can enable higher sensitivity and damage detection due to more effective stress transfer and being in closer vicinity to the developing damage. The CNTs real-time electrical resistance change in response to the applied mechanical stresses is measured in-situ. The purpose is to link the electromechanical behavior of the CNTs to the strain change and damage in the host structure. The quasi-static and dynamic flexural, axial tensile and compression loadings of the composite structures revealed that the CNT sheets exhibited sensitivity, stability and repeatability which are vital properties for any successful health monitoring technique. Furthermore, the reverse loading directions produced opposite signs in the CNTs piezoresistive behavior, all the way until fracture, which allows for easily differentiating the loading direction. The CNT sheets sensitivity to tensile and compression loadings could be tuned by applying post treatment techniques and the optimization of the number of the CNT sheets prior to embedment. Additionally, the CNTs showed increasing continuous linear piezoresistive behavior in response to the growing interlaminar shear with few normalized electrical resistance change spikes corresponding to delaminations inside the laminated structure. Finally, the impact loading of the composite structure demonstrated the sensitivity of the CNT sheets in detecting, locating and quantifying impact damage. The results showed that for multiple low and high impact energy regimes, the CNT embedded in non-impact side exhibited the highest normalized resistance change values of 28% and 560% respectively.