Accidental degeneracy is the only known reason behind the degeneration of 3 or more modes, giving a Dirac cone or Dirac-like cone, depending on the position of the occurrence. The generation of triply degenerate points at the center of the Brillouin zone (where the wave number k→ = 0) is rare and only happens accidentally. In this article, it is proposed to execute triple degeneracy using the simplest geometric microarchitecture of phononic crystals (PnCs). The modulation of the crystals can be performed to demonstrate multiple Dirac-like cones at Γ point by using the nondispersive deaf band obtained from the periodic structure. Thus, a deaf band based predictive model of PnCs can be realized, by proving the existence of the deaf band both numerically and experimentally. The claims have been proved and validated using a squared array of cylindrical polyvinylchloride (PVC) inclusions in an air matrix. This phenomenon yields multiple wave guiding patterns that can be practically used in many research fields.
In this work, reduced order nonlinear state of Lamb wave propagation due to stress-relaxation of composites was experimentally observed. Residual stresses in the composites are developed under tensile-tensile fatigue loading, which reduce over time during relaxation process due to viscoelastic behavior of the polymer matrix. To investigate reduction in nonlinearity of Lamb wave during stress-relaxation, fatigue loading on the composite specimens were conducted at an interval of 75k until 225k cycles for different cyclic frequencies (i.e., 2Hz, 5Hz and 10Hz), and relaxation experiments were conducted for a duration of 8hrs between two successive fatigue loading sequence. Experimental results show a 6-20% reduction acoustic nonlinearity of Lamb wave during relaxation. Reduction in nonlinearity is mainly contributed by stress redistribution at fibers and recovery of plastic strain during relaxation. This technique is imperative to explore long-term performances and conditions of advanced composite structures.
KEYWORDS: Composites, Interfaces, Finite element methods, Systems modeling, Wave propagation, 3D modeling, Structural health monitoring, Computer simulations, Mechanical engineering
In this study, the effect of different void sizes with different void contents are investigated on all coefficients of constitutive coefficients for unidirectional composites. The unidirectional composite can be assumed as a periodic structure. To fulfill this requirement, unit cells with different void contents and different void sizes are simulated. To capture the real effect of void sizes, the unit cells are modeled with different uniform void sizes with a fixed percentage of void content. To quantify all coefficients of material properties in presence of voids, the periodic boundary conditions are applied to the unit cells. The average stresses and strains are obtained using ANSYS interface. The results showed that in the fixed percentage of void content, constitutive coefficients degraded more with the smaller void sizes.
A high percentage of failures and damage propagation in materials and sensors employed in harsh industrial environments and airborne electronics is due to mechanical failure under tension and compression loads. Therefore, it is of paramount importance to test equipment reliability and ensure its survival in long missions in the presence of physical fluctuations. Mechanical testing systems (MTS) employ mechanical load in laboratories and all the scanning tests are performed after removing the sample from MTS machine. However, more precise tracking of failures and damages is possible only the moment the material is under loads. Hence, to systematically characterize and fully understand damage’s behavior, a system capable of Realtime scanning is required. The primary objective of this study is design, fabrication, and testing of a Realtime ultrasonic scanning using hydraulic arms (RUSH), which provides mechanical loads using hydraulic arms on the specimen and simultaneously scans it with ultrasonic scanning system. RUSH consists of two hydraulic pistons (for mechanical loading) and a main control unit that accurately calculates and sets the actuators’ input signals in order to generate desired load on the materials. In this paper, the system’s architecture, its mechanical structure, and electrical components are described. In addition, to verify RUSH’s performance, various experiments are carried out using unidirectional composites.
Nonlinear ultrasonic techniques have shown the prominent potential for assessing progressive damage occurred in the composite materials in their fatigue life cycles. Stress relaxation in composite material is being measured in two ways, in-situ analysis (on-line technique) using Lamb waves and Off-line technique using pressure wave (Scanning Acoustic Microscope). In this article, the progressive damage was investigated by a set of fatigue loading experiments on woven composite samples followed by a specific duration of stress relaxation in room temperature condition. A quantitative measure of stress relaxation is determined using Scanning Acoustic Microscope for a fatigue cycle of 225000 with the loading frequency of 10 Hz. To prove this claim, a well-established reduced order nonlinear state of Lamb wave due to stress-relaxation was compared with SAM data analysis. A good agreement between these two techniques is reported herein.
Extracting improved mechanical properties such as high stiffness-high damping and high strength-high toughness are being investigated recently using high symmetry interlocking micro-structures. On the other hand, development of artificially engineered composite metamaterials has significantly widen the usability of such materials in multiple acoustic applications. However, investigation of elastic wave propagation through high symmetry micro-structures is still in trivial stage. In this work, a novel interlocking micro-architecture design which has been reported previously for the extraction of improved mechanical properties has been investigated to explore its acoustic responses. The finite element simulations are performed under dynamic wave propagation load at multiple scales of the geometry and for a range of material properties in frequency domain. The proposed composite structure has shown high symmetry which is uncommon in fiber-reinforced polymer composites and a desirable feature for isotropic behavior. The existence of multiple acoustic features such as band gap and near-isotropic behavior have been established. An exotic wave propagation feature, wave trapping and attenuation, has shown energy encapsulation in a series of repeating structures in a frequency range of 0.5 kHz to 2 kHz.
With developments in software and micro-measurement technology and finite element analysis software, a threedimensional Basilar membrane finite element (BMFE) model can now be further straightforwardly created to investigate the physics and sound transfer function of basilar membrane. Numerous FE studies of the middle ear have been investigated to date, and each has its own specific advantages and shortcomings. In this conference paper, the latest PVDF-based development of the Basilar membrane and its FE modeling technology in both COMSOL Multiphysics and ANSYS software has been investigated and verified. The output responses versus the sound frequency is recorded and caparisoned.
In spite of many studies concerning the potential of auditory nerve actions, the timing of neural excitation in relation to basilar membrane motion is still not well understood. In this study, therefore, a Piezoelectric Artificial Basilar Membrane (PABM) is fabricated using Denton Explorer evaporator. The proposed dynamical system is made of polyvinylidene fluoride membrane on which 40 chromium electrodes were deposited with thickness close to 104 Å. The PABM sensor was tested with variable engineering parameters that contribute to its frequency selection capabilities. To characterize the frequency selectivity of the PABM, mechanical displacements were measured using a very precise high-resolution data acquisition board. When electrical and acoustic stimuli were applied, the measured resonance frequencies were in the ranges of 600to2000. These results demonstrate that the mechanical frequency selectivity of this PABM is close to the human communication frequency range (300–3000 Hz), which is a vital feature of potential auditory prostheses.
The objective of this study is to investigate the effect of nonlocal precursor damages through modulated constative properties on the Guided wave propagation in composite materials. To understand the effect of lower scale damage on the interaction of wave propagation in composite materials, all the constitutive coefficients need to be evaluated. Hence, a method is developed to investigate the effective material properties of damaged composite materials using the representative volume element (RVE) model. To calculate the full matrix of constitutive coefficients, periodic boundary conditions were applied on the RVE and average stresses and strains were evaluated using a finite element model. In this study, the effect of different percentages of void contents on effective material properties is presented. Further, the effect of modified material properties on the Guided wave propagation in a transversely isotropic composite plate was investigated.
In this article, the concept of simultaneous noise filtering and energy harvesting are fused to propose metamaterial (MetaWall) bricks made of rubber-metal-concrete composite, as an industrial building material. The MetaWall bricks are capable of filtering acoustics noises more effectively than conventional barriers while harvesting the electrical energy from the trapped acoustic pressure generated by the sound and vibration. MetaWall bricks are made of Acousto-Elastic Metamaterial (AEMM) unit cells as energy harvesting component in the wall. A unit AEMM cell of the brick consists of the concrete exterior, soft rubber inclusions and hard metallic resonators. To exploit the local resonance of the resonator and recover the trapped strain energy in the soft constituents of AEMM, piezoelectric wafers are placed inside each AEMM unit cell. The primary objective of the work is to examine the performance of the MetaWall bricks. A prototype of the MetaWall brick is simulated to verify the concept. The results show that the model could generate about 1.73mW power under 10KΩ resistive load.
Energy harvesters primarily depend on on a groups of unit cells to harvest energy at broadband frequencies so that each unit cell is responsible to harvest energy at a distinct frequency. Other design complexity, space, and financial profusion are required for transferring from unit-frequency to multi-frequency energy scavenging. Also, it is very unlikely to obtain expected power output if the available vibration source doesn’t match the designed loading condition (usually, unidirectional) of the device and requires rearrangement of the base structure to have projected output. In this paper we model the unique feature of acoustic metamaterial (AM), which is not only able to harvest energy at multiple frequencies using only a unit cell device, but also able to harvest energy under a variety of uncoupled (unidirectional) and coupled (multi-directional) vibration environments with an identical base structure arrangement.
Primary objective of the work is to design, fabrication and testing of a 3-dimensional Mechanical vibration test bed.
Vibration testing of engineering prototype devices in mechanical and industrial laboratories is essential to understand the
response of the envisioned model under physical excitation conditions. Typically, two sorts of vibration sources are
available in physical environment, acoustical and mechanical. Traditionally, test bed to simulate unidirectional acoustic
or mechanical vibration is used in engineering laboratories. However, a device may encounter multiple uncoupled and/or
coupled loading conditions. Hence, a comprehensive test bed in essential that can simulate all possible sorts of vibration
conditions. In this article, an electrodynamic vibration exciter is presented which is capable of simulating 3-dimensional
uncoupled (unidirectional) and coupled excitation, in mechanical environments. The proposed model consists of three
electromagnetic shakers (for mechanical excitation). A robust electrical control circuit is designed to regulate the
components of the test bed through a self-developed Graphical User Interface. Finally, performance of the test bed is
tested and validated using commercially available piezoelectric sensors.
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