The Bracketweld project, led by the CIDAUT and granted by the clean sky programme JTI-CS2-2014-CFP01-LPA-02-03 GAP, number 686611, presented his final results on 28th frebuary 2019.
The general objective of the BRACKETWELD project has been to develop an innovative technology for the rapid assembly of thermoplastic supports so that there is an improvement in the integration of these materials, in particular supports of thermoplastics material as PEI, with the thermosetting composite components currently used in aircraft structures. It also seeks to be a greener, efficient and cost-effective solution than the current ones.
This innovative assembly technology is based on the idea of using the fast and efficient ultrasonic welding technology to assemble thermoplastic brackets to thermosetting composite structural components.
As thermosetting materials cannot be welded, a thermoplastic surface media has to be strongly attached to the thermosetting composite structure by a co-curing process, being this surface media used as an anchor interface for the latterly welding of the thermoplastic brackets by any fusion bonding technique, and especially by the ultrasonic welding.
The key challenge for the development of this innovative technology will be the development of the appropriate surface media that must be compatible with the typical thermoplastic materials used for the injection of brackets (PA, PEEK, PEI,..) while at the same time achieving a very high adhesion strength to the thermosetting composite laminate during the curing of the structural components.
With the objetive to achieve a succesful results, three great phases was developement:
In the first, an innovative test method was developed combining different mechanical and lab tests to analyze the performance of the co-cured welded joints. This test method was used to assess the compatibility and the behaviour of a large series of commericial references of surface medias, and also of in-house developed formulations for surface media.
The second was focused on the validation of the conclusions obtained in the first part of the project . It was necessary implied a redefintion of the welding procedure and parameters. New samples were welded to validate the performance of the welding tool and ensure that the performance of the welded joints was not affected by this change.
And finally,in the third phase, the curved panels were produced, after the tooling design and manufacture, the chosen surface medias were co-cured and welders were used to manufacture a new series of curved panels. These welded joints were analyzed under different condtions (room temperature and hot and wet) and load cases (axial and transversal) comparing the performances of the joints versus those of the previous stage and the different load cases and situations. This analysis has resulted in a series of recommendations for the future market uptake of the project technologies.
The success in the development of the activities proposed in the BRACKETWELD contributes to the development of the rapid assembly process for the integration of brackets and fittings into structural thermosetting composite components leading to significant advantages related to the reduction in weight (associated to the change from metal to thermoplastic brackets), assembly costs and energy reductions compared to the current adhesive bonding methods, since is that there is no need for surface preparation, adhesives, and curing time nor expensive quality control procedures. To illustrate the magnitude of the benefits achieved by the development of this rapid assembly technology, it has to be considered the number of ten thousand brackets used today in the A350-XWB, and this allow us to contributed to the achievement the innovative assembly technology through the evaluation of the performance of the assembly itself in accordance with the specifications of typical aeronautical requirements used in the joined/bonded supports.
Much knowledge has been generated during the project that will enable the technology update in the future, though higher TRL levels call for larger experimental campaigns, and more robust manual welding tool design. This may be exploited not only for the specific application of System-Structure integration in aviation, but also for the general development of the welding capability for thermosetting composites that may be useful in other applications and sectors.
For instance, the laboratory characterization methodologies and the procedures for the elaboration of suitable surface media that have been developed in the project for the specific application of ultrasonic welding of PEI brackets will be also useful for:
- the rapid assembly of thermosetting composite components with suitable surface medias using the welding techniques already industrialized for thermoplastic composites like resistance or induction welding,
- the assembly of thermosetting composites with suitable surface medias to thermoplastic components or thermoplastic composite components,
- the capability to manufacture hybrid components (thermoset composite-thermoplastic) by over-moulding a thermoset composite component having a suitable surface media with a thermoplastic material by injection moulding for an added functionality.
These potential new capabilities for thermosetting composite components will have a positive impact for the reduction of current assembly times and costs, increasing the number of applications of composite lightweight materials in Transport Sectors for reducing vehicle weights, fuel consumptions and emissions. For instance, in the Automotive Sector, where the need for lightweight materials is increasing and the use of composite materials will grow. The development of rapid and reliable joining technologies for thermosetting composite components may boost the current developments related to the use of carbon fibre composites in vehicle structures, since the assembly and repair of these structures is nowadays a handicap compared to the traditionally used steel and aluminium structures.
More information https://www.cidaut.es/bracketweld-project/about/
The aim of the workshop was to provide practical information on the overall functioning of this European initiative to support transnational cooperative R&D on automotive sector (Clean Sky 2), in particular the 6th call for proposals.
Clean Sky is the largest European research programme developing innovative, cutting-edge technology aimed at reducing CO2, gas emissions and noise levels produced by aircraft. Funded by the EU’s Horizon 2020 programme, Clean Sky contributes to strengthening European aero-industry collaboration, global leadership and competitiveness. This 6th Clean Sky call for proposals is the biggest Call ever launched by Clean Sky 2: 74 topics with a funding of 68.8M€. The deadline is 21 June 2017.
In this Workshop, CIDAUT Foundation exposed its experience in Clean sky programme. Inside the aeronautics sector, CIDAUT Foundation has a long tradition working for leading companies in programmes for the development of components and structures for commercial and military airplanes, focusing on those projects that involve the introduction of new materials or processing technologies. Cidaut Foundation´s offer moves around three concepts: Development Engineering, Advanced Engineering, and Prototyping and Manufacturing.
During the workshop, CIDAUT Foundation deepened its experience in some of the most representative projects, which this technological center has carried about, both on its own and in combination with other partners. In particular, the Cidaut’s representative addressed on those projects financed under the Clean Sky programme:
- PLIT Project. Development of advanced Liquid Infusion Technology for regional wing panels structure: numerical simulation of the process and validation through an innovative test bench.
- NEMESIS Project. New trends and market survey for the end of the life of aircrafts. Eco design guideline.
- REMART Project. Recycling of metallic materials from rotocraft transmissions.
- ACID Project. Advanced composite integrated skin panel structural testing.
- Project. Development of an innovative welding process for the rapid assembly of thermoplastic brackets to thermoset-matrix composite structures.
- ACCUBLADE Project. Low cost design approach through simulations and manufacture of new mould concepts for very high tolerance composite components.
- ALMAGIC Project. Aluminium and Magnesium Alloys Green Innovative Coatings.
CIDAUT has recently started the project BRACKETWELD, a 3 years Research & Innovation Action launched under the Platform 2 of the Large Passenger Aircraft IADP program within Clean Sky 2, which is orientated to the development, assessment and selection of integrative concepts to optimize assembly of elementary parts, sub-components and modules in modern aircrafts. The general objective of the BRACKETWELD project is to contribute to the cost-efficient integration of system and aircraft structures by the development of innovative technology for the rapid assembly of thermoplastic brackets to thermosetting composite components like stringers and frames.
Brackets are small fixation elements used as local links between aircraft structure, systems and cabin. The assembly of these elements to structural components made of thermosetting CFRP (Carbon Fibre Reinforced Polymers) is carried out by time-consuming techniques, like adhesive bonding or mechanical fastening, that add significant labour and tooling costs to the whole assembly process. Usually, brackets are made of metals but they could be made by injection moulding of reinforced thermoplastics leading to significant reductions in weight and costs. However, the use of thermoplastic brackets is still very limited because of the increased difficulty of the adhesive bonding process with thermoplastic materials.
Figure 1 Thermoplastic bracket example
The present research initiative aims to cope with the limitations mentioned above, by the development of an innovative technology for the rapid assembly of thermoplastic brackets to thermosetting CFRP parts using ultrasonic welding. Since the thermosetting materials cannot be welded, a layer of thermoplastic material will be co-cured with the CFRP component, being this layer an attachment area for the ultrasonic welding of the thermoplastic bracket. Figure 2 shows the general concept for the assembly of thermoplastic brackets to CFRP components:
Figure 2 Thermoplastic bracket welding concept
The key challenge will be the development of an appropriate surface media compatible with the thermoplastic materials of the brackets while at the same time achieving a very high adhesion to the thermosetting composite during the co-curing process.
During the project CIDAUT will contribute to the development of this innovative assembly technology addressing the following objectives:
- The development of an efficient test method for the quick evaluation of materials compatibility that will be used for the down selection of surface media alternatives.
- The definition, evaluation and selection of the most appropriate surface media for the selected thermoplastic brackets.
- The final validation by the assessment of assembly performances according to the usual requirements of bonded brackets.
The succeed in the development of this innovative assembly technology will mean significant advantages in terms of reductions in weight, costs and energy consumptions, avoiding the current needs for complex surface preparations, expensive adhesives, curing times and simplifying quality control procedures. The magnitude of the potential benefits is large, just considering the number of ten thousand brackets that are used in the A350-XWB.
One of the most challenging areas of research leaded by the Green Rotorcraft Consortium (GRC) within the Clean Sky Program was the development of Active Rotor Technologies as the Active Gurney Flap (AGF) systems, which enable a helicopter to operate with a reduced tip speed of its main rotor whilst preserving the current flight capabilities. The on-going validation of innovative AGF systems by the GRC required the manufacturing and testing in wind tunnel tests of small scale composite model blades before their implementation at full scale. The integration of the AGF systems into the model blades (figure 1) demanded a precise process and tooling design in order to allow an accurate assembly for an efficient performance.
Figure 1 Scheme of rotorcraft model blade with AFG system
Due to the small scale, the dimensional tolerances of the model rotor blade were very tight (less than +/- 0,1mm on the aerodynamic profile). This fact represented a great challenge for process and tooling design. Actually, to fulfil those tight tolerances, not only the mould had to be machined with very high precision means, but also the cavity design had to be designed with special consideration for minimizing any shape distortion induced by the process due to the different thermal and chemical shrinkage of the materials.
In close collaboration with the GRC consortium, CIDAUT contributed with its expertise in composite materials processing, and an innovative tooling design methodology. This methodology was based in process simulations capable of predicting distortions and solving process related issues from the early design stage. The methodology also avoided the need for expensive and long lasting trial and error procedures. Process simulation models developed by CIDAUT in the ACCUBLADE project included thermal and impregnation simulations for the analysis and optimization of critical process related issues, such as shape distortions caused by the different CTE of the materials, temperature gradients, or potential resin flow defects.
Laboratory characterisation tests were carried out with the selected materials in order to determine the required input data for the modelling of the most relevant causes of distortions when processing composite materials, including the warping and spring-in phenomena. Process simulation models were well correlated with experimental results obtained through the processing of flat and C-profile coupons with different layups and curing conditions (figure 2).
Figure 2 Simulation of process induced distortions: warping and spring-in phenomena
Based on the results of process simulations, the design of the suite of tools required for the processing of the model blades was optimized. This included the selection of the optimum tool materials, the definition of the proper alignment and clamping systems, and the integration of an efficient and homogenous heating and cooling system with in-mould sensors. Also, with the aim of evaluating potential improvements in the manufacturing process in terms of quality and costs, the mould was designed and manufactured allowing the evaluation of two alternative processes, both aimed at producing the same net model rotor blade product: compression moulding and SQRTM.
All tools were manufactured by CIDAUT using very fine milling means, and inspected to guarantee the fulfilment of the requirement specifications before putting them at the disposal of the GRC consortium for the processing of the model blades. The first three model blades produced with the tools (figure 3) were used for the validation of process and tooling designs, being subjected to destructive and non-destructive inspection tests including mechanical substantiation tests with fully satisfactory results.
Figure 3 Rotorcraft model blade produced for the integration of AGF systems
Within the scope of the Cleansky Programme, CIDAUT leads a research project aimed at the manufacturing of composite rotorcraft blades with very tight tolerances that will incorporate an Active Gurney Flap mechanism. Active Gurney Flap (AGF) systems enable the rotorcraft to safely operate with reduced tip speeds whilst preserving high performances with reduced fuel consumption and noise; something that constitutes one of the most challenging research areas leaded by the Green Rotorcraft Consortium in the Cleansky Programme: the development of Active Rotor Technologies.
Active Gurney Flap mechanisms scheme.
In the ACCUBLADE project, CIDAUT researches on the development of a robust and very accurate moulding process for the manufacturing of carbon fibre model blades that will be used for validation of the AGF systems through wind tunnel tests. Due to the small scale of the model blades, the dimensional requirements for the manufacturing are very tight, less than +/- 0,1mm on the aerodynamic profile. In order to fulfil the tight tolerances, not only the mould must be machined with high precision means, but also the cavity design must be defined with special consideration for minimizing any distortion during the process.
The analysis of potential process-induced distortions, such as warping or spring-in, has been carried out by means of process simulations based in accurate material parameters identified through laboratory material characterisation tests. The influence of different material and processing parameters including the weave pattern, ply stacking, mould interaction, curing temperature and pressure has been experimentally characterised and correlated with the simulation models. These have been developed for the prediction of the distortions that would appear during the processing of the model blades. Using this methodology, the design of the cavity can be defined to fulfil the required tolerances.
Mould design optimization methodology by means of process simulations.
During the near to follow validation stage, CIDAUT will partake in the processing of the model blades and the non destructive and destructive inspection tests that will be carried out to demonstrate the functionality of the tools and to validate the design methodology.
In particular, the following outcomes from the ACCUBLADE project will stand out among the technological expected results:
- An optimization methodology for tooling and moulds design based in accurate distortion simulations accounting for different thermal coefficients of the composite materials.
- A reduction in current development costs and lead-times for aeronautic composite parts, that usually need expensive and long lasting trial and error procedures to be carried out before a suitable mould cavity design is experimentally determined.