The use of electromechanical impedance conductance signatures for detection of weak adhesive bonds of carbon fibre–reinforced polymer

Abstract

The joints between structural elements should ensure safe usage of the structure. One of the joining method is based on adhesive bonding. However, adhesive bonding has not replaced riveting yet. Rivets are still present even in newest composite aircraft AIRBUS 350. The reliability of the adhesive bonding limits the use of adhesive bonding for primary aircraft structures and there is a search for new non-destructive testing tools allowing to (1) assessment of the surfaces before bonding and (2) assessment of the adhesive bond. The performance of adhesive bonds depends on the physico-chemical properties of the bonded surfaces. The contamination leading to weak bonds may have various origin and be caused by contamination (moisture, release agent, hydraulic fluid and fuel) or poor curing of adhesive. In this work, the research is focused on the development of the method for assessment of the adhesive bonds. Bonded carbon fibre–reinforced polymer samples were considered. Electromechanical impedance technique was proposed. The technique is based on electrical impedance measurements of a piezoelectric transducer attached to the investigated structure. The piezoelectric effect causes the electrical response of a piezoelectric transducer to be related to mechanical response of the structure. The indexes for comparison of the conductance spectra were proposed. Three different cases of possible weak bonds were selected for the investigation. The same cases were investigated by destructive methods by other authors. Such approach allows for direct comparison of the obtained results. It was shown that the proposed method allows for clear separation of weak bond cases from the cases for other samples and free sensors. In terms of weak bond assessment, the frequency change with weak bond level (contamination and level of poor curing) was observed. The obtained results are promising and encourage to future research.