Direction Of Wave Propagation Research Articles

Wave propagation analysis in pre-stressed incompressible hyperelastic multi-layered plates using a plate theory

Wave propagation analysis of pre-stressed incompressible hyperelastic thin multi-layered plates is developed based on the first-order shear deformation plate theory (FSDT) with second order thickness extensibility. Variables describing the thickness-direction deformation are obtained from the incompressibility condition. The incremental theory of nonlinear elasticity is employed in conjunction with a plate theory. The multi-layered plate is assumed to be symmetric in the thickness direction. Since the plate theory has not been used for the wave propagation analysis of hyperelastic plates, the results of flexural wave modes are compared with results of the straight-crested flexural waves based on the three-dimensional (3D) elasticity theory available in the literature and the results for in-plane wave modes are compared with those developed in this study using the plane-stress elasticity theory. The dispersion curves of both extensional and flexural waves for single-layer and three-layer plates are presented and the effects of different parameters including the homogenous pre-stretch, the wave propagation direction and the plate initial thickness on the dispersion curves are discussed in detail. The results indicate that the phase speed of the extensional waves depends on the wave propagation angle due to the induced anisotropy. As the overall shear modulus of the three-layer plate increases, the phase velocity increases. Also, the effect of pre-stretch and thickness on phase speed of flexural waves is not pronounced for high frequencies.

Monitoring for fatigue crack geometry in orthotropic steel bridge decks by application of reflected Lamb waves

Orthotropic steel bridge decks (OSDs) have been widely used as a crucial load-bearing structures in long-span bridges,. However, the weld connections in OSDs are susceptible to fatigue crack initiation due to the high stress cycle caused by vehicle loads. Reflected Lamb waves have potential in the monitoring of crack geometry, and different sensor arrangements are employed for different crack types in OSDs. Three full-scale specimens were fabricated and tested with the recommended sensor arrangements to continuously monitor two types of cracks in tests. Finite element models were also established and verified by the experimental results. Parametric studies were conducted to investigate the effects of sensor arrangements, crack lengths and angles on monitoring results. A wavelet transform algorithm was applied to analyze reflected waves and extract standardized wave features, based on which the crack geometry was evaluated. The experimental results reveal that the standardized wave features from the reflected Lamb waves change significantly with the increase of fatigue life, indicating that the features are sensitive to fatigue crack growth. The simulation results reveal that the standardized wave features effected by crack geometry are positively correlated with projected crack lengths perpendicular to the direction of wave propagation. The optimal arrangements of wave sensors for crack monitoring at deck-to-rib connections are to place excitation and reception sensors at 200 mm and 60 mm from the welds at crack initiation plates, respectively; while for cracks at rib-to-floorbeam connections, excitation and reception sensors are located at 200 mm and 50 mm distant from the horizontal welds at cut-out, respectively, and all sensors are 50 mm distant from vertical welds.

Effect of spacecraft attitude on radio wave polarization measurements for the radio Receiver instrument on Swarm-E

The characteristics of a polarized radio wave detected by an antenna onboard a spacecraft depend on the relative positions of the transmitting source and the spacecraft, the orientation of the satellite antenna with respect to the wave propagation direction, the electric field of the incident wave, and the properties of the medium traversed by the wave. Precise knowledge of the radio wave propagation direction and the receiver antenna orientation is needed for resolving the received signal properties. In this work, the focus is on the coordinated experiments between the Radio Receiver Instrument (RRI) on Swarm-E and the Natural Resources Canada Ottawa transmitter (45.4°N, 75.6°W). Three experiments, in which the RRI boresight was along the ram (13 December 2017), nadir (20 December 2017) and slew-to-target (21 December 2017) directions were selected for detailed investigations and comparisons. The deviation of the RRI boresight direction from the propagation direction of the wave vector was up to 10° in the slewed pass and varied between 0° and 180° in the two other passes. The effects of the antenna pointing direction were pronounced in the observed powers on each dipole and affected the observed (apparent) polarization characteristics. The observations were not sufficient for describing the polarization state of the original signal during one third of the RRI-ram experiment when only one dipole was receiving the signal with optimal geometry. The methods developed lay the foundation for mitigating the attitude effects to determine the true polarization characteristics of the radio wave.

Life Cycle Assessment of a wave energy device – LiftWEC

The need to move towards a low-carbon economy has brought about the emergence of various renewable energy sectors, including Marine Renewable Energy (MRE). However, after many years of research and development, the MRE industry still faces challenges in achieving commercial viability, especially regarding wave energy. Whilst it remains possible that successful wave energy technologies exist in the traditional research trend, it is also appropriate to explore alternatives that produce energy by different approaches. Wave-induced lift force devices may be the possibility to move beyond traditional wave energy technologies using diffraction and/or buoyancy forces. In this context arises the LiftWEC, a promising configuration of a lift-based wave energy converter. The LiftWEC device couples with the waves through lift forces generated by two hydrofoils that rotate in a single direction aligned orthogonally to the direction of wave propagation.
 To fully evaluate the overall advantages of this new technology, it is necessary to go beyond the techno-economic performance and reliability. While capable of producing electricity from clean sources, MRE devices are not entirely environmentally friendly, since energy is consumed and pollutants are emitted during their various life cycle stages. Accordingly, as the MRE sector expands, it is important to ensure that the technologies prove to be sustainable alternatives in terms of their environmental impact.
 Life Cycle Assessment (LCA) is a widely recognized methodology to evaluate environmental impacts by considering the technology’s performance over its life cycle. This methodology complies with international standards ISO 14040, which specify the general framework, principles, and requirements for conducting and reporting this type of assessment.
 A “cradle to grave” LCA assessment was applied to the LiftWEC device to evaluate the potential cumulative environmental impacts of the system, complete from the extraction of raw materials until decommissioning. Each stage was analysed within the defined system boundaries, and data on the energy, materials, emissions, and waste products associated were gathered. To allow comparison with other MRE technologies and traditional means of electricity generation, carbon dioxide equivalent emissions per produced electricity (gCO2eq/kWh) were calculated for the study.
 Since ocean energy is broadly considered to contribute to a low-carbon energy system, special attention was given to the LCA results on the global warming potential (GWP). Besides a set of 18 impact categories, Cumulative Energy Demand (CED) and Carbon and Energy payback time (CPT and EPT, respectively) were also analysed. The CPT and EPT are important indicators that measure the time required to offset the carbon emission and demanded energy, respectively, accounted across all development phases of the device.
 This work included the comparison of LCA findings for other MRE devices reported in the literature to validate the viability of the LiftWEC in terms of carbon and energy footprint. In addition, the assessment analysed alternative materials, locations, and recyclability allowing the identification of potential improvement opportunities regarding the reduction of environmental impacts.

Excitation and Propagation of Magnetosonic Waves in the Earth's Dipole Magnetic Field: 3D PIC Simulation

AbstractMagnetosonic (MS) waves are common plasma waves in the Earth's magnetosphere. The self‐consistent excitation of MS waves has been studied by 2D particle‐in‐cell simulations in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Therefore, the 3D simulation of MS waves needs to be investigated. In this paper, we investigate the excitation and evolution of MS waves in the Earth's dipole magnetic field based on a 3D general curvilinear particle‐in‐cell simulation. We find that the MS waves are excited primarily within 3° of the equator when the thermal velocity of the ring distribution is much less than the ring velocity of the ring distribution. These waves propagate along both the radial and azimuthal directions nearly perpendicular to the background magnetic field. In the linear stage, the growth rates of MS waves are almost equal in the radial and azimuthal directions. Compared with the waves propagating along the radial direction, the waves propagating along the azimuthal direction can grow for a longer time, resulting in a larger wave amplification in this direction after saturation. The simulation results provide a valuable insight to understand the self‐consistent evolution of MS waves in the Earth's dipole magnetic field, and the findings are useful for understanding the plasma wave‐particle interaction in the Earth's radiation belts.

A Hybrid Theory-Driven and Data-Driven Modeling Method for Solving the Shallow Water Equations

In recent years, mountainous areas in China have faced frequent geological hazards, including landslides, debris flows, and collapses. Effective simulation of these events requires a solver for shallow water equations (SWEs). Traditional numerical methods, such as finite difference and finite volume, face challenges in discretizing convection flux terms, while theory-based models need to account for various factors such as shock wave capturing and wave propagation direction, demanding a high-level understanding of the underlying physics. Previous deep learning (DL)-based SWE solvers primarily focused on constructing direct input–output mappings, leading to weak generalization properties when terrain data or stress constitutive relations change. To overcome these limitations, this study introduces a novel SWE solver that combines theory and data-driven methodologies. The core idea is to use artificial neural networks to compute convection flux terms, and to reduce modeling complexity. Theory-based modeling is used to tackle complex terrain and friction terms for the purpose of ensuring generalization. Our method surpasses challenges faced by previous DL-based solvers in capturing terrain and stress variations. We validated our solver’s capabilities by comparing simulation results with analytical solutions, real-world disaster cases, and the widely used Massflow software-generated simulations. This comprehensive comparison confirms our solver’s ability to accurately simulate hazard scenarios and showcases strong generalization on varying terrain and land surface friction. Our proposed method effectively addresses DL-based solver limitations while simplifying the complexities of theory-driven numerical methods, offering a promising approach for hazard dynamics simulation.

Lever-type inertial amplification plates for low-frequency vibration reduction

In this work, a lever-type inertial amplification (LIA) metastructure is proposed to enhance the vibration reduction capability of thin plates in the low-frequency range. The mechanism consists of a rigid lever hinged on the host plate at two separate locations and a small mass mounted at the end of the lever. Band-structures of the unit cell and vibration transmissibility of the finite LIA plate are computed by using the combination of analytical method and finite element method. Numerical results show that compared with the local resonant (LR) plates with the same added mass, the proposed LIA plates generate wider and deeper complete bandgaps at the same target frequency. The Bragg bandgaps occurring at higher frequencies are also broader compared with those of the LR plates due to a stronger impedance mismatch between the host plate and the attachments. Moreover, the vibration reduction performance of the LIA plate presents unique direction-dependent characteristics: maximum vibration attenuation is achieved when the LIA mechanisms are installed parallel to the wave propagation direction. To address the issue that single-level LIA takes up too much space to install, a multi-level LIA design is proposed. On one hand, the multi-level design makes the LIA mechanisms much more compact without hampering their vibration reduction performance; on the other hand, the combination of sub-levels greatly enhances the design flexibility of LIA mechanisms. Finally, experiments are performed to further confirm the vibration reduction performance of the proposed LIA plates.

Acoustic Waves in Piezoelectric Layered Structure for Selective Detection of Liquid Viscosity.

The acoustic waves of higher orders propagating in a layered structure consisting of a silicon plate coated with piezoelectric ZnO and/or AlN films were used for the development of a sensor with selective sensitivity to liquid viscosity η in the range of 1-1500 cP. In that range, this sensor possessed low sensitivity to liquid conductivity σ and temperature T in the ranges of 0-2 S/m and 0-55 °C, respectively. The amplitude responses insensitive to the temperature instead of the phase were used to provide the necessary selectivity. The sensor was based on a weak piezoactive acoustic wave of higher order. The volume of the probes sufficient for the measurements was about 100 μL. The characteristics of the sensors were optimized by varying the thicknesses of the structure layers, number of layers, wavelength, wave propagation direction, and the order of the acoustic waves. It was shown that in the case of the layered structure, it is possible to obtain practically the same selective sensitivity toward viscosity as for acoustic waves in pure ST, X quartz. The most appropriate waves for this purpose are quasi-longitudinal and Lamb waves of higher order with in-plane polarization. It was found that for various ranges of viscosity η = 1-20 cP, 20-100 cP, and 100-1500 cP, the maximum sensitivity of the appropriate wave is equal to 0.26 dB/cP, 0.087 dB/cP, and 0.013 dB/cP, respectively. The sensitivity of the waves under study toward the electric conductivity of the liquid is much less than the sensitivity to liquid viscosity. These two responses become comparable only for very small η < 2 cP. The waves investigated have shown no temperature responses in contact with air, but in the presence of liquid, they increase depending on liquid properties. The temperature dependence of liquid viscosity is measurable by the same sensors. The results obtained have shown the possibility of designing acoustic liquid viscosity sensors based on multilayered structures. The set of possible acoustic waves in layered structures possesses modified propagation characteristics (various polarization, phase velocities, electromechanical coupling coefficients, and attenuations). It allows choosing an optimal acoustic wave to detect liquid viscosity only.

Analysis of Sharp Eagle Oscillating Surge Wave Energy Converter Based on a Two-Dimensional Numerical Wave Flume Model

To investigate the overtopping and slamming phenomena that occur in the interactions between waves and oscillating surge wave energy converters (OSWECs), a two-dimensional numerical wave flume was established using computational fluid dynamics (CFD) software Fluent by adding the momentum source terms into the original Navier–Stokes equation. Numerical convergence studies of the mesh sizes and time steps were firstly performed to ensure the sufficient accuracy of the numerical model. The variations in the wave heights along the wave propagation direction in the wave-generating area, working area, and wave-absorbing area were analyzed. The dynamics of the flap-type OSWEC were simulated using the overset mesh function embedded in Fluent. In addition, the numerical results were compared with the experimental data, and good agreements were achieved. External torque was applied to the hinge joint of the OSWEC to simulate the forces due to the power take-off (PTO) system, and the identified optimal PTO damping was compared with the numerical results based on the potential flow theory, which verified the correctness of the numerical PTO system. On this basis, nonlinear wave slamming by the sharp-eagle OSWEC was analyzed. The results show that under certain incident wave conditions, the sharp-eagle OSWEC can effectively reduce the maximum rotation angle and angular velocity compared with those of the flap-type OSWEC, and there is no overtopping that occurring for the sharp-eagle OSWEC. Furthermore, the sharp-eagle OSWEC performs better than the flap-type OSWEC.

Wave mode analysis of a turbine guide vane-integrated rotating detonation combustor based on instantaneous frequency identification

Owing to the pressure gain combustion characteristics, the gas turbine can enhance the system thermodynamic cycle efficiency by integrating a rotating detonation combustor (RDC). In this study, an experimental model of a hydrogen/air RDC with a variant installation of turbine guide vanes (TGVs) was developed. A mathematical model was proposed to identify the rotating detonation wave (RDW) propagation direction. The accuracy of the calculated RDW instantaneous velocity was improved, and the range of velocity fluctuations narrowed from 200 to 1700 m s−1 to 800–1600 m s−1. The impact of TGV installation on the rotating detonation characteristics was investigated. Both the RDW frequency and static pressure increased by installing the TGV at the combustor outlet. The propagation direction of the initial detonation wave induced the RDW to propagate along the same direction. TGV installation reduced the discrepancy in the RDW velocity between two opposite directions as well as the propagation direction changing frequency, while the inverse TGV installation achieved better optimization. The high-frequency pressure oscillations attenuated at the downstream of the TGV, and the amplitude varied depending on the TGV installation options. The static pressure decreased by 14%–16% in both TGV installation options when the combustion flow travelled through the TGV.

Experimental investigation on the interaction between mode-2 internal solitary wave and horizontal transverse cylinder

The experimental study on the interaction between a mode-2 internal solitary wave (ISW) and a horizontal transverse cylinder has been carried out in a large gravity-stratified fluid flume. One kind of making-wave method of “rotating-blade-gate” is proposed to achieve the experimental simulation of the standard mode-2 ISW in the laboratory flume. The conductivity probe array and particle image velocimetry are used to measure the mode-2 ISW and its wave-flow structure of the interaction with the horizontal transverse cylinder, and the micro-amplitude force sensor is used to measure the forces of the mode-2 ISW on the cylinder model. It is shown that the mode-2 ISW geometric structure is characterized by a convex and concave oval wave envelope shape and the counterclockwise and clockwise circulations are formed in its convex and concave domains, respectively. The resulting pair of upper and lower antisymmetric circulations moves forward together, which causes a horizontal strong flow at the symmetric center of the wave envelope consistent with the wave propagation direction. The horizontal transverse cylinder is subjected to the combined action of horizontal flow, vertical flow, and density change induced by the mode-2 ISW, in which the horizontal strong flow, induced flow separation, and vortex structure located at the symmetric center of the wave envelope are the signature features that are different from other positions. The theoretical analysis model of the forces exerted by the mode-2 ISW on a horizontal transverse cylinder is established. The spatial distribution characteristic of the force exerted by the mode-2 ISW on a horizontal cylinder is obtained, and the essential reason for the maximum horizontal and vertical loads on the cylinder is revealed. The horizontal maximum load corresponds to the inhomogeneous change of the horizontal velocity along the depth, and the vertical maximum load depends on the maximum density change along the depth, of which variation trend corresponds to the linear change of wave amplitudes.

Comprehensive analysis of band gap of phononic crystal structure and objective optimization based on genetic algorithm

In this paper, a type of acoustic metamaterial structure is proposed, which provides a new idea for the structural design of metamaterials with specific wave propagation direction. The structure is named snowflake-like square phonon crystal structure (SLS). Firstly, based on Bloch theorem and finite element method, the energy band curve of the structure is obtained, and the low frequency band gap in the range of 300–600Hz is obtained, and the band gap is analyzed comprehensively with the main band gap. It includes the vibration modal analysis at the corresponding point of the boundary of Brillouin zone and the establishment of a corresponding simplified model to explain the mechanism of band gap opening and closing more deeply. At the same time, the band gap characteristic of SLS structure is verified by the transmission curve, and the difference of elastic wave transmission results between inside and outside the band gap is given. Not only that, the real propagation direction of the band gap is described by group velocity and equal frequency diagram. The results show that in the range of relatively low frequency elastic waves, the propagation of elastic waves in the structure has a certain direction and region, that is, the structure can play a good effect of vibration and noise reduction. In addition, after the comprehensive analysis, this paper analyzes the influence trend of various factors on the band gap of the structure, and establishes the optimization model of the target band gap based on genetic algorithm, and finally obtains the optimized SLS structure. The results show that the band gap width is more than 2.5 times larger than the initial band gap width. It shows that even under the specific structure, the optimization space of the band gap still has a high lifting upper limit.

Quantifying the anisotropic elasticity of 3D printed phononic artifacts with ultrasound for process monitoring

The layer-wise additive building technique utilized by 3D Printing (3DP)/Additive Manufacturing (AM) processes results in builds and parts with strong anisotropy. Also, with 3DP/AM, the intentional creation of intricate internal geometries and regulated compositions is possible, leading to exciting opportunities for generating anisotropic structures at the material level with optimized properties and superior performance. Accurate characterization of these anisotropic properties for specific applications is essential for optimizing their design and performance, as well as monitoring their quality in real-time during production. The current study introduces a new monitoring mechanism to evaluate the anisotropic characteristics of a build by examining the ultrasonic responses of Phononic Crystal Artifacts (PCAs), which are specially designed calibration pieces that represent the geometric and mechanical complexities of the build. But the PCAs are much simpler and smaller in size, making them easier and more cost-effective to manufacture, monitor, and assess compared to the actual build. Based on the Christoffel equation, the proposed monitoring approach extracts the elastic coefficients from the ultrasonically acquired waveforms and their corresponding speeds in three principal (one build-up and two lateral) and two additional (diagonal) wave propagation directions. The printed Polylactic Acid (PLA) material exhibits anisotropy of 3.07 % to 8.91 % in the reported experiments. The extracted directional elastic parameters are determined as the directional Young's moduli E1 = E2 = 2.59 GPa, E3 = 2.22 GPa; shear modulus, G13 = 0.94 GPa, and Poisson's ratios, υ12 = 0.29, and υ13 = 0.34. The obtained elastic parameters show that the lateral directions dominate the elasticity of the printed material in the build-up direction. The presented approach for non-destructive ultrasonic characterization, which is used to investigate the directional mechanical properties, can also be beneficial in developing a real-time quality monitoring system for 3DP/AM.

Ion-driven Instabilities in the Inner Heliosphere. II. Classification and Multidimensional Mapping

Linear theory is a well-developed framework for characterizing instabilities in weakly collisional plasmas, such as the solar wind. In the previous installment of this series, we analyzed ∼1.5M proton and α particle velocity distribution functions (VDFs) observed by Helios I and II to determine the statistical properties of the standard instability parameters such as the growth rate, frequency, the direction of wave propagation, and the power emitted or absorbed by each component, as well as to characterize their behavior with respect to the distance from the Sun and collisional processing. In this work, we use this comprehensive set of instability calculations to train a machine-learning algorithm consisting of three interlaced components that: (1) predict if an interval is unstable from observed VDF parameters; (2) predict the instability properties for a given unstable VDF; and (3) classify the type of the unstable mode. We use these methods to map the properties in multidimensional phase space to find that the parallel-propagating, proton-core-induced ion cyclotron mode dominates the young solar wind, while the oblique fast magnetosonic mode regulates the proton beam drift in the collisionally old plasma.

Tsunami waveform forecasting at cooling water intakes of nuclear reactors with deep learning model

The Fukushima nuclear disaster highlights the importance of accurate and fast predictions of tsunami hazard to critical coastal infrastructure to devise mitigation strategies in both long-term and real-time events. Recently, deep learning models allowed us to make accurate and rapid forecasts on high dimensional, non-linear, and non-stationary time series data such as that associated with tsunami waveforms. Thus, this study uses a one-dimensional convolutional neural network (CNN) model to predict waveforms at cooling water intakes of nuclear power plant at Uljin in South Korea. The site is particularly vulnerable to tsunamis originating from the west coast of Japan. Data for the CNN model are generated by numerical simulation of 1107 cases of tsunami propagation initiating from fault locations. The time series data for waveforms were predicted at 13 virtual gauges located in the nearshore region of the study area, 10 of which were classified as observation points and 3 gauges situated at the cooling water intakes were categorized as target locations. The performance assessment of the model's forecasts showed excellent results with rapid predictions. The study highlights two main points: (i) deep learning models can be based on sparse waveform in situ data (such as that recorded by deep-ocean assessment and reporting of tsunamis or any locally operating monitoring stations for ocean waves) or numerically simulated data at only a few points along the dominant wave propagation direction, and (ii) deep learning models are fully capable of accurate and fast predictions of complex geo-hazards that prompt rapid emergency response to coordinate mitigation efforts.

Near-field vibration induced by single-hole blasting under different initiation modes

In drilling and blasting, initiation modes determine the propagation direction of detonation wave, which affects the distribution of blast vibration field. In this study, the near-field distribution characteristics of peak particle velocity (PPV) induced by different initiation modes were obtained based on the solution model of the Heelan equivalent elements. The influence law of the initiation mode on the transmission of explosion energy was determined. Through onsite single-hole blasting experiments, the PPV resulting from different initiation modes, such as top initiation, bottom initiation, midpoint initiation, and two-point initiation, were compared and analyzed. The influence of the initiation mode on the transmission of explosion energy was also discussed. The results revealed that the PPV are positively proportional to the blasting load amplitude and the charge length, but inversely proportional to the duration and rising time of the blasting load. The PPV induced by the different initiation modes has the following relationship: bottom initiation > midpoint initiation > two-point initiation > top initiation. The effect of initiation mode on PPV gradually decreased with increasing distance from the blasting centre.

Spin wave selection in an ensemble of laterally and vertically coupled iron-yttrium garnet microwaveguides

Numerical micromagnetic modeling used to study the modes of propagation of spin waves and processes of power transfer in the considered arrays of microwaves in the configuration corresponding to excitation in two central channels of one of the layers of surface and inverse bulk magnetostatic spin waves is conducted. It is shown that the proposed structure performs the functions of spatial-frequency signal selection, the efficiency of which depends on the mutual orientation of the submagnetization field and the direction of wave propagation.

Small amplitude dust acoustic solitary waves in a magnetized dusty plasma with nonthermal distribution for both electrons and ions

The propagation of dust acoustic solitary waves (DASWs) has been investigated in a magnetized dusty plasma consisting of negatively charged dust particles, nonthermal ions and nonthermal electrons. Using reductive perturbation method, the Korteweg-de-Vries (K-dV) equation is derived. It is found that the basic structures (such as polarity, amplitude, width) of DASWs are influenced by nonthermal ions and nonthermal electrons. The results show that both compressive and rarefactive DASWs exist in such dusty plasma. It is observed that by increasing the nonthermality parameter of ions and electrons, the nonlinear wave phase velocity increases. Furthermore, nonthermal distributed ions have more effect on the nonlinear wave phase velocity of the DASWs than nonthermal distributed electrons. The dependence of the angle that the external magnetic field makes with the propagation direction of wave on the amplitude and width of DASWs has been also investigated. It was concluded that this angle strongly affects the DASWs. Finally, the effect of the magnetic field appears only in the width and not in the amplitude of the DASWs. The findings of this study can be helpful in understanding the structures of DASWs in the dusty plasma in which both nonthermal ions and nonthermal electrons exist. Such structures can be found in different regions of space and astrophysics mediums such as the upper ionosphere and Saturn’s E-ring.

Direction-Duplex Janus Metasurface for Full-Space Electromagnetic Wave Manipulation and Holography.

Janus metasurfaces, a category of two-faced two-dimensional (2D) materials, are emerging as a promising platform for designing multifunctional metasurfaces by exploring the intrinsic propagation direction (k-direction) of electromagnetic waves. Their out-of-plane asymmetry is utilized for achieving distinct functions selectively excited by choosing the propagation directions, providing an effective strategy to meet the growing demand for the integration of more functionalities into a single optoelectronic device. Here, we propose the concept of direction-duplex Janus metasurface for full-space wave control yielding drastically different transmission and reflection wavefronts for the same polarized incidence with opposite k-directions. A series of Janus metasurface devices that enable asymmetric full-space wave manipulations, such as integrated metalens, beam generators, and fully direction-duplex meta-holography, are experimentally demonstrated. We envision the Janus metasurface platform proposed here to open new possibilities toward a broader exploration of creating sophisticated multifunctional meta-devices ranging from microwaves to optical systems.

The Influence of the Strain Rate and Prestatic Stress on the Dynamic Mechanical Properties of Sandstone-A Case Study from China.

A series of conventional dynamic uniaxial compressive (CDUC) tests and coupled static dynamic loading (CSDL) tests were performed using a split Hopkinson compression bar (SHPB) system to explore the variable dynamic mechanical behavior and fracture characteristics of medium siltstone at a microscopic scale in the laboratory. In the CDUC tests, the dynamic uniaxial strength of the medium sandstone is rate-dependent in the range of 17.5 to 96.8 s-1, while the dynamic elastic modulus is not dependent on the strain rate. Then, this paper proposes a generalized model to characterize the rate-dependent strength from 17.5 to 96.8 s-1. In the CSDL tests, with increasing initial prestatic stress, the dynamic elastic modulus and dynamic strength increase nonlinearly at first and then decrease. The results show that two classical morphological types (i.e., Type I and Type II) are observed in the dynamic stress-strain response from the CDUC and CSDL tests. By scanning electron microscopy (SEM), microscopic differences in the post-loading microcrack characteristics in the behavior of Type I and Type II are identified. In Class I behavior, intergranular fracture (IF) usually initiates at or near the grains, with most cracks deflected along the grain boundaries, resulting in a sharp angular edge, and then coalesces to the main fracture surface that splits the specimen along the direction of stress wave propagation. In contrast, Class II behavior results from the combined IF and transgranular fracture (TF).