Monitoring of the curing process and thermal failure of adhesive-bonded structures based on damping measurement using quantitative electromechanical impedance

Adhesives are essential materials in engineering and their strength directly affects the safety of structures. The curing and environmental deterioration are critical areas of focus in adhesive researches. However, conventional studies on adhesive curing emphasize only on the characteristics of the adhesive itself, while studies on deterioration of adhesive-bonded structure tend to concentrate on detecting disbonds. In this work, we proposed to monitor the curing process and thermal failure of adhesive-bonded-structures based on damping measurement using a quantitative electromechanical impedance (Q-EMI) method. The performances of two adhesives, i.e. epoxy and cyanoacrylate, were studied via bonding a piezoelectric transducer onto one end of an alumina bar and measuring the admittance spectrum of the transducer-adhesive-alumina system. Results show that the damping steadily decreases during the curing process and eventually stabilizes for both adhesive-bonded structures. A significantly higher curing rate is observed at 60 °C compared to that at room temperature. At high temperatures, the damping will increase dramatically, or no peaks can be observed in the admittance spectrum, indicating adhesive failure. The results indicate that the epoxy adhesive can function normally at temperatures up to 120 °C and can endure heating-cooling cycles up to 180 °C, while the cyanoacrylate adhesive can function normally at 140 °C but fails upon cooling. The proposed Q-EMI method is quite powerful in monitoring the performances of bonded structures.