Application of the distributed point source method to rough surface scattering and ultrasonic wall thickness measurement.

The distributed point source method is commonly used to predict the complex acoustic field emitted by ultrasonic transducers. In this paper, it is presented as an alternative to conventional approaches often used when solving rough surface scattering problems. Surface shadowing and multiple scattering effects are inherently included in the mesh-free semi-analytical simulation method through matrix manipulation making it very efficient and simple to implement. Results are presented which illustrate the improvement in accuracy gained over the Kirchhoff approximation and the decrease in computational load over the finite element method, culminating in greater than an order of magnitude decrease in required simulation time. The method is applied to the practical problem of online wall thickness monitoring within corrosive environments, illustrating the variability in reflected pulse shape that could be expected from rough surfaces with similar statistics. Three commonly implemented time-of-flight algorithms are used to analyze a large number of simulated signals from which it is concluded that those based on first arrival time are more stable under increasing roughness conditions than those which are based on reflected pulse shape.

Energy concentration at the center of large aspect ratio rectangular waveguides at high frequencies.

Waveguides in non-destructive evaluation (NDE) applications are commonly of a regular geometry (e.g., circular and ring cross section) for which analytical solutions exist. In this paper, wave propagation in infinitely long strips of large rectangular aspect ratio is discussed. Due to the finite width of strips, a large number of modes exist within the structure. This complicates the analysis and usually discourages the use of strip waveguides in NDE sensors. However, it is shown that among the many modes of a strip, there are some with very desirable properties. This is highlighted by the example of two guided wave modes of a large aspect ratio rectangular strip whose dispersion characteristics approach those of the fundamental modes of an infinitely wide plate at high frequencies. The energy of these modes concentrates in the central region of the strip and decays toward the edges so that the strip waveguide can easily be mechanically attached to other components without influencing the wave propagation. Dispersion curves and mode shapes were derived by using a semianalytical finite element technique and are presented over a range of frequencies. It is shown that selective excitation of both modes is possible in practice and the experimental setup is described.