Damage detection in non-planar carbon fiber-reinforced polymer laminates via electrical impedance tomography with surface-mounted electrodes and directional sensitivity matrices

Carbon fiber reinforced polymers (CFRPs)have emerged as promising alternatives to traditional metals and alloys in weight-conscious applications such as aerospace due to their superior strength-to-weight properties. However, these materials are vulnerable to complex and difficult-to-predict sub-surface damages. Therefore, damage detection modalities are important for assuring the safety of CFRP-based structures and components. Particularly for in-service structural health monitoring (SHM), it would be desirable to utilize an inherent property of these materials, such as electrical conductivity, as a means of detecting and localizing damages. In this respect, electrical impedance tomography (EIT)is a promising SHM modality. Studies to-date involving the application of EIT to CFRPs are limited due to the challenges associated with high electrical anisotropy. Furthermore, most studies have only considered flat laminates with unrealistic edge-placed electrodes. In this paper, we advance the state of the art by studying the application of EIT to non-planar CFRP geometries using surface-mounted electrodes. Additionally, we present a modified EIT sensitivity matrix formulation which intrinsically incorporates the electrical anisotropy of the material by forming the sensitivity matrix via three approaches - with respect to i)a scalar multiple of the conductivity tensor, ii)the in-plane conductivity, and iii)the through-thickness conductivity. It was found that both through-hole and impact damages can be adeptly identified with a combination of surface-mounted electrodes and a sensitivity matrix formed with respect to either a scalar multiple of the conductivity tensor or the in-plane conductivity. The results presented here are an important step towards the transition of EIT-based diagnostics to actual CFRP structures.