University of Strathclyde

Supervisors: Dr Ehsan Mohseni &  Professor Gareth Pierce 

Carbon Fiber Reinforced Polymer (CFRP) composites formed a large fraction of structural components used in high-value lightweight applications in the aerospace and automotive industries in the past decade. These lightweight components, mainly composed of carbon fibers and resin, can be engineered to offer superior mechanical tensile properties in preferred directions where the application loadings are expected to be the highest. However, due to their intricate multi-stage manufacturing process and lower in-service toughness, as compared to their metallic counterparts, they are prone to both post-manufacturing, and in-service impact-induced discontinuities. Therefore, Non-Destructive Evaluations (NDE) are essential during manufacturing and in-service operation to ensure the quality and integrity of these safety-critical components. 

While ultrasound testing has been dominantly used for inspection CFRPs in the past, owing to its excellent performance for the bulk NDE inspections and detection of delamination defects. However, ultrasound it is not sensitive to all defect types occurring in such components, and it is, in most cases, a contact-based inspection method where a liquid coupling medium is required for the energy transfer. The compound CFRP structure gives rise to the mixed electromagnetic properties where highly conductive fibers are molded in dielectric resin matrix. This provides a unique opportunity to explore the potential of electromagnetic NDE methods such as Eddy Currents (EC) and Capacitance Imaging (CI). The CFRP NDE inspection research using either of these methods is very limited which could be explained by the depth of penetration concerns. However, the non-contact nature, and the rich and broad range of information acquired by these two methods unlocks unique characterization capabilities that can complement and underpin other NDE inspection findings. EC reading, mostly influenced by fiber conductivity, can provide valuable information of fiber texture such as gaps, undulations, wrinkles, orientations, and rapture. The CI, on the other hand, measures electrical permittivity and is mostly impacted by the dielectric properties that can be leveraged to identify indentations, matrix defects, and curing defects such as hot spots and polymer degradation. 

The research aims to use model-based studies for sensor design optimization for EC and CI to balance the trade-off between the penetration depth and the sensitivity for best inspection results. A robotic dual-sensor deployment strategy for inspection of CFRPs will be investigated and implemented, and the data for the two sensors will be fused to gain a more comprehensive NDE understanding of the inspected component.