This paper reports the first set of results from ultrasonic measurements for determining the imaging capability of a plate-type ultrasonic waveguide sensor in 200 • C liquid sodium. This 10-m long plate-type waveguide sensor has been developed for viewing objects in opaque liquid sodium coolant for the applications in a sodium-cooled fast reactor (a next generation nuclear reactor). Various imaging capabilities of the waveguide sensor have already been demonstrated in water including ultrasonic beam steering, high resolution C-scan, and so on. However, water and liquid sodium have different acoustic properties and, more importantly, different wetting characteristics with stainless steelthe material for the waveguide sensor. For applications of the developed waveguide sensor in a real reactor environment, this research performs a set of necessary ultrasonic measurements in liquid sodium. The end section of the waveguide sensor which radiates an ultrasonic beam into the liquid sodium is coated with thin beryllium and nickel layers which can significantly improve the ultrasonic beam quality and wetting property of the stainless steel. A liquid sodium facility that consists of a glove box system, a sodium test tank, and an argon purification system has been built. The resolution and beam property are determined from ultrasonic C-scan experiments; a signal-to-noise ratio of over 10 dB and the resulting detection of a 1 mm wide slit can be achieved.

The scattering of sound waves by an empty, cylindrical, elastic shell in a fluid is considered in the resonance regime. A long-standing difficulty in the application of the resonance scattering theory is that an exact expression of the acoustical background has been elusive to find over the years. The exact expression, named the inherent background coefficient, is presented here. The coefficient is uniquely determined by the generalized fluid-loading-parameter and leads to the impenetrable background coefficient in appropriate limit. Numerical calculations show that it describes the background for a shell of arbitrary thickness and material correctly over the entire frequency range.

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This research investigates second harmonic generation in Rayleigh surface waves propagating in 9%Cr ferritic martensitic steel. Previous experimental results show that nonlinear ultrasound is sensitive to certain microstructural changes in materials such as those due to thermal embrittlement and precipitation hardening. This research measures the ultrasonic nonlinearity parameter as an indicator of microstructural changes due to thermal aging in 9%Cr ferritic martensitic steel specimens. The specimens are isothermally aged for different holding periods to induce progressive changes in the microstructure and to obtain different levels of thermal damage. As thermal aging progresses, the existing dislocations are annihilated in the beginning and precipitates are formed; these microstructural evolutions lead to large changes in the measured 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 as a function of propagation distance, and these amplitudes are used to obtain the relative nonlinearity parameter at different aging stages. A possible scenario for the microstructural evolution during thermal aging is proposed based on the results from the nonlinear ultrasonic measurements, scanning electron microscopy (SEM), and Rockwell HRC hardness. These results indicate a clear trend that the measured nonlinearity parameter is sensitive to variations in dislocation and precipitate density, and thus can be useful in tracking microstructural changes in this material during thermal aging.

For the scattering of acoustic waves by an elastic shell, the acoustical background coefficients are inherent in the scattering coefficients. The background coefficients for elastic empty shells, named the inherent background, can be obtained from the zero frequency limit of the modal accelerance in the scattering coefficients for analogous liquid shells. In this work, the concept of obtaining the inherent background is applied to multilayered elastic cylindrical structures. The inherent background manifests itself in the sound scattering by the liquid structures. The scattering S-function and the modal accelerance for the liquid system are determined by considering the incoming and outgoing waves. The accelerance of liquid layers is generalized so that the scattering function can be obtained by the recurrence relation for the accelerances of the adjacent liquid layers. From the zero frequency limit of the generalized accelerance of liquid structures, the constant modal accelerance is extracted and the general expression for the inherent background coefficients is obtained. The background coefficients depend on the densities of the layers and ambient fluid medium, the relative thickness of each layer, and the normal mode number. The acoustical background coefficients for solid cylinders, empty shells, fluid-filled shells, and double-layered shells can be obtained by the appropriate limit of the density ratio and the relative thickness of layers in the generalized inherent background coefficients. The usefulness of the proposed background is demonstrated for several examples of layered structures.

For problems of resonance scattering of acoustic waves from penetrable targets of canonical geometry, a general approach which yields an exact and simple expression, named the inherent background coefficient, for the acoustical background is proposed. By analyzing the effect of the structural damping of targets, it is found that, within each modal surface admittance of targets, including a negligible effect of the structural damping, there are two interacting contributions: a constant contribution and a resonant contribution. The constant contribution, which corresponds to the inherent background coefficient, can be obtained from the zero-frequency limit of an equivalent fluid target. For targets including a significant effect of the structural damping, the inherent background depends on the damping effect. The inherent background coefficients for empty, elastic, spherical shells not including the structural damping effect are shown explicitly. The coefficients are described by a generalization of the fluid-loading parameter. Also, it is analytically and numerically shown that the inherent background undergoes a transition to the rigid or soft background in the appropriate limit, and correctly describes the acoustical background for any shell over all frequencies.

A Lamb wave in a plate with a finite width has both thickness and width modes, whereas only thickness modes exist in an infinitely wide plate. The thickness and width modes are numerously formed in a finite-width plate, and they all have different cut-off frequencies, wave velocities, and wave structures. These different characteristics can be utilized in various applications, but a selective generation method for a particular Lamb wave mode in a finite-width plate has not been sufficiently studied, and only a method using multiple elements has been reported. This paper presents the selective generation of a certain Lamb wave mode in a finite-width plate by an angle-beam excitation method using single or dual wedges. In the proposed generation method, a specially designed wedge with grooves or a patch having insulation layers is employed for partial acoustic insulation of the ultrasonic energy incident into the plate. The feasibility of the proposed method was investigated through finite element method (FEM) simulations for Lamb wave excitation and propagation, and then experimentally demonstrated by the measurement of Lamb wave propagation using a laser scanning vibrometer.