Preliminary tests for the development of new NDT techniques for the quality of adhesive bond assessment

Although the aerospace industry already has experience with adhesive bonding on carbon fibre reinforced polymers (CFRP) technology, a great potential for the manufacturing of high-loaded structures exists. A prerequisite for such an application is that the quality of the adhesive bond can be controlled. Several non-destructive testing (NDT) methods exist for the characterisation of defects like pores, delamination or debonding within adhesive bond. No existing NDT technique however allows a control of the optimal physical and chemical properties of the adherend surface yet, which is believed to be decisive in the mechanical performance of the adhesive bond. This lack is an obstacle to a wider application of adhesive bonding technology. Therefore, to ensure the optimal properties of the adhesive bond, that are depending on the adhesive and the adherend surface properties, "new" NDT methods for the characterisation of the adherend CFRP surface need to be developed. This paper covers some recent research activities led in this field within Fraunhofer and Airbus Operations GmbH. Preliminary test programs were launched in accordance with Airbus standards in order to reveal the potential of a few pre-selected "new" NDT technologies. They involved also comparison between conventional laboratory measurements and mechanical destructive tests for the correlation of the contamination and damage effects on the CFRP. Tests have been performed on 3 different applications scenarios: moisture uptake in CFRP, thermal damage in CFRP, Skydrol/Water contamination in CFRP. First results demonstrated that portable X-Ray Fluorescence (XRF), Laser-induced Breakdown Spectroscopy (LIBS) were able to detect traces of Skydrol contamination while a portable Infrared Spectrometer was successfully implemented in all experimental cases, from the systematic detection of heat damaged composites to different moisture contents. Interesting results were also obtained from other technologies such as active thermography. This approach relying on the characterisation of properties of the composite material has shown interesting possibilities so that further development of each technology is running and shall provide even better results in the future