Nonlinear Acoustic Interaction of Contact Interfaces

Nonlinear acoustic interactions at contact interfaces can be used to characterize defects or imperfect bonds at these interfaces. Most methods used to characterize nonlinear interactions consider only a portion of the nonlinear signature, such as nonlinearities caused by high-order harmonics or sidebands. We describe a signal processing algorithm that can extract three nonlinear indicators related to amplitude attenuation, phase shift, and harmonics. Two aluminum blocks were mounted together to form a contact interface and subjected to normal compressive loading. Experiments were conducted to investigate the nonlinear interactions between the ultrasonic waves and interfaces, with a focus on the influence of excitation levels, applied pressure, and surface roughness. The ultrasonic signals were then processed using the nonlinear feature extraction algorithm. We show that the nonlinear indicators can characterize the contact condition of interfaces, and that their sensitivities to pressure differ for interfaces of different roughness and over different pressure ranges.