Postgraduate research opportunities Automated Eddy Currents Inspection of Dry Carbon Fibre/Fabric for Composite Manufacturing and Recycling

The world's largest industrial countries' climate change commitments have given rise to a new urge for a green industrial revolution where lighter engineering structures such as composites are needed to reduce Co2 emissions and carbon footprint. Finding tremendous application cases in the fast-growing industrial sectors such as aerospace, automotive, and construction, has created a remarkable growth rate prediction for the composite market size, which is expected to reach 144.5 billion USD by 2028. The market is expected to expand at a Compound Annual Growth Rate (CAGR) of 6.6% from 2021 to 2028. A large part of the market (about 70%), particularly for Carbon Fiber Reinforced Polymer (CFRP) composites, is predominantly driven by the aerospace sector led by Boeing, Airbus, and defense. The UK has become one of the key players in the composite market with an estimated market value of $5.29bn in 2019 where 1,500 companies are involved in the composite industry. Given this outstanding market share, intensive research investment has been dedicated to the production and recycling of CFRPs. Despite the very attractive fuel efficiency, superior mechanical properties, and corrosion properties of the lightweight CFRP components, their manufacturing process is often energy intensive and there is no clear waste management/recycling plan for the aerospace off-cuts, scrappage, and the products at the end of their life.

From the 110,000 tonnes of composites produced in the UK each year, currently, only 15% will be reused or recycled at the end of their life and the rest are transferred to ever-growing landfills. Although recycling CFRP products use only 20% of the energy required to produce them in the first place, the recycled fibre resembles rough wire wool in terms of random alignment and lacks the initial tensile strength. However, this wool-like material can be processed further to closely achieve the appearance and strength of the first-hand fibres. Lightweight Manufacturing Centre (LMC), as a specialist technology centre within the National Manufacturing Institute of Scotland (NMIS), led by Prof. Ian Bomphary, has been leading a Sustainable Composite project to address this issue. NMIS along with National Composite Centre are the only two institutions in the UK pioneering recycling [5]. As promising as the initiative may sound, they don't still have any Non-Destructive Testing (NDT) methods to evaluate the quality of reprocessed fibers, yarns, and woven CFRP mats rigorously.

Moreover, Spirit AeroSystems, as the largest aerospace CFRP manufacturer in the world and a partner of the Centre for Ultrasonic Engineering (CUE) through the RAE/Spirit Chair led by Prof. Gareth Pierce, currently doesn't have any NDT method for inspection of the dry CFRP sheets/plies before the Resin Transfer Molding (RTM) process at their Belfast, Prestwick, and Wichita manufacturing plants. The NDT inspections are only conducted post-process on cured components when the process is not reversible anymore leading to significant material waste and costs associated with repairs. Missing fibre bundles, fibre misorientation, low volume fraction, in-plane waviness, and out-of-plane wrinkles are common ply defects that can be detected and remedied at the early stages of cutting and preforming before resin curing the components.  

Project Scope

While the inspection of dry fibres is almost unfeasible by most of the well-established NDT methods such as a) ultrasound testing due to the lack of coupling medium and strenuous results interpretation, b) X-rays due to safety requirements and concerns on the shop floor, and low resolution and angle-dependent results, c) thermography due to the lack of a monolithic structure and anisotropy of fiber sheets, the highly conductive woven dry carbon fibre plies are formed by a network of fiber yarns with directionally dependent electrical conductivity. This provides a unique opportunity to deploy electromagnetic NDT methods such as Eddy Current (EC) testing which neither require coupling medium nor contact with the test subject. Eddy current testing is a well-established testing method providing a superior detection sensitivity for surface and near-surface defects. The testing method reliability has even grown further with the development and introduction of new EC array technologies in the market. The EC arrays benefit from several coils ordered in rows, which provides a wider inspection coverage for the test subject. They can also use elaborate element excitation sequencing exciting electromagnetic fields in different directions to maximize the resolution and the inspection sensitivity to defects that are oriented differently and located at a range of depths beneath the surface. These probes are also manufactured in form of flexible sensors that can conform to geometric complexities. These flexible sensors are more stable than conventional pencil probes meaning that they produce less lift-off and tilt noise as compared to their predecessors. This makes them greatly suitable for automation, where the sensor can be delivered and manipulated on complex dry carbon fibre stacks (biaxial, triaxial, preforms, etc) using industrial robots.

Aims

Currently, dry fiber inspection research/technology using EC is very limited and seldom commercialized in the industry as the potential has not been extensively explored. However, the non-contact nature, sensitivity to electrically conductive fibre sheets, and the rich and broad range of information acquired by the Eddy currents' wide frequency spectrum could unlock unique characterization capabilities that can underpin the inspection before RTM process and the newly recycled/reprocessed fibres for their thickness, density, and quality consistency. EC reading, mostly influenced by fiber conductivity, can provide valuable information on fiber characteristics such as gaps, undulations, wrinkles, orientations, and rapture. The overarching project aims are to leverage dry fibre inspection capabilities EC to a) prevent the entry of defective fibre fabrics to the process and save cost on scrappage and prolonged repair undertakings after RTM and curing, and b) Inspect the recycled fiber quality for thickness, density, and continuity before reprocessing them into woven sheets and reusing them for manufacturing. As an RAE/Spirit lecturer in sensor development, my research has been particularly focused on the development and promotion of novel electromagnetic sensor technologies with high inspection reliability, which has been previously missing from CUE and UoS, to build on the past pioneering successful automated NDT developments for inspection of CFRPs achieved through several projects with Spirit AeroSystems.

To this end, model-based studies will be used for sensor design optimization for EC to balance the trade-off between the penetration depth and the sensitivity for the best inspection results. Different EC coil designs and layouts in form of arrays will be investigated to ensure the required sensitivity to different desired properties of dry fibres and woven fabrics. Therefore, as the core novelty, this project seeks to develop and robotically deploy a novel automation architecture, and EC sensor technology to yield superior inspection results with higher reliability and robustness. Therefore, a robotic EC sensor deployment strategy for the inspection of fibres/fabrics will be investigated and implemented, and the inspection data will undergo different signal processing stages and will be fused for different frequency channels via an intelligent inference system for defect detection and characterization to gain a more comprehensive understanding of the inspected component. While the novel EC sensor will significantly enhance the overall sensitivity of the NDE system to a broad range of fibre/fabric defects, the intelligent defect detection, and characterization unit will serve as the dedicated multi-tasking brain of the autonomous NDE system to: a) fuse the EC multi-frequency inspection data, b) render the processed signals in form of denoised images, and c) detect, and classify the defect indications.

The project will make extensive use of the £2.5 million cutting-edge Sensor Enabled Automation & Control Hub (SEARCH) hosting several advanced industrial robots and NDE equipment at the Centre for Ultrasonic Engineering (CUE) at the University of Strathclyde. The student will have access to and will work closely with the Aerospace Innovation Centre (AIC) established by Spirit AeroSystems at their Prestwick manufacturing facility and NMIS facilities in Renfrew.

The student will work within an internationally renowned and growing team of diverse and multi-disciplinary researchers and engineers, physicists, and mathematicians and will receive a full NDE training package through FIND CDT and a university training for working with advanced industrial KUKA robots, different NDE controllers and sensor technologies.