Investigation of Kissing Bonds in Adhesive Joints

Kissing bonds are critical defects in adhesively bonded parts. These defects result in a weak bonding in the interface region between the adherent and the adhesive. Unlike delaminations, kissing bonds induce a delicate change in contact properties. Thus, conventional non-destructive testing methods are not able to detect kissing bonds reliably. In this work various non-destructive testing techniques on kissing bonds were investigated. The manufactured kissing bond containing specimens were investigated with different non-destructive testing methods such as conventional ultrasound, aircoupled ultrasound, x-ray and shearography. Unfortunately, those non-destructive testing methods weren’t able to detect the kissing bonds dependably. On the contrary, the application of nonlinear ultrasound demonstrated the most reliable results in detecting kissing bonds. With this testing setup, the specimens were excited with a piezo shaker in a continuous wave mode and the vibration on the surface of the specimen was detected by laser Doppler vibrometry. After fast Fourier transform (FFT), a dramatic increase in the higher harmonic amplitudes were detected in an area with lower bonding force. The low adhesive forces lead to a higher nonlinearity of the kissing bond area, what results in substantial rise of the higher harmonic amplitudes. The measurements on the kissing bonds with their reduced adhesive force show a substantial rise of higher harmonic amplitudes, which makes it a suitable method for the detection of kissing bonds.