Eddy current testing in CFRP production
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Eddy current testing has established itself as a nondestructive test method in CFRP production. By exploiting the electrical conductivity of carbon fibers, it is possible to detect fiber orientation, surface weight, area density and fiber volume content as well as ripples and folds in stacks, preforms and composite components. This article presents recent applications and discusses future perspectives for deployment in CFRP production.
Forming Multi-layer Semifinished Products
Nondestructive testing of multi-layer stacks, preforms and composite components for correct fiber orientation and freedom from defects is a recurrent task in CFRP production. The shearing of textile materials that results when flat semifinished products are draped into multi-curved 3-D geometries, changes local fiber orientation and hence the rigidity of the composite components. What is more, areas with extreme shearing also increase fiber volume content and are prone to the formation of folds and dry spots that need to be eliminated through suitable process control. Fiber orientation must be known in particular for calculating rigidity and deformation.
Optical methods allow fiber orientation to be inspected quickly and precisely for single-layer fabrics or the uppermost layer of multi-layer stacks. However, one of the major challenges in process development is that the forming behavior of multi-layer stacks such as textile sheet materials, prepregs or organic sheets is essentially different from the forming behavior of single-layer semifinished products. Owing to the complex friction and glide conditions between the individual layers, defects occur when forming stacks that are not seen when forming single-layer semifinished products. Similarly, shearing and fiber orientation in the individual layers can differ considerably when compared with single-layer forming . A lack of information about the paths of the fibers in the inner layers is therefore generally reflected in additional safety factors and hence in increased component weight and material costs.
Exploiting Electrical Conductivity
Inline and 3-D Testing
The difference to eddy current testing on metals is the higher testing frequency.
Automatic Detection of Thread Paths
Software automatically reconstructs the local orientation of the individual layers.
Figure 8 (center) shows an example of the automatic import of measured yarn directions in a finite element (FE) model for the second layer. On the right, the result of an FE simulation is depicted.
Eddy current testing is currently starting to be deployed in automotive series production, and online and offline eddy current testing systems are in use in a large number of CFRP production processes. This has led to an improved understanding of eddy current distribution in carbon-fiber materials, and will predictably result in the development of specialized sensors with greater resolution and measurement depth, adapted to the fiber structure and anisotropic conductivity of carbon fiber materials. Typical detection depths are currently between 8 and 12 carbon-fiber layers. A recent paper presented sensors that can detect small fiber waves with an amplitude of 3 mm up to the 18th layer . Another foreseeable trend will be the development of sensor arrays to speed up the testing process similar to those familiar from ultrasonic testing. A challenge that still needs to be tackled in this respect is the implementation of flexible, three-dimensional testing heads that adapt to the surface geometry in order to prevent sensor liftoff for curved component.
The method supplies information that was previously difficult to access.
Furthermore, the range of detectable characteristics in CFRPs is far from being exhausted. All effects that have a direct or indirect influence on the conductivity of the fibers, or on the electrical contact between the fibers, fiber bundles and layers, could potentially be detected with eddy current testing. It has already been shown that local burns — so-called hot spots — can be detected , and also that aging and load history can be determined in the eddy current signal . At the same time, it is expected that the use of eddy current testing for process monitoring and quality assurance will further grow. The possibility of being able to detect fiber orientation and defects in the multi-layer structures of three-dimensional preforms and CFRP components in a nondestructive way provides a method that can supply information for design, process development and quality assurance that was previously difficult to access and will conceivably raise the quality of CFRP structures while continuing to reduce raw material cost.
The work on 3-D fiber orientation detection described here is part of the “3-D Fast” project that is funded by the European Regional Development Fund (ERDF) and the Free State of Saxony (funding code 100224749). The authors would like to thank the aforementioned institutions for the provision of financial resources. Parts of the findings represented here are based on work performed by Martin Schulze and Matthias Pooch (Fraunhofer IKTS) and Matthias Hübner and Andreas Nocke (TU Dresden), whom the authors wish to thank for kindly making them available.
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