Plastic are an indispensable part of daily life. After having replaced other materials in less complex areas, they are increasingly being filled with fibers and employed for more complex purposes, in the aerospace industry and in automotive or civil engineering. One disadvantage of these composites, however, is their low or non-existent electrical conductivity.
The electrical properties of plastic composites can be significantly improved by using extrinsically conductive plastics. To obtain high functionality when using fiber-filled plastics commercially, it’s helpful to not only know the manufacturing process, but also the fiber content, as well as the local and planar distribution and the alignment of the fibers. The inspection methods currently used for composite materials, such as thermography, x-ray and ultrasound, are highly elaborate and difficult to automate.
As part of a research project sponsored by the German Federation of Industrial Research Associations, the company imq- Ingenieurbetrieb für Materialprüfung, Qualitätssicherung und Schweißtechnik GmbH and the Steinbeis Innovation Center Application- oriented Material-, Production-, and Process-Technology have developed a new non-destructive inspection method using eddy current for characterizing metal-filled plastic composites.
This method measures the effect of the eddy current induced in the material on the test sensor. The plastics filled with metallic fibers cause a change in the eddy current which creates a signal consisting of displacement currents and conduction currents from in and between the fibers. Whereas the conduction currents depend on the fibers themselves as well as their volume and contact to each other, the displacement currents are determined by the dielectric properties of the plastic matrix, as well as the diameter and length of the fibers and distance between them. The displacement currents are seen as capacitive components in the measuring signal.
During the experiment, researchers analysed how measurable these effects are and what information can be derived from them. Initial results showed that this testing method makes it possible to determine the fiber content of a composite, as well as the local and planar distribution and alignment of fibers in long fiber composites. More extensive investigations are currently underway. As a result, it is now possible to detect metal fibers in plastics and even locate metal structures. This is particularly useful in the case of nonvisible components, which result in conditional positioning, and for hybrid structures and mixed structures with metal areas such as inserts and load transmission elements.
The eddy current method can be successfully used on composites to non-destructively determine their material properties or find irregularities. It can be easily automated and is suitable for use alongside the manufacturing process. When combined with analysis of carbon fiber reinforced plastics, the eddy current method is incredibly convenient in investigating and characterizing plastic composites.
Dipl.-Ing. Alexandru Söver
Prof. Dr.-Ing. Lars Frormann
Steinbeis Innovation Center Application-oriented Material-, Production-, and Process-Technology (Zwickau)
Dipl.-Ing. (FH) Christopher Seidel
Dr. rer. nat. Antje Zösch
Dr. sc. techn. Dr. rer. nat. Martin Seidel
imq-Ingenieurbetrieb für Materialprüfung, Qualitätssicherung und Schweißtechnik GmbH (Crimmitschau)