Baseline-free damage detection in self-sensing composites via frequency-difference EIT

Abstract

Electrical impedance tomography (EIT) is the process of spatially mapping the conductivity distribution of a domain based on voltage-current relationships observed at the boundary of the domain. Due to its low cost, ease of implementation, and benign nature (i.e. non-ionizing), EIT has received much attention for potential application to damage, strain, and saturation mapping in civil, mechanical, and aerospace venues that employ conductive materials (e.g. nanofiller-modified cements and polymers, carbon fiber composites, etc.). Mathematically, however, EIT is an ill-posed inverse problem predicated on reconstructing the conductivity distribution on a model of the domain. This process is prone to nonconvergence due to factors such as experiment-to-model domain shape discrepancies and electrode misplacement. Time difference imaging is commonly used to overcome this challenge–boundary measurements are collected when the material is in a pristine state and another set of boundary measurements are collected after some conductivity-changing perturbation such that experiment-to-model discrepancies largely subtract out. But this process of time-difference imaging is problematic for several reasons: one, a pristine state is not available for existent or already in-service structures. Two, pristine-state data can be lost over time. And three, the pristine-state data may no longer be applicable if there are significant shape changes, electrodes are reapplied, etc. Thus, it is desirable to have a baseline-free technique for EIT. To that end, we herein present the results of a preliminary investigation on damage detection via frequency-difference imaging (fdEIT). In this approach, we leverage the fact that materials generally exhibit frequency-dependent conductivity such that an EIT image can be formed from the difference between two different frequencies. Experimental measurements of AC conductivity up to 10 MHz were collected from a carbon nanofiber (CNF)-modified glass fiber/epoxy laminate. These measurements were then used in a computational simulation to explore the potential of fdEIT for damage detection and localization. The results of this study suggest that fdEIT may indeed be a viable method of eliminating baseline measurements for EIT thereby providing an important advance towards making EIT a practical inspection modality.