Carbon-Nanotube-Film-Based Electrical Impedance Tomography for Structural Damage Detection of Carbon-Fiber-Reinforced Composites

This article shows a full utilization of the inherent conductive network of a carbon-nanotube (CNT) film as a sensing material and the development of an electrical impedance tomography (EIT) system for structural damage detection of carbon-fiber-reinforced composites. Taking advantage of the piezoresistivity of entangled and randomly oriented CNTs, this CNT-film-based system successfully monitors the evolution of the stress status and indentation damage during quasi-static indentation. The detectability of a carbon-nanotube sensing film is investigated by laminating a CNT film onto the surface or in the middle of the composite laminate, and the corresponding hybrid structures with different ply schemes are denoted as s-CNT/CF and m-CNT/CF, respectively. The CNT-film-based EIT system with a 100 × 100 mm2 sensing film and a 16-electrode design can detect the indentation stress of s-CNT/CF at a displacement of no less than 3 mm, while m-CNT/CF shows an obvious pattern of the indentation stress and damage in the EIT image, even at an indentation displacement of 1 mm. On the basis of the indentation dimension analysis of s-CNT/CF and the through-hole detectability of the CNT film, the dimension threshold is determined for the CNT film to detect stress distribution, damage, and through holes, which is in the range of 3.55–4.54 mm for the current EIT system design. A damage severity metric, ∑N, is defined to quantify the variation of the electrical conductivity in the indentation area. ∑N shows a slow descending tendency in the initial stage, followed by a rapid decrease with damage evolution. According to the microstructure analysis of the CNT film, the piezoresistive response caused by the elastic deformation of the CNT network plays an important role in the EIT signal in the initial stage, while the local CNT orientation and fractures cause a dramatic drop in ∑N. A high sensitivity of m-CNT/CF on the stress status and damage evolution shows great potential in structural health monitoring, especially for the detection of barely visible impact damage.