Material BehaviourRequirements of amount of carbon nanotubes for damage detection in large polymer composite structures

Due to their very high electrical conductivity, the addition of carbon nanotubes (CNTs) into polymers such as epoxies makes these materials conductive. This conductivity has been utilized to provide damage sensing in composite structures. Usually, the amount of CNTs needs to be more than the percolation threshold to assure electrical conductivity. The percolation threshold is usually determined using small samples. For large samples, the amount of CNTs needs to be higher to take into account some non-uniformity of the dispersion. More CNTs would provide better conductivity. One normally expects that more CNTs would also provide better damage detection. However, it was found that this is not the case. Certainly, the amount of CNTs needs to be more than a certain lower limit to assure conductivity throughout the large structures. Once this condition is met, adding more CNTs would reduce the sensitivity for damage detection. The sensitivity of damage detection can be measured by the change in electrical resistance (due to the occurrence of damage) between grid points that are attached on the surface of the composite structure. Higher sensitivity in damage detection would enable coarse grids (larger distance between grid points). Coarse grid points would mean lower number of grid points, less space, less wiring and less weight. This paper describes this phenomenon in detail. It provides models that simulate the conductivity configurations. It also introduces a new term call “Aggregately Conductive Materials” to distinguish the particular conductive characteristics of materials that are made conductive by the addition of nano-particles.