Linear vs nonlinear ultrasonic testing of kissing bonds in adhesive joints

Kissing bonds in adhesive joints are precursors to damage and failure in materials and components used in safety-critical industries. They are zero-volume, low-contrast contact defects widely regarded as invisible in con-ventional ultrasonic testing. In this study, the recognition of the kissing bonds is examined in automotive industry-relevant aluminum lap-joints with standard bonding procedures using epoxy-and silicone-based ad-hesives. The protocol to simulate kissing bonds comprised customary surface contaminants PTFE oil and PTFE spray. Preliminary destructive tests revealed brittle fracture of the bonds with typical single-peak stress-strain curves indicating ultimate strength reduction due to adding contaminants. The curves are analyzed by using nonlinear stress-strain relation with the higher-order terms containing the higher-order nonlinearity parameters. It is shown that the lower-strength bonds manifest a high nonlinearity while the high-strength contacts are candidates for a low nonlinearity. Based on that, the nonlinear approach is set side by side with linear ultrasonic testing for experimental locating of the kissing bonds fabricated in the adhesive lap-joints. The sensitivity of the linear ultrasound is shown to be adequate to detect only a substantial bonding force reduction caused by the irregular interface defects in adhesives, while a minor contact softening due to kissing bonds remains undis-tinguishable. On the contrary, the probing of the kissing bonds vibration with nonlinear laser vibrometry reveals dramatic growth of the higher harmonic amplitudes and thus validates highly-sensitive detectabilty of these troublesome defects.

Automated summary

Kissing bonds in adhesive joints are recognized as invisible defects in conventional ultrasonic testing. This study examines the recognition of kissing bonds in automotive industry-relevant aluminum lap-joints using a nonlinear laser vibrometry approach. The results show that the linear ultrasound method is unable to distinguish minor contact softening caused by kissing bonds, while the nonlinear approach can detect these troublesome defects with high sensitivity.