Planar array capacitive imaging for characterizing subsurface composite damage

Despite the advantages and increasing prevalence of composite use in aerospace structures, they are susceptible to unique damage modes such as cracks, delamination, and manufacturing defects, which are fundamentally very different than those that occur in metals. In particular, subsurface damage can be hidden from plain sight and lead to catastrophic structural failure if left undetected. Thus, this study aims to develop a noncontact, nondestructive inspection technique (NDI) for rapid and in situ subsurface damage assessment of composites. The proposed technique works on the principle of electrical capacitance tomography (ECT). Unlike conventional ECT systems, electrodes were designed and arranged to form a planar array. By propagating electric fields in the volumetric region above the electrode plane and measuring mutual capacitances between electrodes, the volumetric permittivity distribution of the interrogated region could be determined. First, a custom electrode array was built, and different objects were interrogated for validation. The changes in electrical permittivity distributions (and their corresponding locations) due to the presence of these objects were successfully detected. Second, subsurface damage, such as voids, were introduced in 3D-printed test specimens, which were also successfully characterized by the planar ECT system. Last, a carbon fiber-reinforced polymer composite (CFRP) specimen was fabricated and then subjected to impact to introduce subsurface delamination. The specimen was imaged using planar ECT, and the reconstructed permittivity distribution was compared with C-scan images. The results showed that the proposed system was able to detect the location and extent of impact-induced delamination in CFRP panels, thereby demonstrating its potential as a viable NDI technique.