D. E. Chimenti

Leaky Lamb waves in fibrous composite laminates

Results of experimental measurements and theoretical calculations on ultrasonic leaky Lamb‐wave propagation in fiber‐reinforced, unidirectional composite laminates are presented. With the Lamb wave vector oriented parallel to the fiber direction, dispersion curves of phase velocity versus frequency and plate thickness have been constructed from measurements of ultrasonic reflection from fluid‐loaded composite plates. The experimental results are supported by a theoretical model of Lamb‐wave propagation in the composite plate. The model begins with an approximate calculation of the effective, homogeneous, transversely isotropic elastic behavior of a unidirectional composite laminate in the long‐wavelength limit, using a two‐step procedure based on alternating layered media. This intermediate continuum result is then incorporated into a calculation of the ultrasonic reflection coefficient of a fluid‐loaded anisotropic plate, which is assumed to approximate the fibrous composite laminate. Good quantitative a...

Propagation of guided waves in fluid‐coupled plates of fiber‐reinforced composite

Guided wave propagation in fluid‐coupled plates of fiber‐reinforced composites has been investigated by studying ultrasonic reflection in these structures. From measurements of ultrasonic reflection on unidirectional graphite‐epoxy plates over a range of incident angles, experimental dispersion curves using a total‐transmission criterion for leaky plate waves have been constructed. Theoretical calculations with no adjustable parameters demonstrate excellent agreement with the experimental data. Unusual behavior observed in the fundamental total‐transmission curve led to a reexamination of the mode identification criteria. Both the total‐transmission curves and the normal modes of the fluid‐coupled plate differ significantly from the result expected on the basis of the widely used Cremer coincidence condition to identify propagating plate waves. It is found that these differences are particularly pronounced in cases where the ratio of fluid to solid densities is high, as for composite materials. The existe...

Ultrasonic reflection and guided wave propagation in biaxially laminated composite plates

This paper presents the results of numerous ultrasonic reflection measurements on plates of laminated composites in which the angle of incidence of a finite‐aperture acoustic wave and its frequency have been varied. These experiments have been carried out on biaxially laminated graphite–epoxy specimens utilizing water as a fluid‐coupling medium. The stacking sequence of the individual laminae is restricted such that in each layer a principal material axis lies in the incident plane of the acoustic wave. The data are compared to the results of a recent theoretical analysis based on an analytical treatment of wave propagation in orthotropic media in conjunction with a transfer matrix approach. Reflection and transmission coefficients are derived from which the characteristic behavior of the system is identified. Results are presented both as reflection spectra and dispersion curves. In general, very good agreement is found between prediction and experiment. Moreover, significant changes in the reflection sp...

Elastic wave propagation in fluid‐loaded multiaxial anisotropic media

Theoretical investigations supported by extensive experimental comparisons are carried out on the interaction of ultrasonic waves with multilayered media. It is assumed that each constituent of the plate can possess up to as low as monoclinic symmetry. The plate is assumed to be immersed in a fluid and subjected to incident acoustic waves at arbitrary angles from the normal as well as at arbitrary azimuthal angles. Reflection and transmission coefficients are derived from which all characteristic behavior of the system is identified. Solutions are obtained for the individual layers that relate the field variables at the upper and lower layer surfaces. The response of the total plate proceeds by satisfying appropriate interfacial conditions across the layers.

Interaction of acoustic beams with fluid‐loaded elastic structures

This article describes experimental and theoretical results of acoustic beam reflection from curved and planar solid media subject to fluid loading, with emphasis on parametric studies of wave phenomena ascribable to curved and planar fluid–solid interfaces. Measurements on aluminum and stainless steel, in three geometries and two sizes, over many values of incident angle and angular or linear coordinate, have been performed in the extensive experimental program. Simultaneously, theoretical modeling of the acoustic beam–structure interaction has been carried out via a complex source point algorithm that accommodates wave–solid interactions for arbitrarily collimated acoustic sheet beams. The resulting scattering problem is solved in terms of rigorous spectral integrals that are approximated by high‐frequency uniform asymptotics. The expressions obtained in this manner for the reflected field contain the interacting specularly reflected beam and leaky wave contributions, which establish the physical basis ...

Ultrasonic leaky waves in a solid plate separating a fluid and vacuum

The reflection of a finite acoustic beam from a solid plate separating fluid and vacuum half‐spaces has been measured and analyzed. Material viscous damping has been included in the reflection model and the finite beam has been approximated by an integral of the plane wave reflection coefficient times the spatial beam transform over the incident wave vector. The effects of viscosity and the finite beam on the reflected field are separately demonstrated, and generally good agreement is found between the model calculation and experiments on an aluminum plate with water chosen for the fluid. It is shown that the finite beam alone modified the plane‐wave reflection coefficient substantially, whereas the introduction of viscosity exerts a smaller, but still important, influence.

Transducer misalignment effects in beam reflection from elastic structures

In two-transducer, fluid-coupled ultrasonic reflection measurements phase matched to guided modes of elastic solid structures, the effects of misalignment (i.e., differences) between the receiver angle and the incident beam angle on the receiver voltage have been studied. The received voltage is typically due to contributions from the specular reflection and any of the several possible guided wave modes excited by the incident wave field. It is found that misalignment leads to changes in the relative amplitudes of the various contributions. Further, the more highly collimated the beam (or the contributor), the more pronounced are the effects. It is shown that the signal maximum is not a reliable indicator of receiver alignment. These conclusions are based on measurements and on calculations that have been performed at incident angles selected both close to, or far from, phase-matching angles to guided wave modes on plates, curved surfaces, and cylindrical shells. Receiver voltage coordinate scans have bee...

Ultrasonic plate waves in three‐dimensional braided composites

This paper reports on a study of ultrasonic wave propagation in three‐dimensional braided composites; an extensive experimental survey is compared to the results of an approximate analytical model. The model describes the average elastic behavior of composites having continuous fiber weaving in three spatial dimensions. The approximation rests upon the treatment of the composite as a homogeneous medium that possesses the average directional stiffnesses implied by the fiber placement and braid angles. The model is compared to the results of experimental ultrasonic guided wave studies in 3‐D woven AS4/PEEK. This composite has the structure of a three‐dimensional braided fabric infiltrated with thermoplastic polyether ether ketone matrix. Its unit‐cell size dimensions are approximately 8 by 2.5 by 1.2 mm. Some 90% of the fibers are braided in the ±20° directions, with the balance being laid‐in fibers in the warp (0°) direction. It is found that the approximate model yields an acceptably good description of t...

Interaction of Gaussian acoustic beams with plane and cylindrical fluid-loaded elastic structures

Nonspecular reflection effects for ultrasonic beams incident from a fluid onto solid surfaces have been studied continuously since the early investigations by Schoch [1]. He calculated the reflected held for both the fluid-loaded halfspace and the plate using a series expansion for the phase of the reflection coefficient. A more accurate expression for Gaussian beam reflection has been derived by Bertoni and Tamir [2], who approximated the reflection coefficient by leading terms in a Laurent series, performing the resulting integrals analytically. Many researchers have contributed to this literature from the experimental [3–5], theoretical [6,7], and numerical sides [8,9].

Ultrasonic Plate Waves in 3-D Braided Composites

Conventional composite laminates draw their strength from high-modulus fibers placed in a single direction within each of many individual plies comprising a multilayer laminate. The plies are arranged in various directions in the plane of the laminate to tailor the stiffness of the finished part. However, the material is designed so that fibers from different layers do not communicate — or cross each others’ plane. This situation leads to relatively low tensile strength normal to the plane of the plies. In practice, this construction geometry means that the laminate is susceptible to damage in impact, where plies tend to come unbonded. After impact, the overall laminate strength, particularly the in-plane compressional strength, can be seriously impaired.