CNT-based SHM

MDC has partnered with the Technology Laboratory for Advanced Materials and Structures (TELAMS) in the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology (MIT) to provide futuristic SHM technologies by using carbon nanotubes (CNTs) to enable mulit-physics, multi-functional capabilities within composite laminates. Several studies have shown that CNTs possess exceptional mechanical stiffness (as high as ~1 TPa) and strength, as well as excellent electrical conductivity (~1000x copper) and piezoresistivity (resistivity change with mechanical strain). Thus, they can be used to not only to reinforce composite structures to improve impact and delamination resistance, but also to enable novel SHM and NDE techniques.

Specimens provided by MIT have been instrumented by MDC with non-invasive silver-ink electrodes and multiplexing micro-switches using direct write, which can then be connected to custom miniature hardware for resistivity measurements. The painted electrode grid, inspired by flat panel liquid crystal display (LCD) technology, used an “active” layer of electrode columns on one surface of the laminate as positive electrode, and on the other surface, another layer of electrode rows will act as “passive” ground. Damage affects the CNT-link network around the affected zone in the structure, and correspondingly the local resistivity. Thus, by selecting particular combinations of rows and/or columns, local in-plane and through-thickness resistivity changes can be monitored, and subsequently visualized with nearly-unlimited full-field resolution. Experiments on several composite laminates have validated that even small beneath the visible surface impact damage is readily captured using this method.

Beyond resistance-based methods, MDC & MIT have demonstrated that the multi-functional capabilities of CNT-engineered laminates can be exploited for many other purposes ranging from greatly enhancing traditional NDE techniques such as thermography, to being used as in-situ acoustic emission (AE) sensors or de-icing elements. Many of these capabilities further make use of the piezoresistive properties of CNTs, the fact that they dynamically change resistance values under strain. Presently the opportunities are nearly limitless for this exciting new research area, which can be introduced into many typical composite laminates, and MDC is actively collaborating with MIT and other corporate partners to demonstrate and mature new aspects of this technology.