In CFRP bonded repair applications, it is mandatory to ensure surface cleanliness for achieving robustness and reliability of the bonds. Unfortunately, aircraft surfaces can be affected by several potential contamination sources that may hamper the bond strength; such contaminations have to be detected before a bonded repair takes place. Maintenance and repair operating conditions also require that any contamination detection technique should be operated on a portable basis and measurement results be readily available to the operator.
Artificial olfaction (AO) technologies could represent an optimal framework to develop such a detection tool (i.e. an electronic nose). They are based on the integration of gas sensing devices and pattern recognition (PR) algorithms. Their working principle mimics mammal’s olfaction systems: a gas sensor array, being exposed to a gas mixture, produces a distinct response pattern that can be interpreted by software components to produce qualitative and quantitative estimation on the mixture composition.
Our proposal addressed the investigation of AO technology suitability and the development of an e-nose prototype for surfaces contamination detection to be used in bonded repair of aircraft composite structures. The selection of an ad-hoc sensing array was carried out, investigating the use of several gas sensors based on different operating principles. Their capability to reliably react to volatiles emitted by potential aeronautics contaminants (e.g. skydrol, fuels, etc.) and to exhibit limited drift behaviour was firstly investigated in controlled environment. Once a response database was recorded, a PR component was designed and developed to provide qualitative and quantitative readings. Design and development efforts of a final prototype inspection tool was focused on portability in order to achieve operative capability.
Transport industry accounts for a significant percentage of CO2 emissions. Obtaining a lower fuel per mile ratio will not only help to reduce pollution and greenhouse gas emissions, it will also contribute to the competitiveness of aerospace industries and airlines. As such, Europe is significantly investing in research that pursues the achievement of the so called Green Aircraft industry with the CleanSky joint technical initiative targeting, among others technologies, increased reliance on laminar flows and improved usage of composite material for structural components in aircrafts.
CFRP components assembly is not achieved through riveting, it actually relies on adhesive bonding. A significant increase in the adoption of CFRP technology in primary structures will only be achieved if a reliable quality assurance technology will ensure the mechanical properties of the bonded panels. Up to now, no objective technology has been validated to ensure the quality of bonds and that is having a detrimental effect on the quest for the Green Aircraft. With the ICARO proposal (Infield CFRP surfaces Contamination Assessment by aRtificial Olfaction tool, JTI SP1-JTI-CS-2010-05, GA Number 286786) , ENEA was tackling the challenge primarily targeting pre-bond inspection and specifically the search for surface contaminants in maintenance and repair scenarios. Contamination in fact strongly affects the mechanical properties of adhesive joints assembly lowering the amount of force needed to cause joint failures. In fact, more than 20% (and up to 70%) reduction of interlaminar fracture toughness is expected to occur based on exposure to hydraulic fluids and release agents. A number of different fluids can act as surface contaminant both in the assembly and operative life of the aircraft: hydraulic fluids, moisture, release agents (assembly phase) and de-icing fluids being among the most dangerous.
During the ICARO project, benefitting from the CleanSky contribution, the ENEA electronic nose technology had been studied and adapted for the use as a highly portable, fast contaminant detection and quantification tool and eventually developed in the ICARO e-nose prototype. The combined use of different solid-state sensing technologies as well as pattern recognition methodologies have given birth to a tool that is able to detect surface cleanliness state and purposely quantify the amount of contamination especially in the maintenance scenario. After the scarfing process and before the patch adhesion and curing process, the ICARO e-nose can be used to detect contaminations ensuring the quality of the maintenance patch adhesive bond.
Thanks to the JTI efforts the ICARO prototype had been tested versus the capability to detect hydraulic fluids, moisture and anti-ice agents. Cooperation with SFWA partners and specifically with Fraunhofer IFAM institute (as a task responsible) had allowed developing the tool in a realistic framework thanks to appropriate sample preparation methodologies and focusing versus relevant contaminant and relevant contaminants concentrations in the maintenance scenario. During the project some limitations of artificial olfaction technology had been tackled and successfully solved while a long-term test period allowed to finally fuelling the validation of the use of e-nose technology as a CFRP fast pre-bond inspection tool.
After the execution of the test procedures, the ICARO prototype had shown the ability to obtain performance in excess of 96% correct classification when dealing with a test scenario including the detection of Skydrol 500-B hydraulic fluid and Kilfrost as surface contaminants in the range of 2 grams per m^2.