Industry 4.0 Enabler for CFRP Repairs in Vehicles
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KeywordsPoint Cloud Data Management System Damage Detection Damage Assessment Tool Concept
Increasing production figures of FRP-based cars and the manufacturers’ variety of individual components require a state of the art automated repair processes. Together with 60 industry partners, the RWTH Aachen University is currently working on a project for an integrated repair strategy. It is based on standardised damage detection, a database-oriented damage assessment as well as the individualised production of repair patches conforming to Industry 4.0 guidelines.
Carbon-fibre reinforced plastics (CFRP) offer a significant contribution to an increase in energy efficiency of cars. It follows, that the everyday use of CFRP-part containing cars results in a larger amount of damaged CFRP parts which can’t be repaired by using standard methods common in car body construction. In addition, it is difficult to detect damages in CFRP structures in comparison to metal parts. Damages can negatively affect mechanical properties of CFRP parts without showing any indication on the surface of the parts.
The application of different materials and implementation of CFRP components in modern vehicles results in complex repair procedures. Currently, repairs are done by the cars’ manufacturer and require a high amount of model- and brand-specific competence. They include the replacement of entire components or predefined repair sections, without taking the actual extent of damage into account. Repairs via individual repair patches which fit the precise damage geometry and replace the affected area provide a material-efficient and lower-cost alternative. Therefore, at RWTH Aachen University the Institute of Plastics Processing (IKV) and the six research institutes ika, ISF, IBF, WZL, MMI, FIR as well as 60 industry partners develop new technologies for the detection and assessment of damages, the individualised production of repair patches and the installation of individual repair patches with an overarching quality control in place.
All data obtained within the individual repair steps is saved in a centralised data storage and management system within which a digital twin model of the damaged component is created. The data storage system includes interfaces for a partly-automated data exchange with any technical system used during the individual steps of the repair strategy. Therefore, the data management system needs to be able to import and export various kinds of data formats, including but not limited to sensor images, point clouds, material charts and different geometry exchange formats. Additionally, the data storage system features a damage database which makes an easy and quick damage assessment of already known repair cases possible. In the future, the data management system includes the possibility of adding further functions such as assistance systems for the workers involved in the repair process or virtual test beds for training and simulation-based repair optimisations.
Damaging a Component
Geometry and Damage Detection
A singular visual damage assessment is insufficient for an accurate analysis of the damage and its effect.
Damage Assessment and Repair Method
Tool Design and Manufacturing of FRP Repair Patches
The multitude of possible component geometries corresponds to a large diversity of repair patches necessary to recreate the area of damage as fast and flexible as possible. Such flexibility in geometry cannot be covered economically by conventional tool concepts. This is why a new tool concept for the repair strategy is developed. For the development economic efficiency and speed in the production of different patch geometries are key elements and the tools cavity should form based on the geometry detection and patch design of the damage evaluation. It follows, that the setting of the tools geometry is done by the service provider in charge of patch production who is in possession of the geometry data due to the collective data management system.
During the research project, the development of a flexible tool concept is based on different trials with several trial tools. On its cavity side, the mould surface was formed by use of incremental sheet forming (IBU) and embedded in a mix of resin and graphite for stabilisation and heating. Whereby short cycle times even for repair patches with thermoset resins can be achieved.
For the manufacturing of FRP repair patches with a thermoset matrix the dual-diaphragm forming process (DDF) is used. The DDF machine consists of the assembling, heating and forming station. At the assembling station, an FRP semipreg is placed between two diaphragms and fixated by vacuum pressure. A complete consolidation and preliminary cross-linking inside the heating station follow. The vacuum ensures the removal of any air trapped between laminate and diaphragms, which could otherwise lead to a reduction in part quality. After the heating phase is completed the frame moves to the forming station. A circumferential sealing inside the pressure plate creates a pressure-tight space between mould and frame. The laminate is shaped by the negative mould through overpressure from above and fully cross-linked into a finished repair patch. The patch is then milled to match the structural preparations done to the damaged component and sent to the work shop for the subsequent joining process, Figure 6.
Preparation and Joining Procedure
The choice of a repair method and necessary preparations of the damaged part are based on the previous damage assessment. Since the project considers various joining techniques, for example adhesive bonding via scarfing, mechanical joining, employment of doppler for the repair patches, the individual preparations for the damaged part are stored in the data management system. With regard to the economical capabilities of independent workshops - the project’s main target group — research focuses on hand-operated tools.
In this particular case the preparation was done via continuous scarfing. The large angle in the scarfing area offers a larger bonding surface, Bild 8, for better force transmission in compliance with material requirements. Furthermore, this kind of repair is visually and aerodynamically inconspicuous. For the scarfing process grinding is preffered over milling, due to the lower material removal per time which favours the manual handling and guiding of the tool. Industrial vacuum cleaners protect workers from respirable CFRP particles. Other health and safety measures include protective clothing and the enclosure of working areas. Shortly before applying the adhesive to the surface it needs to be pre-treated with an abrasive fleece, which can easily be done in the workshop.
Following the structure preparations, the geometry detection can be repeated on the altered part of the component. The renewed data supports the manufacturing company of the repair patch in the production of a perfectly fitted patch with all necessary joining surfaces so as to avoid any post processing at the work shop.
▸ high mechanical strength for structural adhesive bonding of FRP
▸ viscosity suitable for vertical joints as well as bridging gaps due to geometrical variances
▸ a reproducible curing process under workshop conditions
▸ embedded glass beads to set a specific bonding gap
▸ suitable for sanding/grinding post process.
Summary and Conclusion
In this article interim results of a research project focusing on a repair strategy under Industry 4.0 guidelines for CFRP based vehicle structures are presented. The repair process is outlined through all individual steps with a demonstrative part in style of a side sill.
It is demonstrated that the innovative approach of a decentralised damage detection and repair in workshops, as well as a centralised, database based damage assessment and automated patch production, is expedient in respect of an efficient repair of CFRP components. Any necessary data exchange between the respective repair steps is ensured by a collective data management system in accordance with Industry 4.0 principles.
The individual aspects of the multisensory-based damage detection create a basis for the standardised and quality-assured repair. The subsequent preparation of the vehicle structure and geometry detection of the prepared joining area and consideration for the patch production make the implementation of repair steps possible within the existing tool environment.
The authors express thanks to the co-authors Philipp Nicolas Wagner, Bernd Marx, Katharina Bethlehem-Eichler, Sarah Ekanayake, Markus Gottschalk, Daniel Losch und Kai Fischer.
The research projects 18757N, 18758N and 26 LN in context of the initiative “Leittechnologien für KMU” of the Forschungsvereinigung Kunststoffverarbeitung is sponsored as part of the “Industrielle Gemeinschaftsforschung und —entwicklung (IGF)” by the German Bundesministerium für Wirtschaft und Energie (BMWi) due to an enactment of the German Bundestag through the AiF. The Authors would like to extend their thanks to all organisations mentioned.