The interaction of the low-order antisymmetric (a0) and symmetric (s0) Lamb waves with vertical cracks in aluminum plates is studied. Two types of slots are considered: (a) internal crack symmetrical with respect to the middle plane of the plate and (b) opening crack. The modal decomposition method is used to predict the reflection and transmission coefficients and also the through-thickness displacement fields on both sides of slots of various heights. The model assumes strip plates and cracks, thus considering two-dimensional plane strain conditions. However, mode conversion (a0 into s0 and vice versa) that occurs for single opening cracks is considered. The energy balance is always calculated from the reflection and transmission coefficients, in order to check the validity of the results. These coefficients together with the through-thickness displacement fields are also compared to those predicted using a finite element code widely used in the past for modeling Lamb mode diffraction problems. Experiments are also made for measuring the reflection and transmission coefficients for incident a0 or s0 lamb modes on opening cracks, and compared to the numerical predictions.

This article develops a method for investigating some anisotropic media, such as composites, by the use of ultrasonic waves transmitted through a plate-shaped sample immersed in water. The discussion begins with Christoffel's equations for plane linear anelastic waves under the assumptions that for small angles of incidence the wave modes are plane and inhomogeneous and that the anisotropy is representable by hexagonal symmetry. The water-sample interface is treated using the law of Snell-Descartes for nonabsorbing media and takes into account mode conversion and the generation of acoustic surface waves. The method produces viscoelastic constants and relative attenuation coefficients as a function of the angle of refraction. The experimental measurement apparatus is described and data are given for the 25-layer unidirectional Gr/epoxy composite. Results are presented in terms of slowness, damping vector, and attenuation curves. The results are significant in that they demonstrate the anisotropy both for the elastic stiffness and the attenuation. The method appears to hold promise for characterizing some classes of anisotropic media, including 2-D composites, in terms of their anelastic behavior.

The dispersion curves for guided waves have been of constant interest in the last decades, because they constitute the starting point for NDE ultrasonic applications. This paper presents an evolution of the semianalytical finite element method, and gives examples that illustrate new improvements and their importance for studying the propagation of waves along periodic structures of infinite width. Periodic boundary conditions are in fact used to model the infinite periodicity of the geometry in the direction normal to the direction of propagation. This method allows a complete investigation of the dispersion curves and of displacement/stress fields for guided modes in anisotropic and absorbing periodic structures. Among other examples, that of a grooved aluminum plate is theoretically and experimentally investigated, indicating the presence of specific and original guided modes.

This paper presents an adaptation of the well-known Thomson/Haskell method to introduce anisotropic attenuation into the formulation of the transfer matrix. The expression for the transfer matrix of one elementary ply is modified to take into account the heterogeneity of the generated modes at interfaces between absorbing media. This heterogeneity depends on the direction of propagation and on the anisotropic attenuation of the ply. The transfer matrix of the stratified composite is derived classically from the multiplication of the elementary matrices and leads to the transfer function of the plate. The transmission and reflection coefficients of multilayered viscoelastic anisotropic composite materials can be computed, for any direction of propagation and for any stacking sequence, with a program that requires the viscoelastic characteristics of the ply. The method is tested with PEEK matrix/carbon fibers composites. The viscoelastic properties of the ply were previously measured with a unidirectional composite made of the same elementary ply in a frequency domain where bulk heterogeneous modes can be isolated. To extend these properties to a larger domain, the model assumes that the attenuation is proportional to the frequency. Doing so, the transmission of broadband waveforms can be computed. These waveforms are compared to experimental waveforms transmitted by composites with different stacking sequences.