Nonlinear Ultrasound for detectability of Kissing Bonds

Abstract

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 “in-visible“ in conventional 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 adhesives. 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 for experimental locating of the kissing bonds fabricated in the adhesive lap-joints. 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.