[3A1] Application of eddy currents for inspection of carbon fibre composites
R Gomes¹, E Mohseni¹, S Pierce¹, K Burnham¹, C MacLeod¹, V Tunukovic¹, E Foster¹, T O’Hare² and G Munro³
¹University of Strathclyde, UK
²Spirit AeroSystems, Belfast, UK
³Spirit AeroSystems, Glasgow, UK
Carbon fibre-reinforced plastics (CFRPs) have diverse industrial applications due to their unique mechanical and structural properties. The manufacturing cycle of CFRPs can be summarised in three stages: preforming, moulding and post-cure. During the preforming stage of the composites, where there is cutting, handling and lay-up of carbon fibre fabrics, defects such as fibre waviness, missing bundles and in-plane waviness can occur. These defects are usually detected when the component is inspected after the post-cure stage. Hence, there is a need to inspect these components before the resin is infused into the dry lay-up. Currently, there are no standardised non-destructive evaluation (NDE) protocols for the inspection of these dry fabrics and preforms in the aerospace manufacturing industry. This study investigates the inspection of dry carbon fabrics (DCFs) for fibre orientation, density and defects such as missing fibre bundles, in- and out-of-plane fibre waviness, before the resin infusion manufacturing stage, using eddy current testing (ECT).
Initial experiments were conducted to test the penetration depth of eddy currents in DCF. A sample was built using biaxial fibre cloth with fibre orientation at 0° and 90°. Six layers were used where layers 2, 3, 4 and 5 had a strip of aluminium foil to detect the penetration depth of eddy currents through the sample. A total of four strips were used within the sample. The inspection was carried out at frequencies of 500 kHz and 800 kHz using an eddy current array probe attached to a KUKA robotic arm. Data was gathered in absolute mode for pairs of transmit-receive coils in two transversal and axial topologies. The scans displayed all four strips, indicating that the eddy current had penetrated through all six layers at both test frequencies. To identify the sensitivity to internal defects, a second experiment was conducted. The inspection sample was made by stacking ten sheets of DCF with a piece of preformed carbon fibre to induce fibre waviness. Initial results show that the waviness can be detected at 500 kHz with a strong accuracy in every repetition of the scans. The orientation of the fibres could not be detected at this frequency.
To conclude, initial experiments were conducted on dry carbon fibre fabrics using eddy current testing to detect fibre waviness and the penetration depth of eddy currents. The results show an indication of fibre waviness in a ten-layer sample at 500 kHz in every repetition of the scans, although the orientation of the fibres could not be detected at this frequency.
Initial experiments were conducted to test the penetration depth of eddy currents in DCF. A sample was built using biaxial fibre cloth with fibre orientation at 0° and 90°. Six layers were used where layers 2, 3, 4 and 5 had a strip of aluminium foil to detect the penetration depth of eddy currents through the sample. A total of four strips were used within the sample. The inspection was carried out at frequencies of 500 kHz and 800 kHz using an eddy current array probe attached to a KUKA robotic arm. Data was gathered in absolute mode for pairs of transmit-receive coils in two transversal and axial topologies. The scans displayed all four strips, indicating that the eddy current had penetrated through all six layers at both test frequencies. To identify the sensitivity to internal defects, a second experiment was conducted. The inspection sample was made by stacking ten sheets of DCF with a piece of preformed carbon fibre to induce fibre waviness. Initial results show that the waviness can be detected at 500 kHz with a strong accuracy in every repetition of the scans. The orientation of the fibres could not be detected at this frequency.
To conclude, initial experiments were conducted on dry carbon fibre fabrics using eddy current testing to detect fibre waviness and the penetration depth of eddy currents. The results show an indication of fibre waviness in a ten-layer sample at 500 kHz in every repetition of the scans, although the orientation of the fibres could not be detected at this frequency.
