Even though adhesively bonded composite repairs exhibit significant advantages in terms of mechanical efficiency compared to mechanically fastened ones, they are at the same time extremely sensitive to process parameters variations. Small deviations against the repair specifications (e.g. surface preparation) may lead to disproportionally larger consequences to the final mechanical performance of the repair and, consequently, to the integrity of the structure. Moreover, the inevitable differences between laboratory and repair shop conditions, may lead to the cancellation of promising technological achievements for “practical” reasons, because the required conditions could not be easily met in standard practice.
The scope of this proposal was to examine from a practical application point of view the innovations achieved within GRA Task 1.3.6-02 for bonded composite repairs, in order to facilitate their transfer to production conditions. For this reason, the whole chain of process steps which have to be followed for the performance of a typical repair was executed in standard repair shop conditions, in terms of facilities, equipment, tooling and personnel training. Potential contradictions of innovations developed by the GRA to standard repair practices were identified and alternative solutions were proposed, to facilitate their efficient transfer to production standards and requirements. The project success was guaranteed from the background of the consortium:
- GMI Aero, a bonded composite repair equipment manufacturer and supplier of major airlines and MROs, with over 25 years experience in the development of repair equipment, techniques, advanced solutions and technicians personnel training, and
- NTUA, a leading institute in the research of bonded composite repairs, cooperating with GMI Aero for more than 15 years in the development of advanced bonded repair processes and having participated to several research projects on composite and metal bonding.
EXPECT focused on estimating the potential of vacuum sand blasting as a surface pre-treatment method in composite aircraft repair. Vacuum sand blasting is an abrasive method like grinding, but it may be that it will provide better process reliability than a manual grinding process. Compared to conventional sand blasting, it takes care that the blasted material will not pollute the surroundings.
A surface pre-treatment with a vacuum sand blasting permits to replace the conventional manual grinding, improves bonding strength and permits to remove the eventual surface contamination. Its big advantage in respect to conventional sand blasting methods is the possibility to perform the treatments without the need for any special protective equipment. Thanks to the system design, blasting dust cannot escape even in case of leaks or other defects. The attractiveness to use it in repair process is based on a possibility of solvent and water-free treatment that could be done automatically, without hand contact with a repair zone.
After theoretical analysis and calculation of “process sensitivity factors and constraints”, as derived from an experimental campaign, an assessment of the practical application issues of innovative process was performed within EXPECT. This included the application of all the required procedures (innovative or standard practices) in small scale representative repair cases. According to these results, further evolution of developed solutions and change proposals were recorded and implemented, and a final demonstrator repair was performed.