1,721,361 research outputs found
Frontiers in Materials
No full textFrontiers in Materials is a high visibility journal publishing rigorously peer-reviewed research across the entire breadth of materials science and engineering. This interdisciplinary open-access journal is at the forefront of disseminating and communicating scientific knowledge and impactful discoveries to researchers across academia and industry, and the public worldwide.
Founded upon a research community driven approach, this Journal provides a balanced and comprehensive offering of Specialty Sections, each of which has a dedicated Editorial Board of leading experts in the respective fiel
Ultrafast Wave Finite Element Method for the computation of dispersion properties in periodic viscoelastic waveguides
No full textIn this work a procedure to compute the dispersion curves for one dimensional viscoelastic waveguides exploiting the finite element based global mass matrix and complex stiffness matrix is proposed. The global matrices of a finite length portion of the waveguide, the unit cell, are post-processed by enforcing Bloch-type boundary conditions along the wave propagation direction and next used to formulate a complex k(w) quadratic eigenvalue problem. The roots of the eigenvalue problem at different frequency values w yield in addition to the wavenumber (phase velocity) information also the attenuation dispersion curves for the waveguide. No finite element coding is needed since the global mass and stiffness matrix can be obtained from commercial FE software. To improve the computational efficiency a modal reduction scheme based on the Component Mode Synthesis method has been applied to reduce the di-
mension of the eigenvalue problem. As a result the computational time is enormously reduced without loss of accuracy in the complex roots calculation
Limits of the Kelvin Voigt Model for the Analysis of Wave Propagation in Monoatomic Mass-Spring Chains
No full textIn this study, the effect of energy dissipation on harmonic waves propagating in one-dimensional monoatomic linear viscoelastic mass-spring chains is investigated. In particular, first dispersion laws in terms of wavenumber, attenuation, and wave propagation velocities (phase, group, and energy) for a generic viscoelastic mass-spring chain are derived from the homologous linear elastic (LE) expressions in force of the correspondence principle. A new formula for the energy velocity is introduced to account for energy dissipation. Next, such relations are specified for the Kelvin Voigt (KV) and the standard linear solid (SLS) rheological models. The analysis of the KV mass-spring chain in the high-frequency regime proves that the so-called wavenumber-gap is not related to energy dissipation, as assumed in previous studies, but is due to the nonphysical rigid behavior of the model at high frequencies. The SLS mass-spring chain, in fact, does not show any wavenumber-gap and at high frequencies recovers the wavenumber dispersion curve of the LE system. The behavior of the energy velocity for the different mass-spring chains confirms this conclusion
A reduced Bloch operator finite element method for fast calculation of elastic complex band structures
Full text linkThis article presents an efficient reduced formulation of the Bloch Operator Finite Element method to calculate complex band structures of periodic waveguides. The use of a Bloch operator formulation allows building and solving a Bloch eigenvalue problem along a generic wave direction, thus being not limited to the unit cell Irreducible Brillouin Zone (IBZ) edges, so that band gap directionality and material absorption in elastic and damped waveguides can be fully disclosed. The proposed Reduced-Order Modeling (ROM) exploits a small set of Bloch modes, extracted at relevant frequency locations along one or more wave directions and post-processed with a Singular Value Decomposition, to reduce the dimensions of the eigenvalue problem. The performances of the proposed numerical technique are evaluated in terms of accuracy and computational saving by analyzing a linear elastic and a damped bi-periodic stubbed plate. Results demonstrate that the reduced formulation yields accurate predictions of propagative, evanescent and complex wave solutions with a reduction in computational time of more than one order of magnitude with respect to the full model calculations. Complex band structures can thus be efficiently computed over the whole IB
A demand-capacity approach to define failure thresholds in anomaly detection monitoring systems
Full text linkThis work proposes a procedure to establish the threshold for a structural anomaly detection system using a supervised computation of the demand-capacity ratio (DCR). A truss structure is analyzed with a large dataset of damage scenarios generated through Monte Carlo simulations, varying in types and intensities of damage. For each scenario, a structural model is subjected to ultimate loads to calculate the DCR for each element, as the ratio between the demand forces and the capacity of the element. Scenarios where the DCR of any element exceeds one, are identified as those potentially leading to failure. For these scenarios, a dataset of pseudo features sensitive to damage, such as modal frequencies, is created, incorporating environmental and operational variations, as well as noise. A damage index is then constructed for each damage scenario as the Mahalanobis distance between the damaged and the healthy modal frequencies datasets. The alarm threshold is finally set at the value exceeded by the damage indexes of all scenarios leading to failure. The method is demonstrated with a numerical example of a steel truss structure under a specific load, considering 100,000 damage scenarios. Results show that the proposed threshold is conservative, not easily derived from simple engineering intuition, and effectively minimizes false alarms in the detection scheme
Extended bloch mode synthesis: Ultrafast method for the computation of complex band structures in phononic media
No full textIn this article an efficient numerical technique, named Extended Bloch Mode Synthesis, is proposed for the fast calculation of the elastic complex band structures in phononic media. The Bloch Mode Synthesis approach, originally developed for reducing the computational cost for the calculation of real band structures, is here extended to evaluate also evanescent/complex near field wave solutions by solving a k(ω) Bloch eigenvalue problem. The k(ω) Bloch eigenvalue problem is built by means of a Wave Finite Element (WFE) discretization of the unit cell combined with a Component Mode Synthesis approach. The Component Mode Synthesis approach is based on the Craig Bampton modal reduction of the interior unit cell degrees of freedom and provides a basis to reduce the Bloch eigenproblem dimension allowing for a fast computation of the complex band structures. The performances of the proposed scheme in terms of band structures accuracy and computational cost saving are demonstrated for a phononic stubbed plate, a case already used in literature as a benchmark for band structures calculation. It is shown that the complex band structures computational time can be reduced by two orders of magnitude with respect to the computational time needed to solve the full model with negligible errors for both real and evanescent/complex solutions
Phonons in Diatomic Linear Viscoelastic Chains
No full textAbstractIn this study waves propagating in a diatomic linear viscoelastic periodic system are investigated with the aim of understanding the operative range of some commonly adopted rheological models. Dispersion laws of a diatomic viscoelastic periodic system under prescribed harmonic motion, i.e. real angular frequency and complex wavenumber (wavenumber and attenuation), are derived. It is shown that such relations can be easily obtained from the linear elastic counterpart in force of the correspondence principle. The complex band structures and energy velocity for the one-dimensional diatomic periodic chains are computed considering both the Kelvin Voigt and the Standard Linear Solid models. It is proven that unusual dispersive behaviors already observed by other researchers when using the Kelvin Voigt model, such as wavenumber-gaps and strong band shifting, are only caused by its nonphysical rigid behavior at high frequencies, since they disappear once the Standard Linear Solid model is adopted. The comparison between the energy velocity of the Kelvin Voigt and Standard Linear Solid discrete systems provides a further confirmation of these findings
Design principles of seismic metasurfaces to control love waves
No full textMetasurfaces, consisting in an array of resonant inclusions or resonant elements placed at a waveguide free surface,
can be designed to interact with elastic waves and used to redirect, steer or absorb the elastic wave energy.
Metasurfaces in half spaces are capable to support the propagation of shear horizontally polarized waves and to
open band gaps in the spectrum of vertically polarized Rayleigh waves. In this work, we discuss the possibility of
designing a metasurface to achieve control of seismic Love waves.
We analytically derive the dispersion law of Love waves existing below a resonant metasurface and observe how
surface resonators can significantly modify the mode shapes and phase velocity of Love waves. Love waves can
be controlled at the seismic scale by designing a metabarrier of meter-scale resonators using steel masses restrained
by elastic connectors. We guide the design of such metabarrier using the derived analytical dispersion relation and
show its ability to manipulate Love waves in the frequency range relevant for seismic isolation
Lamb’s problem for a half-space coupled to a generic distribution of oscillators at the surface
Full text linkWe propose an analytical framework to model the effect of single and multiple mechanical surface oscillators on the dynamics of vertically polarized elastic waves propagating in a half-space. The formulation extends the canonical Lamb’s problem, originally developed to obtain the wavefield induced by a harmonic line source in an elastic half-space. In short, our approach utilizes the solution of the classical Lamb’s problem as Green’s function to formulate the multiple scattered fields generated by a cluster of mechanical resonators attached to the surface. For an arbitrary number of resonators, arranged atop the elastic half-space in an arbitrary configuration, the displacement fields are obtained in closed form and validated with numerics developed in a finite element environment. We demonstrate that our approach can correctly model elastic waves interacting with single and couples of resonators, and capture complex dynamics phenomena such as wave conversion and wave localization induced by arrays of resonators, also known as metasurfaces
A design strategy for highly directional piezoelectric transducers for Lamb waves inspections
No full textDrastic hardware simplification and cost reduction of Guided Waves (GWs) based systems can be achieved by using shaped transducers that present inherent directional capabilities when generating and sensing elastic waves. Directional transducers for GW generation and sensing are achieved by patterning the piezoelectric material lay-out and the electrodes. The peculiar electrodes’ shape produces a spatial filtering effect which is frequency-dependent,
so that a direct relationship can be established between the direction of propagation (wavenumber) and the spectral content of the transmitted/received signal. This kind of transducer has been named Frequency Steerable Acoustic Transducers (FSATs). In this work, a transducer’s shape design strategy is presented which is able to enhance the
accuracy of the desired Directivity function approximation. The proposed design strategy is based on Dithering techniques. The effectiveness of the novel transducer design methodology is shown through a numerical validation with application to defect detection in an aluminum plate
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