Volume 118, Issue 4, October 2005
Index of content:
- GENERAL LINEAR ACOUSTICS 
118(2005); http://dx.doi.org/10.1121/1.2011149View Description Hide Description
This paper focuses on increasing the accuracy of low-order (four-node quadrilateral) finite elements for the transient analysis of wave propagation. Modified integration rules, originally proposed for time-harmonic problems, provide the basis for the proposed technique. The modified integration rules shift the integration points to locations away from the conventional Gauss or Gauss-Lobatto integration points with the goal of reducing the discretization errors, specifically the dispersion error. Presented here is an extension of the idea to time-dependent analysis using implicit as well as explicit time-stepping schemes. The locations of the stiffness integration points remain unchanged from those in time-harmonic case. On the other hand, the locations of the integration points for the mass matrix depend on the time-stepping scheme and the step size. Furthermore, the central difference method needs to be modified from its conventional form to facilitate fully explicit computation. The superior performance of the proposed algorithms is illustrated with the help of several numerical examples.
Acoustic scattering from a finite cylindrical shell with evenly spaced stiffeners: Experimental investigation118(2005); http://dx.doi.org/10.1121/1.2011148View Description Hide Description
The influence of evenly spaced ribs (internal rings) on the acoustic scattering from a finite cylindrical shell is examined over the dimensionless frequency range (where is the wave number in water and the outer radius of the cylinder). Experimental results, obtained with a monostatic setup, are discussed in the incidence angle/time and incidence angle/frequency domains. The physical phenomena that give rise to highlights in the experimental spectra (Bragg scattering and scattering from Bloch-Floquet waves) are investigated. Fast Fourier Transform (FFT) processing on different segments of time signals allows us to distinguish influences of these phenomena. Further, comparison is made between frequency based results and numerical results provided by, respectively, a theoretical model using the thin shell theory [Tran-Van-Nhieu, J. Acoust. Soc. Am.110, 2858–2866 (2001)] and a simple scattering/interference calculation.
Estimation of radius and thickness of a thin spherical shell in water using the midfrequency enhancement of a short tone burst response118(2005); http://dx.doi.org/10.1121/1.2040027View Description Hide Description
The midfrequency enhancement phenomenon for tone burst backscattering by thin spherical shells in water has been discussed by several investigators. In their works, it is found that the earliest elastictone burst echo is enhanced relative to the specular reflection, and this enhancement is mainly due to the lowest subsonic antisymmetric Lamb wave. In this paper, the tone burst backscattering obtained from the convolution integral of the incident tone burst and the impulse response by a submerged spherical shell is investigated to display the midfrequency enhancement. The modified ray approximations are used to calculate the echo contributions from different Lamb waves. The numerical results show that the ratio of and the dimensionless echo delay have nearly linear relationship with the frequency of greatest enhancement. Based on this property, two linear approximate equations are formulated to evaluate the radius and thickness of a thin spherical shell. A simple method is developed to estimate the frequency of greatest enhancement and the corresponding echo delay from a short tone burst echo with a higher carrier frequency. The evaluated results show that the present method is effective on determination of the radius and thickness of a thin spherical shell in water.
118(2005); http://dx.doi.org/10.1121/1.2036147View Description Hide Description
We present an analytical-numerical method to simulate time-harmonic ultrasonic scattering from nonhomogeneous adhesive defects in anisotropicelastic laminates. To that end, we combine the quasistatic approximation (QSA) with a very high-order (tens or hundreds of terms) regular perturbation series to allow modeling of nonuniform interfacial flaws. To evaluate each term in the perturbation series, we use a recursive algorithm based on the invariant imbedding method. It is applicable to solve wave propagation problems in arbitrarily anisotropic layered plates and it is stable for high frequencies. We demonstrate examples of convergence and divergence of the perturbation series, and validate the method against the exact solution of plane wave reflection from a layered plate immersed in water. We present a further example of scattering of a Gaussian beam by an inhomogeneous interfacial flaw in the layered plate. We discuss how results of our simulations can be used to indicate the frequencies and angles of incidence where scattering from potential defects is strongest. These parameters, presumably, offer the best potential for flaw characterization.
118(2005); http://dx.doi.org/10.1121/1.2011807View Description Hide Description
The addition of periodically spaced structures to a cylindrical shell causes energy to propagate at additional wave numbers via a phenomenon known as Brillouin folding. As a result, a load representing one axial mode on a cylindrical shell with circumferentially spaced discontinuities, for example stringers, has distinct energy pass bands and stop bands. Similarly, an excitation representing one circumferential mode on a cylindrical shell with axially spaced discontinuities, for example ribs, also has distinct energy pass bands and stop bands. Discontinuities in both directions lead to energy pass bands when a point force is applied. In this paper, periodically spaced point masses in the axial and circumferential directions are added to a submerged, ribbed, finite cylindrical shell and analyzed via the finite element method. It is seen that Brillouin folding occurs and, in certain circumstances, the energy is split into pass bands and stop bands. For the frequency and circumferential modes where the subsonic energy is most narrowly focused in the axisymmetric shell, a distinct peak in radiated power occurs for the shell with point masses. The energy in this peak is about the same for several examples of different amounts of nonaxisymmetric mass.