Impact damage, excessive loading, and corrosion have been identified as critical and long-term problems that constantly threaten the integrity and reliability of structural systems (e.g., civil infrastructures, aircrafts, and naval vessels). While a variety of sensing transducers have been proposed for structural health monitoring, most sensors only offer measurement of structural behavior at discrete structural locations. Here, a conformable carbon nanotubepolyelectrolyte sensing skin fabricated via the layer-by-layer technique is proposed to monitor strain and impact damage over spatial areas. Specifically, electrical impedance tomographical (EIT) conductivity mapping techniques are employed to offer two-dimensional damage maps from which damage location and severity can be easily and accurately quantified. This study deposits carbon nanotube-based sensing skins upon metallic structural plates with electrodes installed along the plate boundary. Based on boundary electrical measurements, EIT mapping captures both strain in the underlying substrate as well as damage (e.g., permanent

This paper describes the application of electrical impedance tomography (EIT) to demonstrate the multifunctionality of carbon nanocomposite thin films under various types of environmental stimuli. Carbon nanotube (CNT) thin films are fabricated by a layer-by-layer (LbL) technique and mounted with electrodes along their boundaries. The response of the thin films to various stimuli is investigated by relying on electric current excitation and corresponding boundary potential measurements. The spatial conductivity variations are reconstructed based on a mathematical model for the EIT technique. Here, the ability of the EIT method to provide two-dimensional mapping of the conductivity of CNT thin films is validated by (1) electrically imaging intentional structural defects in the thin films and (2) mapping the film's response to various pH environments. The ability to spatially image the conductivity of CNT thin films holds many promises for developing multifunctional CNT-based sensing skins.

Cement-based composites (for example, concrete) are brittle materials that crack when loaded in tension. Current strategies for crack detection are primarily based upon visual inspection by an inspector; such approaches are labor-intensive and expensive. Direly needed are sensors that can be included within a structural health monitoring (SHM) system for automated quantification of crack damage. This study explores the use of cementitious materials as their own sensor platform capable of measuring mechanical behavior under loading. Fundamentally, this self-sensing functionality will be based upon electro-mechanical properties. First, the piezoresistivity of cementitious composites is quantified so as to establish the material as a multifunctional system capable of self-sensing. Second, electrical impedance tomography (EIT) is proposed for measuring internal strain fields using only electrical measurements taken along the boundary of the structural element. An inherent advantage of EIT is that it is a distributed sensing approach offering measurement of strain fields across 2D or 3D. Furthermore, the approach is well suited for imaging cracks which appear as conductivity reductions in EIT-derived conductivity maps. Finally, to validate the accuracy of the EIT technique, it is applied to fiber reinforced cementitious composite elements loaded by axial tension-compression cycles and 3-point bending.

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