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The new process was developed by the Fraunhofer Institute for High Frequency Physics and Radar Techniques in Wachtberg, Germany, as part of the FiberRadar project. Ruhr University Bochum, FH Aachen University of Applied Sciences and Aeroconcept GmbH, all in Germany, were consortium partners.
During the manufacturing process of glass fibre reinforced structural components, the fibre structure is fixed with a resin matrix. Irregularities in the alignment or the flow of fibre reinforcement can alter structural properties, reducing the quality of the final composite material.
“When producing rotor blades, layers of glass fibres are layered on top of each other in a shell. If this is not done accurately, it can lead to various defects such as undulation. But the fibre can also become twisted and therefore impact the mechanical properties of the component,” said Dr André Froehly, Fraunhofer project manager.
Until now, monitoring has consisted of visual inspections, the team said. It has not been possible to reliably analyse the direction and layering of the fibres before the resin matrix is applied, they said, meaning that defects were only discovered afterwards through methods such as ultrasound examination. This made it “impossible” to control the process chain and resulted in costly rework or sometimes even the scrapping of components, they added.
The new method allows manufacturers to check the alignment of the lower glass fibre layers in a non-destructive and automated way. The process uses a millimetre-wave scanning system consisting of a radar, a fully polarimetric robot and associated imaging software. The system also harnesses the polarisation of the electromagnetic waves, so it can also identify possible defects through a variation in the direction of polarisation.
The robot scans the component, and at each position the radar performs a measurement. These are then combined into a 3D image by the software.
Radar images (co-polarisation) of a fibre layer package with non-visible undulation below the surface (Credit: Fraunhofer FHR/ André Froehly)
While conventional radars only have one channel and therefore use one polarisation for both transmitting and receiving, the new radar sends and receives signals in two polarisations.
“This not only provides high-resolution fibre structure imaging, but also makes it easy to expose any defects in the deeper layers,” the researchers said. “Additionally, refraction compensation improves the quality of the images – it reduces unwanted effects due to refraction, especially in deeper layers. By using radar to scan the individual layers, researchers can also identify any anomalies in fibre orientation and examine the entire material volume in a non-destructive manner.”
The measurement system enables the production of fibre composite materials and the control of the manufactured components with levels of precision that were previously not possible, the researchers claimed.
Using the experience of testing, repair and maintenance firm Aeroconcept, the technology can be integrated directly into the manufacturing and monitoring process for wind turbine blade production.
“We plan to use follow-up projects to further develop the system so that it is ready for use in production processes,” said Dr Froehly. “We are aiming to improve both the speed and the depth resolution to detect even more potential defects in a shorter period of time.”
The project was funded by the European Regional Development Fund.
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