Quality assurance concepts for adhesive bonding of composite aircraft structures – characterisation of adherent surfaces by extended NDT

In order to ensure the performance of adhesively joined load-critical composite structures, suitable technologies are needed to steadily monitor adherent surfaces prior to bonding and to detect adhesion properties of bonded components. A novel class of non-destructive testing (NDT) techniques, classified as extended non-destructive testing (ENDT), is required to ascertain selected physicochemical properties which are important for the performance of adhesive bonds in place of detecting material defects like conventional NDT methods do. The European FP7 project, 'ENCOMB - Extended non-destructive testing of composite bonds' aims in the identification, development, adaptation and validation of ENDT methods for characterisation of adherent surfaces and adhesive bond quality. Here, recent NDT techniques such as optically stimulated electron emission (OSEE) and aerosol wetting test (AWT) as well as laser-induced breakdown spectroscopy (LIBS) were advanced and applied in field, and without contacting carbon fibre-reinforced polymer (CFRP) surfaces for detecting different contamination layers such as release agent, moisture or hydraulic oil as well as thermal degradation of CFRP adherent surfaces before performing an adhesive bonding process. Sensitivity and accuracy of these techniques allow distinguishing surface states which are suitable for bonding of CFRP adherents from surface states which are unfavourable for bonding. ENDT using OSEE, AWT and LIBS facilitated the detection of layers of release agent as thin as one nanometre and thin layers resulting from hydraulic oil. OSEE investigations of adherent surfaces before adhesive bonding allowed the indication of all surface states of potential CFRP adherents, which according to previous studies, were related to application scenarios reducing the joint strength of resulting adhesive joints by 20-70%.