Characterization of surface properties of a circular tube using nonlinear circumferential guided wave

The feasibility of using the nonlinear effect of primary circumferential guided wave (CGW) propagation for characterizing the surface properties in circular tubes has been investigated. This nonlinear approach will be suitable for some practical applications, where ultrasonic transducers cannot touch the surfaces to be inspected. An appropriate mode pair of primary CGW and double-frequency CGW (DFCGW) has been chosen to enable the second harmonic generation (SHG) of primary CGW propagation in the given circular tube to have a strong nonlinearity, i.e., the second-harmonic amplitude can accumulate along with the propagation circumferential angle. Here, the theoretical predictions, finite element simulations, and experimental measurements were conducted to analyze the influence of changes in the surface properties of the circular tube on the effect of SHG of primary CGW propagation, where a surface coating with the different properties was used to simulate changes in the surface properties of a circular tube. When the changes in the areas and thicknesses of surface coatings took place, the amplitude-frequency curves of both the fundamental waves and the second harmonics generated by CGW propagation were measured. The Stress Wave Factors (SWFs) of CGW propagation were introduced to characterize the changes in the surface properties. It was found that changes in the surface properties might clearly affect the efficiency of SHG of CGW propagation. There is a direct correlation between changes in the surface properties and the SWFs of CGW propagation. The consistency of theoretical predictions, finite element simulations, and experimental results validate the feasibility of characterizing changes in the surface properties of the given circular tube using the second-harmonic SWF of CGW propagation.