Review on quality assurance along the CFRP value chain – Non-destructive testing of fabrics, preforms and CFRP by HF radio wave techniques

Abstract

a b s t r a c tEddy current testing is well established for non-destructive testing of electrical conductive materials [1]. The development of radio frequency (RF) eddy current technology with frequency ranges up to 100 MHz made it possible to extend the classical fields of application even towards less conductive materials like CFRP [2][3]( Table 2). It turns out that RF eddy current technology on CFRP generates a growing number of valuable information for comprehensive material diagnostic. Both permittivity and conductivity of CFRP influence the complex impedance measured with RF eddy current devices. The electrical conductivity contains information about fiber texture like orientations, gaps or undulations in a multilayered material. The permittivity characterization influenced by dielectric properties allows the determination of local curing defects on CFRP e.g. hot spots, thermal impacts or polymer degradation. An explanation for that effect is seen in the measurement frequency range and the capacitive structure of the carbon rovings. Using radio wave frequencies for testing, the effect of displacement currents cannot be neglected anymore. The capacitive structures formed by the carbon rovings is supposed to further strengthen the dielectric influences on eddy current measurement signal [3]. This report gives an overview of several realized applications and should be understood as a general introduction of CFRP testing by HF Radio Wave techniques.

Process Induced Defects in Liquid Molding Processes of Composites

“…They range from gaps and overlaps, to inclusions and contaminations, or even errors in following the prescribed injection and/or cure procedures. The prevalent quality control practices in the industry and strict adherence to process specifications significantly limit the occurrence of these types of defects (Heuer et al, 2015;Lukaszewicz et al, 2012;Potter et al, 2008). Depending on the manufacturing process, other types of defects can prove to be much more difficult to eliminate.…”

“…Careful choice of LCM process parameters such as resin viscosity, pressure, and molding temperature can minimize the fiber movement during injection (Lightfoot et al, 2013). However, most fiber misalignment defects encountered in the industry originate from the labor-intensive preform placement into the mold cavity prior to resin injection (Heuer et al, 2015). Therefore, automation has been viewed as a viable option to prevent fiber misalignment in LCM on a large industrial scale.…”

“…For automated mass production facilities based on LCM processes, current focus is on acquiring quality parameters prior to the resin infiltration step (Heuer et al, 2015, Lukaszewicz et al, 2012. When defects like misaligned fiber bundles, fiber undulations, or contaminations are detected, the subsequent LCM process step can be stopped and readjusted.…”

“…This in-time process recalibration and/or repair of the fiber preform helps prevent fiber undulation. Heuer et al (2015) proposed a high frequency eddy current technique for the determination at the dry stage of fiber undulation, misalignments, gap size distribution, and local fiber areal densities. Currently, this technique is being integrated into a fully automated production system of airplane parts (e. g. frames in the fuselage) produced by RTM at the German Aerospace Center (Heuer et al, 2015).…”

“…Recently, significant developments of robotized preform placement allowed LCM processes to be more competitive (Fedulov et al, 2015;Heuer et al, 2015;Lukaszewicz et al, 2012). Currently, LCM variants, such as Resin Transfer Molding (RTM) and Vacuum Assisted Resin Transfer Molding (VARTM), are increasingly used to fabricate structural parts in a wide variety of industrial applications.…”