Anisotropy Of Wave Propagation Research Articles

Acoustic emission failure detection and localisation in historic fibrous plaster ceiling

Fibrous plaster (FP) is a composite material that has been used for suspended decorative ceilings in historical buildings since the late 19th century. Such ceilings are susceptible to failures, particularly in hard-to-access structural hangers (‘wads’). Due to a limited understanding of structural behaviour of FP ceilings, maintenance and reconstruction decisions are based on visual and tactile inspections. Such inspections are costly, intrusive and ultimately subjective. Non-destructive techniques are needed to detect and localise failures in FP ceilings. An Acoustic Emission system was recently developed to detect and localise damage sources in FP ceilings. The method processes data from AE sensor clusters with one-dimensional convolutional neural networks(1D-CNNs) trained to account for wave propagation anisotropy. This paper presents a case study conducted at the Institution of Civil Engineers (ICE) building in London (UK) to validate the method. The results confirm the ability of the method to localise failures under real site conditions in 450 mm-by-450 mm monitoring areas. The method may enable non-intrusive detection of failures from the underside of ceilings during periodic inspections. The possibility of conducting pre-training in laboratories and covering large monitoring areas up to 1500mm-by-1500mm is established and may facilitate practical applications in the future.

ANISOTROPY AND MECHANICAL CHARACTERISTICS OF ULTRA-HIGH PERFORMANCE CONCRETE AND ITS INTERPENETRATING PHASE COMPOSITE WITH TRIPLY PERIODIC MINIMAL SURFACE ARCHITECTURES

Abstract This work numerically explores the anisotropy, impact phase wave propagation, buckling resistance, and natural vibration of ultra-high performance concrete (UHPC) and UHPC-steel interpenetrating phase composite (IPC) with triply periodic minimal surfaces (TPMSs), including sheet and solid Gyroid, Primitive, Diamond, and I-WP. The experiment is conducted verifying the accuracy of the numerical model in terms of Young's modulus of polylactic acid (PLA)-based TPMS lattices and PLA-cement IPCs with TPMS cores, with the highest percent difference of 15% found for IPCs and 17% found for lattice. The results indicate that UHPC material with sheet Gyroid exhibits the least extreme anisotropy in response to the varying orientation among other lattices regardless of the change of solid density, making it the ideal candidate for construction materials. Interestingly, compared to UHPC-based TPMS lattice, IPCs possess a much smaller anisotropy and exhibit almost isotropy regardless the variation of solid density and TPMS topology, offering a free selection of TPMS type to fabricate IPCs without much care of anisotropy. The phase wave and buckling resistance of UHPC- and IPC-based beams with TPMSs nonlinearly decrease with a drop of TPMS solid density, but it is the almost linear pattern for the case of natural vibration frequency. UHPC material and IPC with sheet Gyroid lattice are found to possess the lowest phase wave velocity and exhibit the least anisotropy of wave propagation, showing it as an ideal candidate for UHPC material to suppress the destructive energy induced by the external impact.

Use of periodic 2D pillar array for performance enhancement of AlN-based SMR BAW resonators

This paper discusses the applicability of a two-dimensional (2D) periodic pillar array as the first layer of an acoustic mirror for AlN-based solidly mounted resonators of bulk acoustic waves. It is based on the idea that the periodic pillar array behaves like a uniform layer when the period is much smaller than the wavelength and its behavior is given by the ensemble average of pillar materials. The simulation shows that the effective electromechanical coupling factor can be enhanced close to that of AlN-based film bulk acoustic resonators when the slot ratio is large. It is also shown that the use of a 2D pillar array can enhance the reflectivity of the Bragg reflector. It should be noted that the periodic 2D pillar array can also control the anisotropy of wave propagation along the surface. Owing to this, transverse mode resonances can be suppressed well by adjusting the wave dispersion curve properly.

Phase coherence weighted ultrasound total focusing method towards the improved imaging of CFRP defects

Defect imaging in carbon fiber reinforced polymer (CFRP) combining the phase coherence imaging (PCI) and the total focusing method (TFM) is investigated in this study, in order to overcome the adverse influence on defect imaging from wave propagation anisotropy, inter-ply reflection, and high attenuation. Three kinds of coherence factors are proposed first to be combined with the wave velocity corrected TFM for defect imaging in CFRP. Then, the much stronger phase coherence (PC) from the signals scattered from the defect than those from the inter-ply is discovered experimentally. Finally, with the proposed PC-weighted-TFM, the noise of imaging from the inter-ply reflection is largely suppressed, resulting in largely improved signal-to-noise ratios of imaging with defects at different depths and close to each other. The findings demonstrate that the proposed PC-weighted-TFM is of great application potential in nondestructive testing of CFRP.

Influence of Melt‐Peridotite Interactions on Deformation and Seismic Properties of the Upper Mantle Beneath a Destroyed Craton: A Case Study of the Damaping Peridotites From the North China Craton

AbstractExtensive melt‐peridotite interactions had been documented in the lithospheric mantle beneath the North China Craton (NCC), a prime example of destroyed cratons in the world. Yet the impacts of melt‐peridotite interactions on the deformation and seismic anisotropy of the NCC upper mantle remain unclear. Here we studied in detail the microstructure, crystallographic preferred orientation (CPO) of minerals, and seismic properties of 26 peridotite xenoliths from the Damaping area of the NCC. The studied samples can be classified into two groups: weakly to nonfoliated and strongly foliated. Petrographic and microstructural observations suggest that multiple melt‐peridotite interactions and at least two stages of deformation had influenced samples from both microstructural groups. Dislocation creep in response to a transpression deformation led to the [010]‐fiber type olivine CPOs in most samples. Variable degrees of annealing followed the last stage of deformation. Due to a higher degree of melt‐peridotite interactions, which had promoted nondislocation creep, and more extensive annealing, olivine and pyroxene in the strongly foliated samples developed weaker CPOs. This in turn leads to weaker maximum P wave propagation anisotropy and S wave polarization anisotropy for this microstructural group. Our data, therefore, cast light on a strong control of intensity of melt‐peridotite interactions on deformation and seismic properties of the upper mantle beneath the NCC. If foliation and lineation are vertical and horizontal, respectively, the measured SKS splitting parameters can be well explained by the “fossil” anisotropy frozen in the lithospheric mantle, with no need to invoke asthenospheric flow as a source of the anisotropy.

Development of a Numerical Simulator for 3-D Dynamic Fracture Process Analysis of Rocks Based on Hybrid FEM-DEM Using Extrinsic Cohesive Zone Model

The combined finite-discrete element method (FDEM) has attracted significant attention for numerical simulations of complex fracture process of rock-like materials as one of the promising hybrid methods. The mainstream of FDEM simulators developed to date is based on the intrinsic cohesive zone model (ICZM) in which cohesive elements are inserted into all the boundaries of continuum solid elements at the onset of simulations and an artificial elastic behavior must be incorporated to model the intact deformation of rock-like materials. However, the effect of introduction of the artificial elastic behavior on the precision of intact stress wave propagation has not been discussed in previous literatures and this paper discusses this issue. As an alternative for the ICZM-based FDEM, we apply the FDEM based on the extrinsic cohesive zone model (ECZM). An advantage of the ECZM-based FDEM is presented through the 3-dimentional (3-D) numerical modelling of dynamic tension test. In addition, the effect of considering the anisotropy of wave propagation in granite, which has been neglected in all the previous works using FDEM, is investigated through the ECZM-based 3-D FDEM simulation of dynamic Brazilian test with a split Hopkinson pressure bar apparatus. Through the presented numerical simulations, it can be concluded that the ECZM-based FDEM may be an alternative for numerical simulations of complex dynamic fracture process of rock-like materials instead of the ICZM-based FDEM.

Dynamics anisotropy in a porous solid with aligned slit fractures

Crustal rocks are commonly permeated by aligned fractures which may control the wave anisotropy and permeability pattern. In the presence of pore fluid the mechanical and hydraulic features of such rocks become more complex. Understanding the dynamic anisotropy in fluid-saturated fractured rocks is important for detecting and characterizing fractured reservoirs and fault zones with applications in geomechanics, hydrogeology, exploration geophysics and reservoir engineering. For waves propagating normal to the fractures, the effects of wave-induced fluid flow (WIFF) due to the presence of permeable fractures on seismic dispersion and attenuation are significant and have been quantified in earlier studies. But previous literatures are restricted to low frequency range within which the fracture size is much smaller than the incident wavelength. In this paper, we extend low-frequency normal incidence results to full-frequency oblique incidence. We first derive exact solutions of the scattering problem of obliquely incident plane waves by a single slit fracture in a poroelastic solid. Based on previous analysis, for ideal fractures with infinitesimal thickness, the fracture fluid can be modelled as an incompressible one. Then, based on the solutions and Foldy's scattering theorem we develop a dynamic-effective-medium model to estimate frequency-dependent anisotropy of wave propagation in a fluid-saturated poroelastic rock with a sparse set of aligned fractures. We find that for the oblique incidence problem apart from WIFF there exist another two important attenuation mechanisms, i.e., the elastic scattering (scattering into fast P and S waves via mode conversion at the fracture faces) and Biot's global flow, in causing velocity dispersion and attenuation. The mixed-boundary problem reveals that the WIFF is controlled by the normal displacement discontinuity that is determined by effective normal stress applied on the fracture faces, while the scattering effects by the tangential displacement discontinuity that is determined by effective shear stress. Because the effective normal and shear stresses depends on incident angles and frequency, the dispersion and attenuation of both P and S waves are anisotropic and frequency-dependent. In contrast, Biot's global flow is an intrinsic energy loss mechanism that can play a role in causing velocity dispersion and attenuation at higher frequency range but it is independent of the presence of fractures or incident angle.

Electrical resistivity and elastic wave propagation anisotropy in glancing angle deposited tungsten and gold thin films

We report on experimental investigations of electrical resistivity and surface elastic wave propagation in W and Au thin films sputter-deposited by glancing angle deposition. For both metals, the angle of deposition α is systematically changed from 0 to 85°. Dense and compact films are produced with a normal incident angle α = 0°, whereas inclined and porous columnar architectures are clearly obtained for the highest angles. Group velocities and DC electrical resistivities of W and Au films are significantly changed in x and y directions as α increases from 0 to 85°. Isotropic behaviors are always observed for Au films, whereas anisotropic velocity and resistivity are clearly measured in W films prepared with incident angles higher than 60°. The anisotropic coefficient reaches 1.8 for velocity and 2.0 for resistivity in W films, while it remains in-between 1.0 and 1.2 in Au films whatever the incident angle. This directional dependence of electrical resistivity and elastic wave propagation is connected to the anisotropic structural characteristics of W inclined columns (fanning of their cross-section), while Au columns exhibit a more symmetric growth as a function of the incident angle.

Nonlinear dynamical analysis of 3D textiles based on second order gradient homogenized media

The general objective of this contribution is the analysis of wave propagation phenomena within architecture media, relying on an effective substitution continuum obtained by homogenization. The proposed methodology is quite general and applicable to any 3D repetitive network of beam-like structural elements, considering beams undergoing large transformations. Based on the writing of the equations of motion of a nonlinear second order effective gradient continuum, we analyze the nonlinear wave propagation in the obtained homogenized nonlinear second order gradient continuum. The resulting wave equations are of Boussinesq type, the solution of which being elliptic functions. The influence of the degree of nonlinearity on the dispersion relations is analyzed, highlighting subsonic and supersonic modes propagating respectively with a velocity lower (resp. higher) than the velocity of linear non-dispersive waves. Subsonic and supersonic modes correspond respectively to regimes of high and low nonlinearity characterized by the so-called universal constant. The three modes of propagation (longitudinal, vertical and horizontal shear) are compared in terms of dispersion relations, phase and group velocity diagrams. The existing anisotropy of wave propagation becomes more marked when the degree of linearity increases. The horizontal and vertical shear modes disappear successively when increasing the wavenumber.

Wave propagation in 2D random granular media

Numerical simulations are performed to study wave propagation in an infinite domain of square packed spheres with intruders at interstitial locations. Randomness is uniformly distributed in the densities of the spheres and the packing contains no precompression. The simulations reveal the two regimes of force amplitude decay: exponential and power law, similar to those previously reported in 1D random granular chains. The source of transition between the regimes is identified as a function of the amplitude of the leading pulse and the backscatter. An analysis of the contours of force amplitude demonstrates the anisotropy of wave propagation with respect to the distance traveled by the wave and the level of randomness. An investigation of the evolution of ensemble kinetic energy shows a gradual increase with increasing randomness, and the results are compared with the corresponding 1D random chains. The study of angular decay provides a way to identify suitable sphere properties to design optimized granular structures.

Sound Wave Propagation Anisotropy in Silver Nanoprisms: Characterization of Photoinduced Multiple Modes Using the Symmetric Molecular Dynamics Method

We present both theoretical and experimental studies of the sound velocity anisotropy and laser-induced multiple acoustic phonon oscillations in silver nanoprisms. A large-scale simulation method based on molecular dynamics and group theory for nanoprisms with C3v symmetry was developed. We observed the most prominent planar modes and the less studied vertical modes and found that the anisotropic vibrational properties of silver nanoprisms strongly depend on the particle’s aspect ratio (bisector length over thickness). For the case of a smaller aspect ratio, we identified these modes as the A1 modes with a larger in-phase radial atomic displacement. These modes can be classified into three main types: breathing modes and totally symmetric modes which are planar modes and thickness-related vertical modes. Due to the strong coupling with laterally confined modes, a broad band of thickness-related modes is found (in contrast to a single sharp peak for thin films), which leads to a fast decaying component in the oscillation spectrum. The calculated results and the sound velocity anisotropy along different crystal axes are in good agreement with our transient optical absorption experiments.

Effects of wave propagation anisotropy on the wave focusing by negative refractive sonic crystal flat lenses

In this study, wave propagation anisotropy in a triangular lattice crystal structure and its associated waveform shaping in a crystal structure are investigated theoretically. A directional variation in wave velocity inside a crystal structure is shown to cause bending wave envelopes. The authors report that a triangular lattice sonic crystal possesses six numbers of a high symmetry direction, which leads to a wave convergence caused by wave velocity anisotropy inside the crystal. However, two of them are utilized mostly in wave focusing by an acoustic flat lens. Based on wave velocity anisotropy, the pseudo ideal imaging effect obtained in the second band of the flat lens is discussed.

Influence of iron on the elastic properties of wadsleyite and ringwoodite

[1] We investigate by first‐principles the influence of iron on the elastic properties of the b–phase (wadsleyite) and g–phase (ringwoodite), polymorphs of olivine, the most abundant minerals of the upper and lower parts of the transition zone, respectively. Our study aims to complement experiments to understand details of the 410 km and 520 km discontinuities. The full elastic‐tensor Cij, bulk (K), and shear (G) moduli are determined under static conditions for b–g–(Mg1–xFex)2SiO4 with x = 0.125. Wave propagation anisotropy in single crystals and polarization anisotropy in aggregates with preferred orientation are investigated and compared with those of iron‐free wadsleyite and ringwoodite for a thorough understanding of the effect of iron. We examine the effect of iron on velocity contrasts due to phase changes and conclude that iron enhances DVP and DVS across the a → b transition but suppresses them across the b → g transition. The latter might contribute to suppress locally the 520 km discontinuity if this has a significant contribution from the b → g transition. We show that lateral variation of iron, dx, produces lateral velocity heterogeneity ratios similar to those produced by lateral variations of temperature, dT, both producing ratios comparable to values extracted from seismic tomography studies. However, in contrast with dT, dx produces negative values for density to longitudinal and shear wave velocity ratios. This might be considered the fingerprint of lateral variations of iron concentration. These negative ratios appear similar to results inferred from geodynamical models compiled by Karato and Karki (2001) for the upper mantle and transition zone.

Dynamic response of phenolic resin and its carbon-nanotube composites to shock wave loading

We investigate with nonreactive molecular dynamics simulations the dynamic response of phenolic resin and its carbon-nanotube (CNT) composites to shock wave compression. For phenolic resin, our simulations yield shock states in agreement with experiments on similar polymers except the “phase change” observed in experiments, indicating that such phase change is chemical in nature. The elastic–plastic transition is characterized by shear stress relaxation and atomic-level slip, and phenolic resin shows strong strain hardening. Shock loading of the CNT-resin composites is applied parallel or perpendicular to the CNT axis, and the composites demonstrate anisotropy in wave propagation, yield and CNT deformation. The CNTs induce stress concentrations in the composites and may increase the yield strength. Our simulations suggest that the bulk shock response of the composites depends on the volume fraction, length ratio, impact cross-section, and geometry of the CNT components; the short CNTs in current simulations have insignificant effect on the bulk response of resin polymer.

Anisotropy of wave propagation in the heart can be modeled by a Riemannian electrophysiological metric

It is well established that wave propagation in the heart is anisotropic and that the ratio of velocities in the three principal directions may be as large as v(f)v(s)v(n) approximately 4(fibers)2(sheets)1(normal). We develop an alternative view of the heart based on this fact by considering it as a non-Euclidean manifold with an electrophysiological(el-) metric based on wave velocity. This metric is more natural than the Euclidean metric for some applications, because el-distances directly encode wave propagation. We develop a model of wave propagation based on this metric; this model ignores higher-order effects like the curvature of wavefronts and the effect of the boundary, but still gives good predictions of local activation times and replicates many of the observed features of isochrones. We characterize this model for the important case of the rotational orthotropic anisotropy seen in cardiac tissue and perform numerical simulations for a slab of cardiac tissue with rotational orthotropic anisotropy and for a model of the ventricles based on diffusion tensor MRI scans of the canine heart. Even though the metric has many slow directions, we show that the rotation of the fibers leads to fast global activation. In the diffusion tensor MRI-based model, with principal velocities 0.25051 m/s, we find examples of wavefronts that eventually reach speeds up to 0.9 m/s and average velocities of 0.7 m/s. We believe that development of this non-Euclidean approach to cardiac anatomy and electrophysiology could become an important tool for the characterization of the normal and abnormal electrophysiological activity of the heart.

Extreme anisotropy of wave propagation in two-dimensional photonic crystals

We demonstrate that electromagnetic waves propagating in square and hexagonal photonic crystals can have fundamentally different anisotropy properties. The wave frequency and group velocity can be functions of the propagation direction even for vanishingly small wave numbers (near the gamma-point). This anisotropy, present in square but absent in hexagonal lattices, can be so extreme that the group velocity can be either parallel or antiparallel to the phase velocity depending on the propagation direction. An analytic explanation of this effect based on the k.p perturbation theory and group-theoretical considerations is confirmed by electromagnetic simulations. One manifestation of the extreme anisotropy is the divergent van Hove singularity in the density of photonic states at the gamma-point. New applications, including surface-emitting quantum cascade lasers, are proposed.

Elastic properties of zinc tris(thiourea) sulphate single crystals

Zinc tris(thiourea) sulphate (ZTS) single crystals possess excellent nonlinear optical properties and are nearly 1.2 times more efficient than potassium dihydrogen phosphate in optical harmonic generation. Large single crystals of this material have been grown from aqueous solution. All nine second order elastic stiffness constants of this orthorhombic crystal have been determined by measuring the velocity of propagation of ultrasonic waves of longitudinal and transverse polarizations along specific symmetry directions. The McSkimin Δt correction has been applied for better accuracy of velocity values. The corresponding elastic compliance constants and Poisson’s ratios have also been evaluated. Planar plots of phase velocity, group velocity, slowness, Young’s modulus, and linear compressibility have been drawn to display the anisotropy of wave propagation in the crystal. The bulk modulus and volume compressibility of this crystal have been evaluated from the elastic constant data. Variation of a selected number of elastic constants with temperature in a limited range have also been measured and reported.

New Approaches to Nondestructive Evaluation of a Filament-Wound Composite Motor Case Using Circumferential Wave.

For reliable quality assurance of filament-wound composite rocket motor (FCRM) cases by use of acoustic emission during hydroproof (AE/H) testing, it is necessary both to detect defects introduced in fabrication process and to monitor damage done due to hydraulic pressurization. The circumferential wave, which has strong directivity and weak anisotropy in wave propagation in the particular FCRM case under present investigation, has high potential to address such a need. To explore these outstanding capabilities of the circumferential wave, two kinds of experiments were conducted. The pitch-catch measurements of the circumferential wave that propagated through the artificial slits demonstrated its high potential for detection of flaws. The experiments for AE source location using a particular, triangular layout of AE sensors declared its capability for monitoring of damages. Inspired from the experiments, new approaches to nondestructive evaluation of the given FCRM case were proposed using the circumferential wave: 1) an ultrasonic pitch-catch scanning and 2) an AE/H testing with a suitable triangular layout of AE sensors.

Poroelastic Backus averaging for anisotropic layered fluid‐ and gas‐saturated sediments

A homogeneous anisotropic effective‐medium model for saturated thinly layered sediments is introduced. It is obtained by averaging over many layers with different poroelastic moduli and different saturating fluids. For a medium consisting of a stack of vertically fractured horizontal layers, this effective medium is orthorhombic. We derive the poroelastic constants that define such media in the long‐wavelength limit as well as the effective large‐scale permeability tensor. The permeability shows strong anisotropy for large porosity fluctuations. We observe pronounced effects that do not exist in purely elastic media. At very low frequencies, seismic waves cause interlayer flow of pore fluid across interfaces from more compliant into stiffer layers. For higher frequencies, the layers behave as if they are sealed, and no fluid flow occurs. The effective‐medium velocities of the quasi‐compressional waves are higher in the no‐flow than in the quasi‐static limit. Both are lower than the high‐frequency, i.e., ray‐theory limit. Partial saturation affects the anisotropy of wave propagation. In the no‐flow limit, gas that is accumulated primarily in the stiffer layers reduces the seismic anisotropy; gas that is trapped mainly in layers with a more compliant frame tends to increase the anisotropy. In the quasi‐static limit, local flow keeps the anisotropy constant independent of partial saturation effects. For dry rock, no‐flow and quasi‐static velocities are the same, and the anisotropy caused by layering is controlled only by fluctuations of the layer shear moduli. If the shear stiffness of all layers is the same and only the compressive stiffness or saturation varies, only the ray‐theory velocity exhibits anisotropy.

Progressive microcrack development in tests on Lac du Bonnet granite—II. Ultrasonic tomographic imaging

Thorough three-dimensional raypath coverage of transmitted ultrasonic waves was used to assess the development of microcrack damage in a cylinder of Lac du Bonnet grey granite subjected to uniaxial cyclic loading. The peak load for each cycle was increased gradually until volumetric strain reversal occurred during the 9th cycle. For the remaining 19 cycles, the peak load corresponded approximately to the onset of volumetric strain reversal, as determined from real-time strain measurements. Acoustic emission (AE) counts were monitored during loading and unloading. Ultrasonic wave travel time scans were done prior to the test and then following nine of the load cycles. More than 4500 travel time measurements were made for each scan. The data show an increase in slowness (the inverse of velocity) and a change in orientation of the anisotropy of wave propagation with continued load cycling. Slownesses of raypaths propagating normal to the sample axis increase the most. Slownesses for raypaths propagating parallel to the axis increased minimally. Tomographic images constructed from slowness differences (relative to the pre-test scan) show the progressive development of damage with continued cycling, primarily in a donut-shaped zone near the outer surface around the middle of the cylinder. The relatively constant axial velocities suggest the damage was primarily due to axial splitting cracks.