Carbon nanotubes (CNTs) have a unique set of properties that may be useful in the production of next generation structural health monitoring composites. This research introduces a novel CNT based material system for strain and damage sensing applications. An aligned sheet of interconnected CNTs was drawn from a chemical vapor deposition grown CNT array and then bonded to the surface of glass fiber/epoxy composite coupons. Various types of mechanical tests were conducted, accompanied by real-time electrical data acquisition, in order to evaluate the electro-mechanical behavior of the developed sensing material. Specimens were loaded in the longitudinal and transverse CNT sheet orientations to investigate the anisotropy of the piezoresistive effect. The CNT sheets exhibited good sensing stability, linearity, sensitivity and repeatability within a practical strain range; which are crucial sensor features for health monitoring. It was also demonstrated that the CNT orientation in the sheet had a dramatic effect on the sensitivity, thus validating the usefulness of this sensing material for directional strain/damage monitoring. Finally, pre-straining of the CNT sheet sensors was conducted to further enhance the linearity of electro-mechanical response and long-term stability of the sensors during cyclic loading.

A novel nanofabrication method is demonstrated to produce large size, elastically resilient, ultra-low density carbon nanotube foams (3.8 mg cm⁻³) with anisotropic and tunable properties. Potential applications of this unique material are explored.

In practical situations, the flexural loading of fiber reinforced composites is one of the most common types of loading for structures. Nonetheless, carbon nanotubes, which have been used extensively to study in-plane strain sensing in composites, have rarely been employed to monitor flexural induced strains. With this motivation, this paper introduces a novel method for localizing aligned carbon nanotube sheet layers in three selected locations inside a laminated composite structure. These top, middle, and bottom locations are more prone to damage since they experience maximum flexural induced tension, compression, and interlaminar shear stresses. The composite structural performance is monitored by establishing an electromechanical coupling between the developing strain and the three carbon nanotube sensing elements’ in-plane electrical resistance changes that are measured simultaneously. The results of the monotonic and dynamic flexural loading tests suggest that carbon nanotube sensing materials exhibit a high level of sensitivity. The results also show that the resistance change of the three embedded carbon nanotube layers appeared to track the mechanical state of the host structure well. The carbon nanotube layer embedded in the middle section showed a piezoresistive behavior in response to the growing complex stress state with a few electrical resistance change spikes corresponding to damage developing inside the laminated structure. This flexural sensing behavior may considered as useful for real applications because in addition to sensing strain, this technology may help in predicting the failure of the composite component before the actual end of service life.