Early damage detection of metallic plates with one side exposed to water using the second harmonic generation of ultrasonic guided waves

Metallic plates are the main structural components in a wide range of thin-walled structures under extreme operational conditions, such as nuclear cooling pipes, pressure vessels, rocket fuel tanks, and submarine hulls. These structures operate in extreme environments and are subjected to transient and repetitive loads. Real-time health monitoring of these structures is indispensable because they are vulnerable to fatigue and corrosion damage. Second harmonic generation is one of the reliable damage detection approaches to evaluate microstructural evolution and has been successfully applied to characterize initial damage on different structures in gaseous environments. However, there have been very limited studies on the second harmonics generation on the structures submerged in liquid. This paper experimentally and numerically investigates the feasibility of using second harmonic generation to evaluate the material degradation of metallic plates with one side exposed to water. The fundamental leaky symmetric Lamb wave mode (leaky S0) at low frequencies is selected because it has low-attenuation and weakly dispersive features, enabling approximate internal resonance. The experimental results show that the second harmonics of leaky S0 waves grow linearly with the propagation distance. The growth rate of the relative nonlinearity parameter (β′) can be related to the material nonlinearity of the one-side water-immersed plate. In addition, this study proposes a three-dimensional (3D) finite element (FE) model to simulate the generation of second harmonics in the one-side water-immersed plate. The material properties of the plate are modeled by the Murnaghan strain energy function. The Murnaghan constants of aluminum that describe the material nonlinearity at different fatigue levels are obtained from a previous experimental study. The simulation results demonstrate that the values of β′ change significantly with the material properties of the one-side water-immersed plates in the initial stages of fatigue damage. Thus, it can be concluded that the second harmonic generation techniques can be used for early damage detection of metallic plates with one side exposed to water.