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Among the various types of guided acoustic waves, acoustic wedge waves are non-diffractive and non-dispersive. Both properties make them susceptible to nonlinear effects. Investigations have recently been focused on effects of second-order nonlinearity in connection with anisotropy. The current status of these investigations is reviewed in the context of earlier work on nonlinear properties of two-dimensional guided acoustic waves, in particular surface waves. The role of weak dispersion, leading to solitary waves, is also discussed. For anti-symmetric flexural wedge waves propagating in isotropic media or in anisotropic media with reflection symmetry with respect to the wedge’s mid-plane, an evolution equation is derived that accounts for an effective third-order nonlinearity of acoustic wedge waves. For the kernel functions occurring in the nonlinear terms of this equation, expressions in terms of overlap integrals with Laguerre functions are provided, which allow for their quantitative numerical evaluation. First numerical results for the efficiency of third-harmonic generation of flexural wedge waves are presented.

Zerstörungsfreie Verfahren zur Messung von Eigenspannungen
erfordern, abhängig vom gewählten Verfahren, die Kenntnis gewisser
Kopplungskonstanten. Im Falle von Ultraschallmessverfahren sind das neben den
elastischen Konstanten zweiter Ordnung (SOEC) vor allem die Konstanten dritter
Ordnung (TOEC). Elastische Konstanten fester, metallischer Bauteile werden in der
Regel in Zugversuchen bestimmt. Zur Ermittlung der TOEC werden diese mit
Ultraschallmessmethoden kombiniert. Durch äußere Einflüsse, wie etwa mechanische
Nachbehandlungen der zu untersuchenden Bauteile können sich diese Konstanten
jedoch ändern und müssen folglich direkt am veränderten Material bestimmt werden.
Mithilfe von Simulationen wird die Ausbreitung der zweiten Harmonischen und
der nichtlinear erzeugten Oberflächenwellen in Wellenmischexperimenten analysiert
und der akustische Nichtlinearitätsparameter (ANP) bzw. der Kopplungsparameter
aus der Amplitudenentwicklung berechnet. Insbesondere wird untersucht, welchen
Einfluss ein gegebenes Tiefenprofil der TOEC auf den ANP hat (Vorwärtsproblem)
und inwiefern sich aus den Messungen des ANP auf ein vorliegendes Tiefenprofil der
TOEC schließen lässt (inverses Problem). Außerdem wird diskutiert, welchen
Einfluss lokale Änderungen der SOEC auf den ANP haben können und wie groß diese
Änderungen sein dürfen, um die TOEC dennoch bestimmen zu können. Die
Untersuchungen hierzu wurden auf der Basis eines 3D-FEM Modells mit zufällig
orientierten Mikrorissen durchgeführt. Die numerischen Rechnungen zeigen dabei
auch eine gute Übereinstimmung mit einem aus der Literatur bekannten und für dieses
Problem erweiterten, analytischen Modell. Neben der rissinduzierten Nichtlinearität
kann bei diesem auch die Gitternichtlinearität berücksichtigt werden.

In the present work, nonlinearities in temperature
compensating (TC) SAW devices are investigated. The materials
used are LiNbO3-rot128YX as the substrate and Copper electrodes covered with a SiO2-layer as the compensating layer.
In order to understand the role of these materials for the
nonlinearities in such acoustic devices, a FEM simulation model
in combination with a perturbation approach [1] is applied.
The nonlinear tensor data of the different materials involved
in TC-SAW devices have been taken from literature, but were
partially modified to fit experimental data by introducing scaling factors. An effective nonlinearity constant is determined
by comparison of nonlinear P-matrix simulations to IMD3
measurements of test filters. By employing these constants in
nonlinear periodic P-matrix simulations a direct comparison to
nonlinear periodic FEM-simulations yields the scaling factors for
the material used. Thus, the contribution of different materials
to the nonlinear behavior of TC-SAW devices is obtained and
the role of metal electrodes is discussed in detail

A theoretical description is given for the propagation of surface acoustic wave pulses in anisotropic elastic media subject to the influence of nonlinearity. On the basis of nonlinear elasticity theory, an evolution equation is presented for the surface slope or the longitudinal surface velocity associated with an acoustic pulse. It contains a non-local nonlinearity, characterized by a kernel that strongly varies from one propagation geometry to another due to the anisotropy of the substrate. It governs pulse shape evolution in homogeneous halfspaces and the shapes of solitary surface pulses that exist in coated substrates. The theory describing nonlinear Rayleigh-type surface acoustic waves is extended in a straightforward way to surface waves that are localized at a one-dimensional acoustic waveguide like elastic wedges.

Nonlinearity can give rise to intermodulation distortions in surface acoustic wave (SAW) devices operating at high input power levels. To understand such undesired effects, a finite element method (FEM) simulation model in combination with a perturbation theory is applied to find out the role of different materials and higher order nonlinear tensor data for the nonlinearities in such acoustic devices. At high power, the SAW devices containing metal, piezoelectric substrate, and temperature compensating (TC) layers are subject to complicated geometrical, material, and other nonlinearities. In this paper, third-order nonlinearities in TC-SAW devices are investigated. The materials used are LiNbO 3 -rot128YX as the substrate and copper electrodes covered with a SiO 2 film as the TC layer. An effective nonlinearity constant for a given system is determined by comparison of nonlinear P-matrix simulations to third-order intermodulation measurements of test filters in a first step. By employing these constants from different systems, i.e., different metallization ratios, in nonlinear periodic P-matrix simulations, a direct comparison to nonlinear periodic FEM-simulations yields scaling factors for the materials used. Thus, the contribution of the different materials to the nonlinear behavior of TC-SAW devices is obtained and the role of metal electrodes, substrate, and TC film are discussed in detail.

For an elastic medium containing a homogeneous distribution of micro-cracks, an effective one-dimensional stress-strain relation has been determined with finite element simulations. In addition to flat micro-cracks, voids were considered that contain a Hertzian contact, which represents an example for micro-cracks with internal structure. The orientation of both types of micro-cracks was fully aligned or, for flat micro-cracks, totally random. For micro-cracks with Hertzian contacts, the case of random orientation was treated in an approximate way. The two types of defects were found to give rise to different degrees of non-analytic behavior of the effective stress-strain relation, which governs the nonlinear propagation of symmetric (S0) Lamb waves in the long-wavelength limit. The presence of flat micro-cracks causes even harmonics to grow linearly with propagation distance with amplitudes proportional to the amplitude of the fundamental wave, and gives rise to a static strain. The presence of the second type of defects leads to a linear growth of all harmonics with amplitudes proportional to the power 3/2 of the fundamental amplitude, and to a strain-dependent velocity shift. Simple expressions are given for the growth rates of higher harmonics of S0 Lamb waves in terms of the parameters occurring in the effective stress-strain relation. They have partly been determined quantitatively with the help of the FEM results for different micro-crack concentrations.

Existing ultrasonic stress evaluation methods utilize the acoustoelastic effect for bulk waves propagating in volume, which is unsuitable for a surface treated material, possessing a significant variation in material properties with depth. With knowledge of nonlinear elastic parameters – third-order elastic constants (TOEC) close to the surface of the sample, the acoustoelastic effect might be used with surface acoustic waves. This work is focused on the development of an independent method of TOEC measurement using the effect of nonlinear surface acoustic waves scattering – i.e. the effect of elastic waves interaction in a nonlinear medium.
In this paper, the possible three wave interactions of surface guided waves and bulk waves are described and formulae for the efficiency of harmonic generation and mode mixing are derived. A comparison of the efficiency of surface waves scattering in an isotropic medium for different interaction types is carried out with the help of nonlinear perturbation theory. First results for surface and bulk wave mixing with known second- and third-order elastic constants are shown.

Micro-cracks give rise to non-analytic behavior of the stress-strain relation. For the case of a homogeneous spatial distribution of aligned flat micro-cracks, the influence of this property of the stress-strain relation on harmonic generation is analyzed for Rayleigh waves and for acoustic wedge waves with the help of a simple micromechanical model adopted from the literature. For the efficiencies of harmonic generation of these guided waves, explicit expressions are derived in terms of the corresponding linear wave fields. The initial growth rates of the second harmonic, i.e., the acoustic nonlinearity parameter, has been evaluated numerically for steel as matrix material. The growth rate of the second harmonic of Rayleigh waves has also been determined for microcrack distributions with random orientation, using a model expression for the strain energy in terms of strain invariants known in a geophysical context.

In a recent paper it has been shown that the effective nonlinear constant which is used in a P-Matrix approach to describe third-order intermodulation (IMD3) in surface acoustic wave (SAW) devices can be obtained from finite element (FEM) calculations of a periodic cell using nonlinear tensor data [1]. In this paper we extend this FEM calculation and show that the IMD3 of an infinite periodic array of electrodes on a piezoelectric substrate can be directly simulated in the sagittal plane. This direct approach opens the way for a FEM based simulation of nonlinearities for finite and generalized structures avoiding the simplifications of phenomenological approaches.

Elastic constants of components are usually determined by tensile tests in combination with ultrasonic
experiments. However, these properties may change due to e.g. mechanical treatments or service conditions during
their lifetime. Knowledge of the actual material parameters is key to the determination of quantities like residual
stresses present in the medium. In this work the acoustic nonlinearity parameter (ANP) for surface acoustic waves is
examined through the derivation of an evolution equation for the amplitude of the second harmonic. Given a certain
depth profile of the third-order elastic constants, the dependence of the ANP with respect to the input frequency is
determined and on the basis of these results, an appropriate inversion method is developed. This method is intended
for the extraction of the depth dependence of the third-order elastic constants of the material from second-harmonic
generation and guided wave mixing experiments, assuming that the change in the linear Rayleigh wave velocity is
small. The latter assumption is supported by a 3D-FEM model study of a medium with randomly distributed microcracks as well as theoretical works on this topic in the literature.

The existence of acoustic waves with displacements localized at the tip of an isotropic elastic wedge was rigorously proven by Kamotskii, Zavorokhin and Nazarov. This proof, which is based on a variational approach, is extended to rectangular anisotropic wedges. For two high-symmetry configurations of rectangular edges in elastic media with tetragonal symmetry, a criterion is derived that allows identifying the boundary between the regions of existence for wedge modes of even and odd symmetry in regions of parameter space, where even- and odd-symmetry modes do not exist simultaneously. Furthermore, rectangular edges with non-equivalent surfaces are analyzed, and it is shown that at rectangular edges of cubic elastic media with one (110) surface and one (001) surface, a tip-localized guided wave always exists, apart from special cases that are characterized.

Laser pulses focused near the tip of an elastic wedge generate acoustic waves guided at its apex. The shapes of the acoustic wedge wave pulses depend on the energy and the profile of the exciting laser pulse and on the anisotropy of the elastic medium the wedge is made of. Expressions for the acoustic pulse shapes have been derived in terms of the modal displacement fields of wedge waves for laser excitation in the thermo-elastic regime and for excitation via a pressure pulse exerted on the surface. The physical quantity considered is the local inclination of a surface of the wedge, which is measured optically by laser-probe-beam deflection. Experimental results on pulse shapes in the thermo-elastic regime are presented and confirmed by numerical calculations. They pertain to an isotropic sharp-angle wedge with two wedge-wave branches and to a non-reciprocity phenomenon at rectangular silicon edges.

Silicon edges as one-dimensional waveguides for dispersion-free and supersonic leaky wedge waves
(2012)

Acoustic waves guided by the cleaved edge of a Si(111) crystal were studied using a laser-based angle-tunable transducer for selectively launching isolated wedge or surface modes. A supersonic leaky wedge wave and the fundamental wedge wave were observed experimentally and confirmed theoretically. Coupling of the supersonic wave to shear waves is discussed, and its leakage into the surface acoustic wave was observed directly. The velocity and penetration depth of the wedge waves were determined by contact-free optical probing. Thus, a detailed experimental and theoretical study of linear one-dimensional guided modes in silicon is presented.