Taking a look at the inner values: terahertz inspection of fibre

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Spectroscopy by means of ultra-short waves in the terahertz range is a very promising method for volumetric testing in non-conductive materials. It has been integrated for the first time as an additional technology in a multimodal Accubot non-destructive testing (NDT) system from Fill as part of the three-year "Attic" project with international involvement run by Eureka.

Glass-fiber-reinforced plastics (GFRPs) and renewable natural materials, such as wood and cork, are being used ever more widely in industrial lightweight construction. Damage invisible to the naked eye after forming processes and mechanical processing makes continuous quality control of lightweight components indispensable.

The Attic project has focused on the development of a fully automated drilling, monitoring and inspection solution for glass fibre composites. High-resolution laser profilers were used to monitor composite drilling tools to ensure that the level of wear was acceptable prior to each operation. This has been combined with machine learning techniques to optimise tool-processing parameters such as drilling speeds.

The monitoring was included as an automated part of the process, reducing the need for operator interpretation. Once holes had been drilled, inspection of the parent material around the hole was conducted using terahertz imaging.

Reliably and accurately inspecting thick glass fibre composites using a non-contact method is a step change in the NDT sector and a key part of the Attic project.

The project partners are Far-UK, TWI, Brunel University London, Recendt and Fill.

One relatively new technique for volumetric non-destructive testing (NDT) in non-conductive materials is spectroscopy by means of ultra-short waves in the terahertz spectrum (THz-Imaging). This frequency band extends from approx. 100 GHz to 10 THz (0.05 – 3.0 mm wavelength) and forms the boundary between radio and light waves. THz-Imaging enables contactless examinations of GFRP or wooden parts without any special safety precautions. In addition, it can be used for examining foams or for determining the weld quality of thermoplastics.

Alongside production systems for the automotive, aviation, sports, and construction industries, Fill develops and produces automated solutions for non-destructive materials testing. In addition, the company has developed a solution with the high-precision Accubot robots that can operate both independently and jointly on linear axes running in parallel. These systems can perform inspections with different methods. An additional rotatory servo axis at the tool center point also enables component testing in small, highly contorted areas.

The possible uses of terahertz spectroscopy for plastic applications have previously been restricted to laboratory operations. Fill participated in the three-year Attic (Automated TeraherTz Imaging of Composites and tooling profiling) Eureka project to develop a robot-based automated process for the production and subsequent inspection of drill holes in glass-fiber-reinforced composites. This process involves fully-automatic inspection of the material around the drill hole for defects using THz-Imaging. Integration of a THz spectrometer on a Flex Change quick connector flange ensured swift implementation of this technology in a robot-based inspection system from Fill. The flexible system enables use of THz-Imaging for fiber-composite parts, even if these are contorted or freely formed. The same applies to inspection of the internal supporting structures of complex formed parts from additive production.

"In the Attic project, THz-Imaging achieved Technology Readiness Level (TRL) 5 as another technology for our multimodal automatic NDT systems," explains Harald Sehrschön, Research and Development Team Leader at Fill. "Integration of THz-Imaging spectrometry in fully-automatic NDT systems would enable a change in methodology in automated non-destructive component testing for many manufacturers. However, there is still much to do before application in a customer project."

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