Quality assurance concepts for adhesive bonding of aircraft composite structures by advanced NDT

Project Information

ComBoNDT

Grant agreement ID: 636494

DOI

10.3030/636494

Project closed

EC signature date 14 April 2015

Start date 1 May 2015

End date 30 April 2018

Funded under

• SOCIETAL CHALLENGES - Smart, Green And Integrated Transport

Total cost

€ 4 617 226,25

EU contribution

€ 4 617 225,75

Coordinated by

FRAUNHOFER GESELLSCHAFT ZUR FORDERUNG DER ANGEWANDTEN FORSCHUNG EV
Germany

Periodic Reporting for period 2 - ComBoNDT (Quality assurance concepts for adhesive bonding of aircraft composite structures by advanced NDT)

Reporting period: 2016-11-01 to 2018-04-30

Summary of the context and overall objectives of the project

The overall objective of ComBoNDT is to develop a quality assurance concept for adhesive bonding of load-critical Carbon Fibre Reinforced Plastics (CFRP) primary aircraft structures, which is applicable within the whole life cycle of the aircraft to overcome the current limitations regarding certification of composites. Therefore, a detailed approach regarding in-line extended non-destructive testing (ENDT) for CFRP materials with increased technological readiness level (TRL), in-process and post-process, will be established.

The work performed and results will lead to both industrial and societal benefits. For the aeronautics industry, it is important to develop reliable testing methods to ensure the bond quality during the entire life of an aircraft; and the certification of (structural) adhesive bonding marks a key step for full integration of CFRP, as adhesive bonding is the optimum CFRP joining technology. Surface assessment techniques which are being developed will allow reduction of time and cost, because they would permit identifying which contaminated areas need to be cleaned. Adhesive bonds assessment techniques which are being examined could lead to reduction of time and cost related to repairs caused by undetected weak bonds. Thus, the downtime of the aircraft will be reduced leading to costs reduction.

In summary, the positive results provide promising expectations with regards to the potential of the developed technologies to enable time and cost efficiency, increased safety and reducing as well production and maintenance, repair and overhaul (MRO) costs. Time savings of up to 70 % and cost savings of up to 50 % in related production, MRO and retrofit processes are expected from the application of ENDT technologies.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

• Definition of test scenarios, sample preparation and overview of sample measurements; it was performed a concise listing of all relevant contaminants identified by the ComBoNDT project partners and specifically those to be examined within the project. Production and repair use-cases to be considered; size of specimen, mechanical tests scenarios and codification system were defined.
• Sample preparation for the test scenarios, sample characterization by reference methods and mechanical testing. The contaminated CFRP samples were prepared, characterized by means of conventional laboratory surface analytical methods regarding their physico-chemical properties and by means of non-destructive testing methods regarding their structural integrity (conventional defect inspection). Mechanical performance of the samples was characterized by mechanical testing. Ultrasound inspection results led to the conclusion that true weak bonds were successfully produced in most of the cases as defects were rarely revealed. Finally, the negative effect of the contamination on the bondline integrity was mainly revealed by the results of the fracture toughness tests. The study performed is a stepping stone towards understanding and expanding the use of adhesive bonds in aerostructures.
• Development and specific adaption of ENDT techniques for quality assurance of surfaces parts. The work performed included the improvement of sensitivities and reliability of detection for each technique. Singly and multiply contaminated coupon samples were subjected to surface analytical investigations. The techniques were then adapted and improved in order to meet the requirements of better quantification and reproducibility in both the repair as well as the production scenarios.
• Development and specific adaption of ENDT techniques for quality assurance of adhesive bondline. Singly and multiply contaminated coupon samples were investigated. The first results for the laser adhesion tests are promising and show that the technology could be used to test and detect weak bonds.
• Validation of ENDT techniques and technological assessment which is an important step towards the certification of the techniques. In the first 9 months the TRL criteria were defined and for each concerned technology the initial assessment took place. The status of each method is monitored and made measurable. The technologies have reached TRL 3 to 4 which was the expected range to start the innovations planned in ComBoNDT.
• Visual identity of the project was created; leaflets and posters elaborated as well the project website (www.combondt.eu) the development of a project website, the elaboration of a plan for dissemination and exploitation.
• ComBoNDT have been disseminated in 16 scientific events, 1 trade fair and 2 open events were addressed to the general public. A series of open-access publications are planned to be realized within 2017. ComBoNDT research and results are being incorporated within academic material and support the development of academic thesis, as several young scientists and engineers are getting involved in the project, through the participating institutions.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The following outputs can be highlighted as steps forward in relation to the current state of the art:

• Definition of concrete fields of application, sample geometries, surface states and relevant contamination types and levels relevant for production and repair use-cases of high relevance and representative for aerospace industry

• For many of the contaminants studied there has not been any previous mechanical characterization in the literature. A fully experimental characterization and explanation is now provided for the effect that various commonly encountered in aeronautics contaminations have on the integrity of the bondline of adhesive composite joints, thus moving a step forward to understanding and expanding the use of adhesive bonds in aerostructures.

• Assessment of surface: techniques which are being developed allow reduction of time and cost, once they would permit identifying which contaminated areas need to be cleaned. The data from the ENDT methods were successfully compared to the results from laboratory based reference characterisation methods and correlated to the degree of surface contaminations.

• Assessment of adhesive bond: New signal processing algorithms are being developed in order to assess the bonds and correlate the response with bond quality. For some cases equipment is also modified and new devices are being developed to obtain efficient tools of bond assessment. If the techniques under development are proven to be effective it will allow to reduce time and cost related to repair caused by undetected weak bond. Moreover, the downtime of the aircraft will be reduced leading to costs reduction.

• Findings of the project may be interesting for other fields. For example in wind turbines adhesive bonding is also used in blades.

• Findings may lead to new research topics for which funding may be sought in future calls on national and international level.

• Gained knowledge and experience can be included in the teaching and training activities conducted at the partner institutions.

• Effective bond assessment tools can lead to wider acceptance (industrial and societal) of adhesive bonding for joining of the composite parts.

• Validation of ENDT techniques and technological assessment contributes to the awareness of how to evaluate the maturity of technologies within the academic and research world to brighten the views of which requirements need to be fulfilled for a successful development of a product to make it mature for the market.

Aerosol-Wetting-Test BoNDTinspect (AWT), one of the techniques applied within ComBoNDT project for q