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Non-destructive flaw detection in in-situ carbon fibre reinforced composites using eddy-current

: Heuer, H.; Schulze, M.; Heere, R.

El-Hacha, R. ; International Institute for FRP in Construction -IIFC-:
7th International Conference on FRP Composites in Civil Engineering, CICE 2014. Book of abstracts : Vancover, British Columbia, Canada, August 20 - 22, 2014
Kingston, Ontario: IIFC, 2014
ISBN: 978-1-77136-308-2
International Conference on FRP Composites in Civil Engineering (CICE) <7, 2014, Vancouver>
Fraunhofer IKTS ()

Increasingly, externally applied Carbon Fibre Reinforced Polymers (CFRPs) are used to reinforce structural elements. Quality control measures typically verify that the CFRP installed has the potential to achieve its specified performance. Imperfections during the installation process, and post-installation damage, may degrade in-situ CFRP. Visual inspections, tapping, thermal imaging, or laser scanning vibrometry may detect some of these deficiencies. However, none of those methods can reliably detect kinks and cuts in lower plies of multi-ply CFRPs. The authors are researching the use of mobile high-frequency eddy-current scanners to detect misaligned or discontinuous carbon fibres in coated and un-coated single- and multi-ply CFRPs under site conditions. Practical experiments with a portable scanner developed by Fraunhofer Institute have commenced. Eddy-current (EC) technology is a non-destructive method suitable for characterizing materials by their variations in conductivity and permeability. An alternating current (AC) flowing through an induction (excitation) coil generates a magnetic field. This primary field induces eddy-currents in any conductive specimen placed sufficiently close to the excitation coil. The eddy-currents generate a secondary magnetic field opposing the primary field. Geometrical imperfections in the specimen influence the secondary field, causing a complex impedance shift in a detector (pick-up) coil also situated in close proximity to the specimen. The signal detected by the pick-up coil is then analysed. An important parameter for EC measurements is the frequency of the excitation current. With increasing frequency, the skin effect decreases the penetration depth of the AC signals. This decrease follows an e-function. Thus, a tunable instrument may discriminate between the secondary fields emanating from the different layers of CFRP. Detectors for CFRPs typically require frequencies in the 1 to 50 MHz range.