Effect of shear-wave polarization on defect detection in stainless steel weld metal

Ultrasonic inspections of austenitic stainless steel weld metal are particularly difficult because of the dendritic structure and anisotropy of the material. The acoustic properties of stainless steel weld metal are discussed. Data on frequency spectra and variations in longitudinal and shear velocities with wave propagation direction are presented. Differences in shear velocities as great as 25% have been observed as the polarization direction is changed. The difference in detectability of artificial reflectors using shear waves of varying polarization is presented, and it is demonstrated in some cases that horizontally polarized shear waves can ‘detect’ a reflector in the weld metal where the traditional vertically polarized shear waves cannot.     

Numerical analysis of bulk conical waves in anisotropic cylinders; application to stiffness tensor measurement

A point source-point receiver technique, based on laser generation and laser detection of acoustic waves, allows determination of mechanical properties of an anisotropic cylinder. The nature of the material and the geometry of the sample give a dispersive behaviour to the diffracted waves and make the acoustic signature difficult to interpret. To overpass the intricacies, wave fronts (conical waves in the volume and helical waves on the surface) are synthesized from signals provided by scanning the primitive line of the cylinder with a laser point source. In order to distinguish between direct bulk conical waves and other contributions in the acoustic response, some considerations on line surface waves and on reflected bulk conical waves are supplied. The identification of the stiffness tensor components, based on the inversion of the bulk waves phase velocities, is applied to signals simulated for a composite material.     

A model and experimental study of fiber orientation effects on shear wave propagation through composite laminates

The strong elastic anisotropy of the discrete unidirectional plies in a composite laminate interacts sensitively with the polarization direction of a shear ultrasonic wave propagating in the thickness direction. The transmitted shear wave can therefore be used to detect errors in the ply orientation and stacking sequence of a laminate. The sensitivity is particularly high when the polarization directions of the shear wave transmitter and receiver are orthogonal to each other. To understand the interaction between normal-incident shear waves and ply orientations in a laminate, a complete analytical model was developed using local and global transfer matrices. The model predicted the transmitted signal amplitude as a function of polarization angle of the transmitter and time (or frequency) for a given laminate and input signal. To alleviate the experimental problems associated with shear wave coupling, electromagnetic acoustic transducers (EMATs) and metal delay lines were used in the angular scan of the transmitted signal. The EMAT system had the added advantage of being applicable to uncured composite laminates. Experiments were performed on both cured and uncured laminates with common layups for model verification. The sensitivity of the measured shear wave signals to fiber misorientation and stacking sequence errors was also demonstrated.     

Acoustic wave propagation through the boundary between a liquid and the Heusler ferromagnetic alloy

Incidence of an acoustic wave upon a plane boundary between a liquid and a ferromagnetic crystal is considered. The ferromagnet is the Ni_2+x+y Mn_1−x Ga_1−y Heusler alloy with a shape memory, which is in the region of the premartensite or martensite phase transition in temperature. The directions of propagation and polarization and the amplitudes of the reflected and transmitted quasilongitudinal and quasitransverse waves in the (110) plane of the crystal are determined. Starting from a certain critical angle of incidence, a longitudinal wave in the crystal becomes inhomogeneous and gliding along the boundary with an accompanying surface oscillation. In the vicinity of the phase transition point, this wave may be radiated into the crystal bulk. Proceeding from the experimental data by Trivisonno for ultrasonic velocities and absorption in a Ni₂MnGa crystal, numerical estimates are obtained for the aforementioned acoustic effects.     

The anisotropy of elastic waves in a tellurium crystal

For acoustic waves propagating in an acoustooptic tellurium crystal, the dependence of their polarization on the propagation direction with respect to the crystal axes is discussed. The characteristic features of waves propagating in the crystal are considered; these features manifest themselves in an excess of the phase velocity of shear acoustic modes over the velocity of longitudinal modes. The change in the wave type from quasi-longitudinal to quasi-transverse as a result of the variation in the propagation direction of ultrasound is investigated. It is shown that such a behavior of bulk acoustic waves is caused by the specific relation between the elastic moduli, which differs from the corresponding relations observed in other acoustooptic materials.     

Dispersion curves of bulk acoustic waves in an elastic body with a two-dimensional periodic structure of circular holes

The dispersion curves of bulk acoustic waves in systems of circular holes made in an isotropic elastic material are calculated by the finite-element method for the cases of the square and hexagonal symmetries of the hole arrangement. The presence of total band gaps for acoustic waves is demonstrated, and the presence of inverse quasi-transverse first-order modes is revealed. For the hexagonally symmetric system of holes, total band gaps are found in the region of higher-order modes. For waves with a purely shear polarization, the imaginary part of the wave number in the first band gap is calculated.     

Acoustic properties of biaxial crystal of double lead molybdate Pb2MoO5

The acoustic properties of crystalline double lead molybdate, which are promising for acoustooptics, have been investigated. All elasticity coefficients of Pb₂MoO₅ crystal, the phase velocities of acoustic waves, and the angles between the vectors of phase and group velocities of this material have been determined. The acoustic parameters of the crystal have been calculated based on experimental data from the Schaefer-Bergmann diagrams. The phase velocity of acoustic waves along some directions has also been determined by the standard (in acoustics) echo-pulse method.     

谱有限元数值分析各向异性复合板中导波色散特性

提出谱有限元方法研究层状各向异性复合板中导波的色散特性和波结构。基于三维弹性动力学方程,用有限元方法离散波导截面,波传播方向的位移用简谐波表示,得到了导波色散的特征方程。分析了单层和双层复合板中导波沿不同方向传播的色散特性和波结构,讨论了双层复合板中层厚比对相速度的影响。数值研究结果表明:导波的对称模态沿纤维方向传播时在较宽的频率范围内保持弱色散状态。双层复合板中导波基本模态的相速度在低频时受层厚比的影响较明显,随着频率的增加趋向于相速度较低的材料。数值模拟结果为导波用于复合材料定量无损检测和性能评价提供理论依据。      

Investigation of the phase velocities of guided acoustic waves in soft porous layers

A new experimental method for measuring the phase velocities of guided acoustic waves in soft poroelastic or poroviscoelastic plates is proposed. The method is based on the generation of standing waves in the material and on the spatial Fourier transform of the displacement profile of the upper surface. The plate is glued on a rigid substrate so that it has a free upper surface and a nonmoving lower surface. The displacement is measured with a laser Doppler vibrometer along a line corresponding to the direction of propagation of plane surface waves. A continuous sine with varying frequencies was chosen as excitation signal to maximize the precision of the measurements. The spatial Fourier transform provides the wave numbers, and the phase velocities are obtained from the relationship between wave number and frequency. The phase velocities of several guided modes could be measured in a highly porous foam saturated by air. The modes were also studied theoretically and, from the theoretical results, the experimental results, and a fitting procedure, it was possible to determine the frequency behavior of the complex shear modulus and of the complex Poisson ratio from 200 Hz to 1.4 kHz, in a frequency range higher than the traditional methods.     

Microwave excitation of ultrasound in graphite-fiber reinforced composite plates

In this paper is demonstrated the effect of microwave beam polarization on the thermal generation of acoustic waves in continuous fiber-reinforced composite laminates. It is found that beam polarization strongly influences the dielectric interaction that leads to thermal losses, bulk expansion, and acoustic wave generation. The oriented graphite fibers in the composite laminate effectively short the microwave fields and reduce the generation efficiency nearly to zero. Ultrasonic waves at several hundred kHz generated in the composite are detected by air-coupled acoustic transducers located on the opposite side of the plate specimen from the 9.41 GHz incident microwave beam. With some averaging signal-to-noise ratios of better than 26 dB are obtained. Applying a conventional model of electromagnetic wave scattering in anisotropic media to this experiment yields good agreement between calculations and measured data. Implications for microwave-acoustic testing of graphite-reinforced composites are also discussed.     

Properties of surface waves in an elastic cylinder filled with a liquid

The properties of harmonic surface waves in an elastic cylinder filled with a liquid are studied. The case of elastic material for which the shear wave velocity is higher than the sound velocity in a liquid is considered. The wave motion is described based on the complete system of equations of the dynamic theory of elasticity and the equation of motion of an ideal compressible liquid. The asymptotic analysis of the dispersion equation in the region of large wave numbers and qualitative analysis of the dispersion spectrum showed that in such a waveguiding system there exist two surface waves, the Stoneley and the Rayleigh waves. The lowest normal wave forms the Stoneley wave on the internal surface of the cylinder. In this waveguide phase, velocities of all normal waves, except for the lowest one, have the velocity of sound in the liquid as their limit. Therefore, the Rayleigh wave on the external surface of the cylinder is formed by all normal waves in the range of frequencies and wave numbers in which phase velocities of normal waves of the composite waveguide and the lowest normal wave of the elastic hollow cylinder coincide.     

Second-order and third-order elastic constants of B95 aluminum alloy and B95/nanodiamond composite

All independent second-order and third-order elastic constants in B95 aluminum alloy and B95/nanodiamond composite have been determined. To determine the second-order elastic constants, the densities and velocities of longitudinal and shear bulk acoustic waves in the materials under study have been measured. To quantitatively characterize the nonlinear elastic properties, the third-order elastic constants (TOECs) of B95 alloy and B95/nanodiamond composite have been determined. The Thurston-Brugger method has been used to experimentally determine the TOECs. For this purpose, the relative changes in the bulk wave velocity have been experimentally measured depending on the uniaxial compression applied to the samples under study and all independent TOECs have been calculated. The elastic wave velocities have been measured by the ultrasonic pulse method at a frequency of 10 MHz. The results obtained have been discussed.     

Elastic constants measurement of anisotropic Olivier wood plates using air-coupled transducers generated Lamb wave and ultrasonic bulk wave

A hybrid elastic wave method is applied to determine the anisotropic constants of Olive wood specimen considered as an orthotropic solid. The method is based on the measurements of the Lamb wave velocities as well as the bulk ultrasonic wave velocities. Electrostatic, air-coupled, ultrasonic transducers are used to generate and receive Lamb waves which are sensitive to material properties. The variation of phase velocity with frequency is measured for several modes propagating parallel and normal to the fiber direction along a thin Olivier wood plates. A numerical model based mainly on an optimization method is developed; it permits to recover seven out of nine elastic constants with an uncertainty of about 15%. The remaining two elastic constants are then obtained from bulk wave measurements. The experimental Lamb phase velocities are in good agreement with the calculated dispersion curves. The evaluation of Olive wood elastic properties has been performed in the low frequency range where the Lamb length wave is large in comparison with the heterogeneity extent. Within the interval errors, the obtained elastic tensor doesn’t reveal a large deviation from a uniaxial symmetry.     

Acoustic identification of the anisotropic nanocrystalline medium with non-dense packing of particles

A two-dimensional model of the anisotropic nanocrystalline (granular) medium being a rectangular lattice of elastically interacting elliptical particles with translational and rotational degrees of freedom was considered. In the long-wave approximation a system of linear equations in partial derivatives describing the propagation of the longitudinal, transverse, and rotational waves in such a system was obtained. The dependences of the wave velocities on the grain size and form were analyzed. It was shown how to determine the moduli of elasticity of the granular material from the change of the velocities of the acoustic waves propagating along different crystallographic directions.     

Generation of acoustic solitons by a single-mode electromagnetic field

Mechanisms of acoustic pulse generation by a single-mode electromagnetic field propagating in a photoelastic material are analyzed. The anisotropy induced by acoustic excitations in an isotropic medium leads to nonlinear coupling between the polarization components of a single-mode electromagnetic field. For different conditions, it is shown that the acoustic-electromagnetic wave interaction due to mixing of the polarization components of light and acoustic waves can give rise to soliton-like coherent acoustic excitations in a thin crystal plate. When spatial dispersion is ignored, the governing system of equations for unidirectional acoustic solitons can be reduced to an integrable model. It is shown that qualitatively different scenarios of formation of acoustic solitons are possible, depending on the directions of deformation and field polarization.     

Polarization of elastic waves in tellurium single crystal

We study special cases of elastic bulk wave propagation in a crystal of tellurium, in which the phase velocity of shear waves can exceed those of longitudinal waves. The impact of the piezoelectric effect on the phase velocity and polarization of waves is discussed.     

Effect of laser beam incidence angle on the thermoelastic generation in semi-transparent materials

When a laser beam is absorbed in a semi-transparent material, a volume acoustic source is created owing to penetration of the laser beam inside the material and to thermoelastic transduction. Many experimental and theoretical studies have been conducted to better understand this ultrasound generation process with normal laser light incidence on the material surface. The purpose of this paper is to analyze the effects of the asymmetry caused by oblique incidence of a laser line source on the generation of acoustic waves in semi-transparent isotropic materials. Experiments on a glass plate demonstrate that such an obliquely incident laser light strongly affects bulk acoustic waves generation. Compressional and shear waves are enhanced and the loss of symmetry of the acoustic source causes asymmetrical behavior of the acoustic waves. Surprisingly, compressional-wave amplitude decreases whereas shear-wave amplitude increases in the region where the electromagnetic energy is refracted. This feature is explained by semi-analytical calculations.     

A direct method for measuring the temperature coefficients of bulk acoustic velocities in crystals

A method for direct measurement of the temperature coefficients of the velocities of bulk acoustic waves propagating along arbitrary directions in crystal planes is described. It represents a specific version of the Schaefer-Bergmann acoustooptic diffraction in which the effect of the temperature is precisely compensated by biasing the acoustic frequency. The method is equally applicable to the determination of other kinds of velocity coefficients.     

Specificity of the phenomenon of acoustoelasticity in a two-dimensional internally structured medium

A two-dimensional model of a microstructured medium is considered in the form of a square lattice consisting of elastically interacting circular particles with translational and rotational degrees of freedom. The interactions between the particles are modeled by a set of elastic springs. Differential equations are derived to describe the propagation and interaction of acoustic waves in such a medium. The relation between the velocities of wave propagation and the small strain arising in the structure under external action is determined. Analytical expressions that determine the difference between the squares of the velocities of both longitudinal and shear waves propagating in two mutually perpendicular directions in a medium with an externally induced anisotropy are derived and analyzed.