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Fiber-reinforced polymer (FRP) composites are widely used in the aerospace industry, but they are also susceptible to different types of damage. In particular, impact is a serious concern, since they can cause subsurface damage that can be hidden from visual inspection and jeopardize structural safety and performance. In addition, the complex geometries of FRP components can limit the use of current nondestructive inspection techniques. Thus, the objective of this study is to develop a portable, noncontact, surface scanning tool for technicians to rapidly detect, characterize, and localize subsurface structural features and damage in structural components. The technique is centered around the theory of electrical capacitance tomography (ECT). Unlike conventional ECT, an array of noncontact electrodes is arranged in a planar fashion. Electric field is propagated between electrodes to interrogate the sensing region, which is defined as the region of space underneath the plane of electrodes. Meanwhile, the corresponding mutual capacitance between excitation and other electrodes are measured. The recorded datasets are then used as inputs to solve the ECT inverse problem in which the electrical permittivity of the 3D sensing region can be reconstructed. In this work, numerical modeling was employed to demonstrate the sensing performance of the planar array capacitive imaging system. First, several composite components with different types of damage were simulated using finite element modeling. Damage was introduced by selectively changing the electrical permittivity at different regions. Second, the planar capacitive electrode array, which was placed near the composite component, was also modeled. Electric field was propagated between different electrodes while the corresponding mutual capacitance of other electrodes were determined. The simulated data was then used as inputs to solve the ECT inverse problem. The numerical simulation results showed that the planar array capacitive imaging system was able to detect, quantify, and locate damage-induced changes in electrical permittivity distributions in composite structures, thereby demonstrating its potential for use as a nondestructive inspection tool.
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