In-line Eddy Current Testing of Carbon Fibre Reinforced Polymers

Abstract

Carbon Fibre Reinforced Plastic (CFRP) materials are of particular value in the aerospace industry due to their superior physical properties, including their high strength-to-weight ratio and corrosion resistance. Among the various methods for manufacturing CFRPs, automated processes are preferred due to higher production rates than are possible via manual methods. Automated Fibre Placement (AFP) is particularly notable for its speed and efficiency, presenting a promising avenue for enhancing CFRP production efficiency. However, the rate at which AFP can be manufactured can be further improved by reducing the time required for repair and inspection. Specifically, in-line inspection during AFP, one of today's most commonly applied automated composite manufacturing methods, remains an unresolved issue; currently, inspection processes rely heavily on visual methods, which are somewhat subjective and limited to detecting only surface defects. This particular - and rather significant - challenge necessitates the adoption of Non-Destructive Evaluation (NDE) techniques tailored to the specific process parameters of AFP for in-line inspection to ensure CFRP quality during production.

This thesis addresses the vital need for NDE inspection methods tailored to the in-line assessment of AFP-based composites, with a particular focus on AFP machine process parameters. To fully exploit the benefits of AFP, this study introduces a directional eddy current (EC) probe designed to address such manufacturing challenges. Experimental studies were conducted on both cured and uncured CFRP materials, including: a) a frequency selection study exploring Near Electrical Resonance Signal Enhancement (NERSE) for each probe configuration to identify the optimal frequency with regard to signal-to-noise ratio (SNR); b) an investigation into the most sensitive probe configuration for detecting targeted defects by varying the aspect ratio of the sensing coils; and c) a sensitivity evaluation of probe configurations by varying the lift-off distance. The Finite Element Method (FEM) was used to complement these studies, providing insight into the induced eddy current (EC) density in the top layer and at the interface within the composite material.

The experimental studies validate the FEM approach by developing and testing various directional transmit and differential receiver (Tx-dRx) sensor configurations for the in-line detection of defects in composite prepreg. Preliminary investigations identified optimal excitation frequencies and evaluated the impact of the relative orientations of driver and pickup coils on wrinkle detection. Further research indicated a correlation between wrinkle height and the SNR for different probe configurations. Given the low conductivity between uncured AFP-based composite layers, which necessitates to generate a more intense primary magnetic flux, an elongated figure 8-shaped transmitter coil is proposed to overcome this challenge. A novel transmitter coil alongside a conventional single transmitter, paired with the most optimal aspect ratio differential receiver coils, was also tested. High-speed inspection up to 1 m/s was conducted using the selected probe for wrinkle detection. The findings indicate that the probe configuration featuring an elongated figure eight-shaped receiver and square-shaped differential pickup coils represent the optimal configuration for detecting common manufacturing flaws, such as out-of-plane wrinkles, in-plane waviness, gaps, and overlaps, during AFP manufacture.

Date of Award	18 Aug 2025
Original language	English
Awarding Institution	University of Bristol
Sponsors	Bolashak International Scholarship
Supervisor	Paul D Wilcox (Supervisor) & Robert R Hughes (Supervisor)