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Defect detection in carbon fiber non-crimp fabrics and CRFP with high-frequency eddy current technique

Fehlererkennung an glatten Kohlenstofffasergeweben und CFRP mittels Hochfrequenzwirbelstrom-Technik
: Bardl, Georg; Nocke, Andreas; Cherif, Chokri; Kupke, Richard; Klein, Marcus; Heuer, Henning

Technische Textilien 57 (2014), Nr.4, S.E142-E143
ISSN: 0323-3243
Fraunhofer IKTS ()
Kohlenstofffaser; Textil; Wirbelstrommessverfahren; Prüfmethode; Textilprüfung; Gewebeeigenschaft

In the production of carbon fibre reinforced plastics (CFRP), the quality assurance along the whole production chain currently poses a major challenge. At the moment, there is no testing technique available that would allow a cost-efficient full-depth inspection of the (multi-layered) textile base materials, the 3-dimensional preforms, and the cured composites. The high-frequency eddy current measurement technique developed at the IKTS-MD Dresden is based on the induction of high frequency eddy currents in the electrically conductive carbon fibres. In order to assess the detectability of typical quality faults both in the textile and in the cured composite, defined defects were introduced into the carbon fibre biaxial non-crimp fabrics (layer orientation ±45° and 0°/90°, yarn count 800 tex), which correspond to typical production defects. After generation of the defects, one group of the non-crimp fabric specimen was analyzed by high frequency eddy current testing, while the other group was further processed to composite sheets by vacuum infusion technique and subsequently analyzed as well. Bright regions in the measurement scans represent areas of high conductivity (high fibre content), whereas dark regions indicate zones of low conductivity (local reduction of fibre content). From the scan images, the correct fibre orientation of the different layers and the presence of gaps, as well as the missing of individual reinforcement ravings can be clearly detected. Since these parameters have a crucial influence on the mechanical properties and the infiltration behaviour, the eddy current technique can be used for the quality control of textile base material. Wrinkles and dry spots in the composite sheets can also be identified due to meet precisely stipulated parameters. It was shown that with high-frequency eddy current technique, quality defects could be detected both in carbon fibre non-crimp fabrics as well as in cured CFRP composite sheets. It was further shown that the detection was possible even in the inner, optically non-visible layers of the CFRP. The results therefore prove the high potential of the high-frequency eddy current technology as a technique for the quality control for the manufacturing of textiles, preforms and composites based on carbon fibres. Further research for the qualification of the novel testing technique is currently underway.