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GFRP part inspection by robot with terahertz sensor

Glass-fibre-reinforced plastics and renewable natural materials, such as wood and cork, are being used ever more widely. lmaging by means of ultra-short light pulses in the terahertz range is a very promising method for volumetric testing in non-conductive materials. lt has been integrated for the first time as an additional technology in a multimodal ACCUBOT (ACCurate roBOT) robots non-destructive testing (NDT) system from Fill as part of the three-year Automated TeraherTz lmaging of Composites and tooling profiling (ATTlC) project with international involvement run by EUREKA. This article has been published in the JEC Composites Magazine N°153.

GFRP part inspection by robot with terahertz sensor
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Plastics have replaced the materials used previously in many application areas. Above all, fibre-composite materials are very well suited to producing components with precisely defined properties. Thanks to their significantly lower mass in most cases, they also contribute to improving the CO2 balance of the devices and systems made from them.

Versatile material GFRP

Extremely heavily stressed structural components for the automotive and aviation industries are manufactured primarily from carbon-fibre-reinforced plastics in order to reduce their weight. Glass-fibre- reinforced plastics (GFRPs) are just as high quality, but cheaper. With a lower modulus of elasticity, they are less stiff than carbon-fibre-reinforced plastics, offer high breaking elongation values, and enable highly elastic energy absorption. At the same time, they provide good electrical insulation and exhibit excellent corrosion behavior, even in chemically aggressive environments. GFRP has therefore developed into the preferred material for many different products in which one or more of these properties are required. These include leaf springs, profiles, and reinforcements, as well as bathtubs, car body parts, boat hulls and sports equipment. Rotor blades for helicopter sand wind turbines, pipes and tanks for the chemical, pharmaceutical and food industries, as well as control cabinets, circuit boards, and insulators are frequently made of this versatile material. In Europe, more than a million metric tons of GFRP are processed annually.

Automated Terahertz Imaging of Composites and tooling profiling

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Renewable alternatives

The search for sustainable alternatives to fibre-reinforced composite materials is increasingly focusing on renewable natural materials, such as wood and cork. This is the case, for example, in the Austrian CARpenTiER project. The artificial word stands for Computer Aided Research (CAR) and automotive component supplier (TiER), together yielding the joiner (Carpenter). The aim is to develop production technologies for wood-based hybrid designs in automotive engineering, plant manufacturing and machine building. The main focus here is hybrid structures made of laminated wood, plywood or parallel strand lumber, which can be reinforced with natural fibres. Wood-based and particlefoams will also play a significant role in future as a replacement for petroleum based foams in the construction of lightweight components.

Material inspection indispensable

The forming processes are followed mostly by additional machining of the lightweight components. Damage may occur in either of these process steps, the severity of which cannot be assessed by the naked eye. This makes ongoing quality control indispensable. Alongside visual inspection, the various imaging techniques of non-destructive materials testing can be used here to detect such irregularities as delamination, cracks or porosity.

The fl exible, robot-based inspection system enables use of terahertz imaging for fi bre-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 © RECENDT
The flexible, robot-based inspection system enables use of terahertz imaging for fi bre-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 © RECENDT

Between radio and light waves

One promising, relatively young, technique for volumetric testing in non-conductive materials is imaging by means of ultrashort light pulses in the terahertz spectral range, which starts at  approx. 100 GHz and transitions to the infrared range at approx. 10 THz (0.05 mm to 3.0 mm wavelength). This frequency band forms the boundary between radio and infrared light waves. Terahertz waves are a non-ionizing type of radiation, so they are harmless to humans.
While they cannot or barely pass through water, metals, or conductive materials (including carbon-fibre-reinforced plastics), materials such as silicon, glass, and non-polar materials (including paper, textiles, and ceramics) are almost transparent for THz waves. They can pass through plastics, whereby they are hardly absorbed at all by non-polar polymers such as PP, PE or PTFE, but are easily absorbed by polar polymers such as PA.
With THz-Imaging it is possible to perform contactless examinations of GFRP or wooden parts without any special safety precautions. The process allows things such as the resolution of multilayer products, the detection of content and distribution of filler materials, and the detection of fibre direction in fibre-composite materials. In addition, it can be used for examining foams or for determining the weld quality of thermoplastics.

Fill develops and produces automated solutions for non-destructive materials testing on an industrial scale based on high-precision, six-axis, articulated ACCUBOT that can operate both independently and jointly on linear axes running in parallel © raumpixel

Multimodal NDT systems

The globally active machine and plant engineering company Fill Gesellschaft m.b.H., based in Gurten (see Focus), develops and manufactures highly complex production systems for the automotive, aviation, sports, and building industries. These systems also include automated solutions for non-destructive materials testing on an industrial scale.
In addition, the company has developed a solution with high precision, six-axis, articulated robots that can operate both independently and jointly on linear axes running in parallel. The robot kinematics obtain the necessary absolute positioning accuracy for high-performance contactless NDT and metrology applications from high-precision linear axes, designed and produced by Fill, and from additional rotary encoders installed on the drive side on the movement axes. These implants turn standard commercial robots into the ACCUBOT, probably the most accurate robot on the market.

FOCUS
Fill is a leading international ma chine engineering company. With sophisticated high-tech systems and custom manufacturing solutions for metal, plastics and wood, Fill makes its customers the best in their fields. The automotive, aviation, sports and building industries all benefit from Fill’s expertise. Since its foundation in 1966, the company has distinguished itself through enormous innovative strength, strong values and the best jobs. “If you are seeking the best solution, shape your future with Fill” is the guiding principle of the company’s more than 975 employees. “Wir Sind 1! We are one!” is their vision and stands for togetherness with the team, customers, suppliers and partners. The company is 100% family-owned and is managed by Andreas Fill (CEO), Martin Reiter (CSO), Alois Wiesinger (CTO) and Günter Redhammer (COO). In 2022, Fill recorded sales of around €187,6 million.

Many tests without re-clamping

The robot systems can perform inspections with different methods, without re-clamping the device under test, thanks to automatic tool changing. For this purpose, FILL has developed the FlexChange tool changer based on a standard clutch. Its connections include 196 electrical HF contacts for multichannel testing (phased array), which allow insertion of all the currently conceivable inspection heads with complete compatibility. These also include a powered inspection head from Fill with an additional rotatory servo axis at the tool center point, which enables component testing in small, highly contorted areas.
This multimodality saves so much time that a component can, for example, first be inspected by ultrasound in through-transmission technology and then be subjected to extensive phased array pulse-echo tests. Naturally, X-ray, tomography, thermography, and various contactless measuring methods for  geometry measurement can also be integrated, thereby avoiding furthertime-consuming manipulations. The Fill multimodal NDT systems, including robot kinematics, are controlled by a market leading NC controller, which is operated conveniently in the graphics-based Fill Studio environment.

Terahertz testing for industrial use

The possible uses of terahertz imaging for plastic applications have previously been restricted to laboratory operations. ATTIC (Automated TeraherTz Imaging of Composites and tooling profiling) brought a significant step towards industrial suitability. The Austrian participants in this three-year EUREKA project were Fill and the Research Center for Non-Destructive Testing GmbH (RECENDT), while the British partners were Brunel University London, lightweight construction specialist Far-UK Ltd, and engineering company TWI Ltd.

In the course of the three-year ATTIC EUREKA project, Austrian project partners RECENDT and Fill mounted a THz imaging system on a FlexChange quick connector flange, in order to integrate this technology in a robot-based inspection system from Fill © Fill

The aim of ATTIC was to develop a robot-based automated process for the production and subsequent inspection of drill holes in glass-fibre-reinforced composites, as well as for detecting the wear of the drilling tool. Tool measurement data from a high-resolution laser camera is evaluated by means of artificial intelligence and used to adapt the process parameters, such as drilling speed. After drilling, the material around the hole is inspected for any defects by means of THz-Imaging. All processes are intended to run fully automatically.

RECENDT is a specialist for the development of process integrated NDT solutions and has more than 10 years of experience in the field of THz technology. This enables the Austrian project partners to integrate a THz imaging system on a Flex Change quick coupling flange. This enabled a  swift implementation

of this technology in a robotic inspection system from Fill. The flexible system enables use of THz-Imaging for fibre-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.

lmportant step towards product maturity

The results achieved in Fill’s NC Robot Lab were not only convincing, they also aroused the interest of industry partners. “There is still much to do before application in a customer project, says Harald Sehrschön, Research and Development Team Leader at Fill, asking for patience. Nevertheless, THz-Imaging has reached Technology Readiness Level (TRL) 5 as another technology for our multimodal automatic NDT systems.” This allows all project partners to accurately assess the risks that industrial use of this innovative technology involves and to develop the expertise required for addressing the next steps towards product maturity. It is already clear that integration of THz-Imaging in fully-automatic NDT systems would enable a change in methodology in automated non-destructive component testing for many manufacturers.

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