Conductivity-Based Damage Detection in Carbon Fiber Composites

Fiber-reinforced polymer (FRP) composites have become a primary structural material in many new structures, particularly in the aerospace, wind turbine, automobile, and marine industries, due to their higher strength-to-weight ratios, corrosion resistance, and ease of manufacturing. However, these composite materials have complex damages modes that are different from typical monolithic metallic alloys, such as delamination, fiber breakage, matrix cracking, and fiber-matrix debonding. These avenues of damage tend to manifest internally to the composite structure, making them nearly invisible to visual inspection. Several damage detection approaches have been introduced for the purpose of in situ non-destructive evaluation (NDE) of composites; however, many of these approaches require complex analysis methods, data interpolation for achieving spatial sensing, and/or embedding invasive sensors into the composites themselves. To allow for widespread implementation of a next-generation NDE approach for composites, an easily discernible, highly visual, and fast approach that does not adversely affect the structural performance of the composite laminate is needed. This study introduces the use of a spatially distributed electrical conductivity distribution mapping method called electrical impedance tomography (EIT). EIT reconstructs a material’s 2D or 3D electrical conductivity within a series of boundary electrodes. A 100 mA current is injected between two opposing electrodes while the adjacent differential voltages are measured at the remaining electrodes; this process is repeated for all opposing electrode pairs. Using a linear reconstruction algorithm, changes in electrical conductivity are spatially resolved and plotted for easy detection, localization, and evaluation of damage. This approach is validated by applying EIT to a set of carbon fiber-reinforced polymer composite laminates. First, damage has been simulated in composite parts by selectively removing portions of the structure and then verifying that EIT has captured this occurrence. After validation of the EIT method, pristine composite laminates have been subjected to low velocity impact damage. Before and after impact EIT readings have been taken. The differential conductivity reconstruction is presented. This work demonstrates the value of adopting electrical impedance tomography for in situ NDE of FRP composites.