Defect detection in adhesive joints using the impedance method

Adhesive joints distribute a load over a larger area than mechanical joints, but they are very sensitive to surface treatment, service temperature, humidity and other environmental conditions. The ultrasonic method is well known as a non-destructive approach to evaluate defects in adhesive joints, but it cannot detect joint strength degradation due to surface defects or contaminations. In this paper, we evaluated the defects of adhesive joints using the impedance method, which measures the electrical impedance of the adhesive joint. To increase the electrical conductivity of aluminum-to-aluminum single lap joints, 2wt% of carbon nanotubes were dispersed in the adhesive. The impedances of adhesive joints that were modified with artificial defects were measured by using an LCR meter, and the strengths of the joints were evaluated.