This research uses nonlinear Rayleigh surface waves to characterize stress corrosion cracking (SCC) damage in sensitized 304 Stainless Steel (304 SS). 304 SS is widely used in reactor pressure vessels and fuel pipelines, where a corrosive environment in combination with applied stress due to high internal pressures can cause SCC. SCC poses great risk to these structures as it initiates cracks late in the lifetime and often unexpectedly. The initiated microcracks grow and accumulate very quickly to form macroscopic cracks that lead to material failure. Welds and the nearby heat affected zones (HAZ) in the vessels and pipework are particularly affected by SCC as welding induces sensitization in the material. SCC damage results in microstructural changes such as dislocation movement and microcrack initiation that in the long term lead to reduced structural integrity and material failure. Therefore, the early detection of SCC is crucial to ensure safe operation. It has been shown that the microstructural changes caused by SCC can generate higher harmonic waves when excited harmonically. This research considers different levels of SCC damage induced in samples of sensitized 304 SS by applying stress to a specimen held in a corrosive medium (Sodium Thiosulfate). Nonlinear Rayleigh surface waves are introduced in the material and the fundamental and the second harmonic waves are measured. The nonlinearity parameter that relates the fundamental and the second harmonic amplitudes, is computed to quantify the SCC damage in each sample. The results obtained are used to demonstrate the feasibility of using nonlinear Rayleigh waves to characterize SCC damage.

This research investigates second harmonic generation in Rayleigh surface waves propagating in 9%Cr ferritic martensitic steel. Previous experimental results show that the nonlinearity parameter is sensitive to certain changes in a material's properties such as thermal embrittlement and hardness changes. Therefore, the nonlinearity parameter can be used as an indicator of thermal damage due to changes in dislocation density and precipitations. The specimens are isothermally aged for different holding times to create progressive changes in the microstructure and obtain different levels of thermal aging damage. As aging progresses the dislocation density decreases and precipitations are formed; these microstructural evolutions lead to changes in the nonlinearity parameter β. Nonlinear ultrasonic experiments are conducted for each specimen using a wedge transducer for generation and an air-coupled transducer for detection of Rayleigh surface waves. The amplitudes of the first and second order harmonics are measured at different propagation distances, and these amplitudes are used to obtain the relative nonlinearity parameter for each specimen at different aging stages. Conclusions about microstructural changes are drawn based on the nonlinear Rayleigh surface wave measurement and complementary measurements including scanning electron microscopy (SEM) and Rockwell HRC hardness. The results indicate that the nonlinearity parameter is very sensitive to the dislocation density and precipitate formation, and thus can be used to track the microstructural change in this material during the process of thermal aging.

This research investigates the modeling of randomly distributed surface-breaking microcracks and their effects on higher harmonic generation in Rayleigh surface waves. The modeling is based on micromechanical considerations of rough surface contact. The nonlinear behavior of a single microcrack is described by a hyperelastic effective stress-strain relationship. Finite element simulations of nonlinear wave propagation in a solid with distributed microcracks are performed. The evolution of fundamental and second harmonic amplitudes along the propagation distance is studied and the acoustic nonlinearity parameter is calculated. The results show that the nonlinearity parameter increases with crack density and root mean square roughness of the crack faces. While, for a dilute concentration of microcracks, the increase in acoustic nonlinearity is proportional to the crack density, this is not valid for higher crack densities, as the microcracks start to interact. Finally, it is shown that odd higher harmonic generation in Rayleigh surface waves due to sliding crack faces introduces a friction nonlinearity.

Although several studies have proven the accuracy of using a non-contact, air-coupled receiver in nonlinear ultrasonic (NLU) Rayleigh wave measurements, inconsistent results have been observed when working with narrow specimens. The objectives of this research are first, to develop a 3D numerical finite element (FE) model which predicts nonlinear ultrasonic measurements and second, to apply the validated model on the narrow waveguide to determine causes of the previously observed experimental issues. The commercial FE-solver ABAQUS is used to perform these simulations. Constitutive law and excitation source properties are adjusted to match experiments conducted, considering inherent effects of the non-contact detection, such as frequency dependent pressure wave attenuation and signal averaging. Comparison of “infinite” and narrow width simulations outlines various influences which impair the nonlinear Rayleigh wave measurements. When the wave expansion is restricted, amplitudes of the fundamental and second harmonic components decrease more significantly and the Rayleigh wavefronts show an oscillating interaction with the boundary. Because of the air-coupled receiver’s finite width, it is sensitive to these edge effects which alter the observed signal. Thus, the narrow specimen adversely affects key factors needed for consistent measurement of material nonlinearity with an air-coupled, non-contact receiver.