Mechanical Impedance Mismatch Research Articles

Torsional Wave Propagation and Vibration Reducing of Phononic Crystal Pipe With Periodic Torsional Support

Abstract In this paper, torsional vibration band gap properties of a fluid filled pipe were studied by using the transfer matrix method (TMM). By comparing the results obtained from the fem software, the established torsional dynamic model and the proposed method were verified. The effects of pipe wall's material and parameters of support on the torsional vibration band gap properties were analyzed. Furthermore, the relationship between torsional displacement and vibration band gaps was investigated. These attenuation regions of responses show good agreement with the frequency of Bragg band gaps. Explained the locally resonant (LR) phononic crystals (PCs) band gaps form mechanism from the point of mechanical impedance mismatch theory, the results show that the peak frequency of impedance mismatch defines the beginning of both LRs and Bragg band gaps. In essence, the locally resonant is the same as periodic support from the impedance theory. The results of this paper could give some valuable suggestions on the vibration control of the pipeline system.

Numerical Study on Impact Testing of Low-Strength Materials Using Flanged Split-Hopkinson Bar Method

In this study, we investigated the effect of flange part on stress wave propagation in the compression test of low strength material using flanged Split Hopkinson Bar method. Due to the flange with a larger radius than the input/output bar, a pair of reflected waves of compression and tension with apparently different shapes from the one-dimensional reflected wave depending on mechanical impedance mismatch was observed. Depending on the relationship between the elastic wave propagation speed in the input/output bar and the gauge position, the precursive reflected wave of compression corresponds to the rising time of the stress wave, and the tensile reflected wave propagated subsequently corresponds to the falling time. It was found that when the flange is used for impact compression test of low strength material, the effect of the flanges on the reflected wave becomes small. However, it was suggested that the transmitted wave may be affected in the impact compression test of low strength material.

Dense nanopowder composites for thermal insulation

We show in this paper that composites of mechanical impedance mismatch nanopowders yield competitive thermal insulation values to aerogels. The powders are connected by a polymer matrix assuring mechanical integrity. A simple calculation for the thermal boundary resistance has been conducted based on established models.

Micromechanical Actuators Driven by Ultrasonic Power Transfer

Advances in miniature devices for biomedical applications are creating ever-increasing requirements for their continuous, long lasting, and reliable energy supply, particularly for implanted devices. As an alternative to batteries, energy harvesting and wireless power delivery are receiving increased attention. Although the former is generally only suited for lowpower diagnostic microdevices, the latter has greater potential to extend the functionality to include more energy demanding actuation such as drug release or mechanical adjustment of prosthetic devices. With this aim, we present an ultrasonic method of wireless power delivery for actuation, the novelty being that the actuation is powered by ultrasound directly, rather than via piezoelectric conversion. This paper describes a coupled mechanical system remotely excited by ultrasound at 200 kHz and providing conversion of acoustic energy into motion of a micromechanical mechanism using a receiving membrane coupled to a discrete oscillator. We address the problem of acoustic and mechanical impedance mismatch and simulate harmonic and frequency response of the system. The fabrication method is described, and the resulting devices are characterized, revealing hysteresis in their frequency response that is related to nonlinear behavior in the membrane–oscillator coupling. The experimental samples were successfully driven by an ultrasonic source, at various separations, and demonstrated maximum oscillator vibration amplitudes in the range of 9.6–9.9 $\mu{\rm m}$ . This provided a mechanical amplification with respect to the membrane in the range of 240–250.

Energy losses of nanomechanical resonators induced by atomic force microscopy-controlled mechanical impedance mismatching

Clamping losses are a widely discussed damping mechanism in nanoelectromechanical systems, limiting the performance of these devices. Here we present a method to investigate this dissipation channel. Using an atomic force microscope tip as a local perturbation in the clamping region of a nanoelectromechanical resonator, we increase the energy loss of its flexural modes by at least one order of magnitude. We explain this by a transfer of vibrational energy into the cantilever, which is theoretically described by a reduced mechanical impedance mismatch between the resonator and its environment. A theoretical model for this mismatch, in conjunction with finite element simulations of the evanescent strain field of the mechanical modes in the clamping region, allows us to quantitatively analyse data on position and force dependence of the tip-induced damping. Our experiments yield insights into the damping of nanoelectromechanical systems with the prospect of engineering the energy exchange in resonator networks.

Material characterization at high strain rate by Hopkinson bar tests and finite element optimization

In the present work, dynamic tests have been performed on AISI 1018 CR steel specimens by means of a split Hopkinson pressure bar (SHPB). The standard SHPB arrangement has been modified in order to allow running tensile tests avoiding spurious and misleading effects due to wave dispersion, specimen inertia and mechanical impedance mismatch in the clamping region. However, engineering stress–strain curves obtained from experimental tests are far from representing true material properties because of several phenomena that must be taken into account: the strain rate is not constant during the test, the specimen undergoes remarkable necking, so stress and strain distributions are largely non-uniform, and the temperature increases because of plastic work. Experimental data have been post-processed using a finite element-based optimization procedure where the specimen dynamic deformation is reproduced. Optimal sets of material constants for different constitutive models (Johnson–Cook, Zerilli–Armstrong and others) have been computed by fitting, in a least mean square sense, the numerical and experimental load–displacement curves.

Probabilistic Superposition of Energy Modes for Treating 2N-Layered Mechanical Impedance Mismatch System

A piezoelectric transducer with 2N (N=1,2,3,...) mechanical impedance mismatch layers is systematically analyzed by considering 2N energy modes or complex dynamical variables on the circuit structure representing the 2N-layered system and by preparing matrices representing energy propagation and energy exchange along probabilistic paths that are considered in a spatially alternating manner for odd-th and even-th spatial energy modes. The superposition of these energy modes, which is expressed with the infinite geometric series of a matrix or Neumann series, provides the resonance frequency and resonance intensity of the system. Each of the 2N modes is interpreted as a “local” energy mode, and their sum simply provides the total characteristics of the system. By adopting 2N=2,4,8,16,... mathematically, multiresolution analysis can be applied for the system with a spatially inhomogeneous structure.

P2-1 Probabilistic Superposition of Energy Modes for Treating 2n-Layered Mechanical Impedance Mismatch System(Short oral presentation for posters)

P2-1 Probabilistic Superposition of Energy Modes for Treating 2n-Layered Mechanical Impedance Mismatch System(Short oral presentation for posters)

Coherent to incoherent ultrasonic wave conversion in heterogeneous media

An initially coherent acoustic wave propagating in a heterogeneous medium becomes progressively incoherent. The diffusion of the acoustic wave on randomly distributed heterogeneities leads to a progressive frequency-dependent conversion from coherent to incoherent waves along the propagation direction. A simplified statistical model for this propagation of ultrasonic waves is presented. This model is valid when both the mechanical impedance mismatch and acoustical frequency are sufficiently small to approximate the effect of each heterogeneity to a single-phase lead or lag. The propagation of acoustical waves is studied by a phase gradient model which shows the progressive loss of phase coherence of the wave, i.e. the progressive transition from the coherent to the incoherent state. A critical length beyond which the loss of coherence is so high that the classical phase reconstruction methods of imaging are no longer applicable is defined and evaluated. An experimental set-up is described and the experimental results are presented and discussed and good fit between experiment and theory is found.