Experimental Verification of Contact Acoustic Nonlinearity at Rough Contact Interfaces

When a longitudinal wave passes through a contact interface, second harmonic components are generated due to contact acoustic nonlinearity (CAN). The magnitude of the generated second harmonic is related to the contact state of the interface, of which a model has been developed using linear and nonlinear interfacial stiffness. However, this model has not been sufficiently verified experimentally for the case where the interface has a rough surface. The present study verifies this model through experiments using rough interfaces. To do this, four sets of specimens with different interface roughness values (Ra = 0.179 to 4.524 μm) were tested; one set consists of two Al6061-T6 blocks facing each other. The second harmonic component of the transmitted signal was analyzed while pressing on both sides of the specimen set to change the contact state of the interface. The experimental results showed good agreement with the theoretical prediction on the rough interface. The magnitude of the second harmonic was maximized at a specific contact pressure. As the roughness of the contact surface increased, the second harmonic was maximized at a higher contact pressure. The location of this maximal point was consistent between experiments and theory. In this study, an FEM simulation was conducted in parallel and showed good agreement with the theoretical results. Thus, the developed FEM model allows parametric studies on various states of contact interfaces.

Scattering analysis of nonlinear Lamb waves at delaminations in composite laminates

This study investigates the higher harmonic generation of the Lamb wave at a delamination due to contact acoustic nonlinearity, which is a clapping phenomenon between sublaminate surfaces due to the Lamb wave interaction with the delamination. In this study, higher harmonics of the Lamb wave induced at the delamination in composite laminates are studied in detail. This study performs both numerical and experimental studies. A three-dimensional finite element model is proposed for predicting the propagation of nonlinear Lamb waves in composite laminates and is verified using experimentally measured data. The results show the proposed numerical model can reasonably predict higher harmonic generated by contact acoustic nonlinearity. It is found that the delamination is the major source of contact acoustic nonlinearity in the composite laminates. A mode conversion study is also carried out to gain further physical insight into the higher harmonic generation of the Lamb wave at the delamination.