Nonlinear Transfer of Ultrasound by Adhesive Joints - a Theoretical Description

Quality assessment of bond interface in composite materials in an important task in nondestructive material testing. Techniques to characterize or even measure the bond strength are required. In general, binding forces are nonlinear and cause a nonlinear modulation of transmitted or reflected ultrasonic waves. Therefore, the generated higher harmonics of an incident monochromatic wave will give informations about the quality of an adhesive bond. This paper presents an ultrasonic measurement procedure that exploits the nonlinear transfer of ultrasound to distinguish between strong and weak bonds. The theoretical fundamentals of this technique are described; a measure of the bond strength of an adhesive joint and a calibration procedure are derived. Experimental results are shown, and limitations of the method and modifications to eliminate nonlinear error effects are discussed