Half-power Bandwidth Research Articles

Sound absorption of space-coiled metamaterials with soft walls

Integrating soft components into acoustic metamaterials is an extremely promising way to enhance their adjustability and reconfigurability. Herein, we report a type of space-coiled metamaterials using soft walls to construct internal channels, denoted as soft-wall space-coiled metamaterials (SSM), exhibiting superior sound absorption performance at low frequencies. The physical mechanism of the SSM is revealed through finite element simulations and sound impedance tube-based experiments. It is demonstrated that the exceptional sound absorption of the SSM originates from the strong coupling effect between the friction-like dissipation along the internal channel and the deformation-like dissipation in the soft wall. The experimental results show that the fabricated SSM sample with a subwavelength thickness (<1/25 wavelength) features near-perfect sound absorption (reaching 0.97 at 615 Hz) and a broad half-power bandwidth (1.6 times that of the rigid-wall space-coiled metamaterials sample with the same geometric dimensions). Moreover, the SSM has excellent sound absorption performance under oblique incidence even when the incident elevation angle reaches 75°. This work may bring inspiration for developing soft-components-integrated acoustic metamaterials with compact, adjustable, and reconfigurable characteristics.

Characterization of damping in 3D metal printed gear body using parameter optimization

With the rapid development of the global economy and the widespread use of high-speed machinery, vibration and noise pollution is becoming increasingly serious. The increased energy costs, environmental pollution, and carbon emissions have prioritized sustainable development for industry. As an important link of mechanical transmission, the gear transmission vibration and noise adversely affect the physical and mental health of operators and interfere with the normal use of machinery. This paper proposes a new metal 3D printing-made vibration-damping lightweight gear model. The solid gear web is replaced with a lightening hole structure that is expected to ensure rigidity and torsional compliance while controlling the damping effects of the gear web structure. Gears were manufactured from an additive manufacturing material using a metal 3D printing technique. This work advanced an experimental analysis of the dissipation properties of metal printed gear exploiting vibration tests. A gear frequency-sweeping excitation test bench was designed and developed to simulate in-plane excitation to produce a circumferential torsional mode of the specimen. Circumference damping estimates are guided by executing vibration exciter excitation on the setup. The half-power bandwidth and modal fitting methods were arranged and compared, relying on the measured frequency response function (FRF) single-degree-of-freedom (SDOF) approximation.

Preparation of a superior damping coating and study on vibration damping properties

In this paper, a superior damping coating was prepared, which can be easily sprayed onto the samples, showing a promising application in the vibration reduction for aircraft. This paper aims to investigate the effect of filler content, size and coating thickness on the damping properties of aluminum flake samples. It is found that the vibration of the sample is significantly suppressed with the vibration reduction rate of the coating reaching 63.23%. Moreover, the damping ratio of the coating was calculated using the half-power bandwidth method, based on which the finite element simulation model was established. The simulation results are in perfect agreement with the test ones, and the maximum vibration amplitude error stayed within 9.06%. These results fully demonstrated the effectiveness and practicality of the damping coating.

Free vibration behaviour and some mechanical properties of micro particle reinforced epoxy composites

Abstract The epoxy composite specimens of certain sizes were produced by adding graphite, silicon carbide and boron carbide microparticles separately at the rates of 5, 10 and 15 wt% to the epoxy resin. Free vibrations of these composite samples were investigated experimentally in a simple supported vibration test setup. Frequency spectrums were obtained by modal analysis method in this experimental setup. The damping ratios were calculated using the half power bandwidth method. The mechanical properties of these composite specimens were also determined by applying the tensile test. By controlling the obtained vibration values theoretically, the effects of microparticle contributions to these specimens at certain rates on natural frequency and damping properties were investigated in detail. The maximum reduction in natural frequency was observed at 5 wt% of each reinforcement. Only the first natural frequency of the 15 wt% boron carbide reinforced epoxy composite was slightly increased. The damping ratio was increased at 10 wt% graphite added and 5 wt% boron carbide added epoxy composites compared to pure epoxy, but decreased in other composite specimens.

Development and Verification of Wireless Vibration Sensors

Structural vibration testing is an effective guarantee for the Structural Health Monitoring (SHM) of large-scale civil engineering. Traditional vibration testing has drawbacks such as difficulties in wiring and picking up low-frequency signals, low communication speed, and susceptibility to testing site conditions. In order to improve the universality of wireless vibration sensors, this article develops a wireless vibration sensor, introduces the module composition and basic principles of the sensor, and conducts standard vibration table performance comparison tests between wired acceleration sensors and wireless vibration sensors, verifying the accuracy of wireless vibration sensors. In order to further explore the feasibility of wireless vibration sensor applications, the wired acceleration sensor and wireless vibration sensor were used to analyze the structural dynamic characteristics of the four-layer steel frame structure model in the laboratory, and the comparison was made based on ABAQUS finite element simulation. Finally, the field vibration test was carried out outdoors. The results show that the natural frequency identification results of the wireless vibration sensor and the wired acceleration sensor for the four-story steel frame structure through fast Fourier transform, short-time Fourier transform, and wavelet transform are basically the same, the half-power bandwidth method and logarithmic decrement rate method are used to identify the damping, and wavelet transform is used to identify the vibration mode with minimal error and high accuracy. It shows that the wireless vibration sensor is feasible in practical engineering, has stable and reliable transmission capacity, and can provide certain reference values for earthquake monitoring, building Structural Health Monitoring, etc.

Performance enhancement of electromagnetic vibration energy harvesters via bi-stable structure based on MEMS 3D coils

In this study, we conducted a systematic exploration of a micro-electromagnetic vibration energy harvester with a bi-magnet structure and microelectromechanical systems 3D coils. First, we establish a physical model of stiffness and damping characteristics based on the superposition principle for the bi-magnet structure which has also been verified experimentally. Then, we investigate the influence of magnet gap and air gap on the stiffness and the magnetic flux change rate, mainly focusing on the distance and the potential barrier between the two potential wells. Finally, we fabricate and assemble the bi-magnet VEH prototype and tested the output performance under 1 and 6g excitation, which correspond the intra-well vibration and inter-well vibration, respectively. The tested results show that under 1g excitation, the prototype can output 155.38 μW power with 32 Hz half-power bandwidth and 4939.06 μW·Hz integrated power, which are obviously higher than those for mono-magnet under same excitation. When the excitation raises to 6g, the output power is improved to 362.98 μW with the half power bandwidth and integrated power enhanced to 56 Hz and 9289.96 μW·Hz, respectively. The tested results also prove that the structure proposed in this study can significantly enhance the output performance compared with a mono-magnet structure and other published data.

Experimental and Numerical Analysis of the Performance of Various Damper Materials

The vibration in ships causes structural fatigue, damage to electrical and mechanical devices, excessive level of noise and discomfort to the passengers and crews. Sometimes the increase in ship vibration leads to unsafe operating conditions and thereby discarding it. Thus, the research on vibration reduction is essential in the maritime field and this paper deals with the reduction of vibrations in marine machinery using elastomeric bearings. The numerical analysis of the performance of passive vibration isolators with different damping materials was conducted using ANSYS and the results are validated experimentally. With the help of a vibration exciter, vibration transmissibility at different frequencies is plotted and the hysteresis curve was generated numerically by applying a cyclic load to the bearing. Then, the best damping material was deducted by comparing the calculated loss factor of different elastomeric materials such as natural rubber, styrene-butadiene rubber and polybutadiene Rubber (PBR) and their combinations. The natural rubber blended with styrene-butadiene rubber (SBR) and polybutadiene rubber (PBR) shows an increase in the damping property of the elastomer and can be used for high-frequency damping applications. HIGHLIGHTS Study of damping property of various elastomers. Experimental as well as numerical analysis were done to find the loss factor of elastomers The transmissibility curve of the elastomers was plotted at different frequencies ranging from 10 - 4000 Hz using a vibration exciter and the loss factor was calculated using half power band width method Hysteresis curve of elastomers were generated numerically using ANSYS software and the loss factor of the elastomers was calculated theoretically GRAPHICAL ABSTRACT

Unusual nonlinear switching in branched carbon nanotube nanocomposites

In this experimental study, we investigate the nonlinear dynamic response of nanocomposite beams composed of polybutylene terephthalate (PBT) and branched carbon nanotubes (bCNTs). By varying the weight fraction of bCNTs, we obtain frequency response curves for cantilever specimens under harmonic base excitations, measuring the tip displacement via 3D scanning laser vibrometry. Our findings reveal a surprising nonlinear softening trend in the steady-state response of the cantilevers, which gets switched into hardening for higher bCNT weight fractions and increasing oscillation amplitudes. The interaction of bCNTs with the thermoplastic hosting matrix results in stick-slip hysteresis, causing a softening nonlinearity that counteracts the geometric hardening associated with the nonlinear curvature of the first mode of the cantilever. However, when the weight fraction of bCNTs is greater than 1%, the bridging of the branched CNTs leads to the formation of a strong network that contributes to the hardening response at higher oscillation amplitudes. This mechanical behavior is detected by the trend of the nonlinear harmonic spectra and the equivalent damping ratio estimated using the half-power bandwidth method. To predict the observed unusual experimental behavior, we use a nonlinear mathematical model of the nanocomposite cantilever samples derived from a 3D mesoscale hysteretic model of the PBT/bCNT material. Our results suggest that the presence of bCNTs in a thermoplastic matrix is the main driver of the highly tunable nonlinear stiffness and damping capacity of the material. The reported experimental and modeling results provide valuable insights into the nonlinear dynamic behavior of PBT/bCNT nanocomposites and have potential applications in the design of advanced materials with tailored mechanical properties.

Analysis and design of power and bandwidth of nonlinear vibration energy harvesters with hardening restoring forces

This paper presents a theoretical foundation for designing the power and bandwidth performance of an electromagnetic nonlinear vibration energy harvester with a hardening resonator. To this end, the steady-state solution derived via first-order approximation is investigated to establish a graphical approach, which gives a clear perspective on how the resonance peak point on the frequency–displacement curve is determined in terms of the excitation amplitude, nonlinear restoring force function, and other design parameters. Then, a ν-power bandwidth, a generalized version of the half-power bandwidth, is newly introduced and its approximate analytical formulation is derived. Based on the findings of the parameter study on the ν-power bandwidth, a design scheme is proposed, which begins with specifying the location of the resonance peak point that guarantees the existence of the high-energy branch under possible variations of the excitation, followed by determining the linear natural frequency that makes the ν-power bandwidth as large as reasonably possible. Design examples, assuming a specific class of hardening nonlinearity of the restoring force, are shown to demonstrate how the proposed design scheme yields vibration energy harvesters with maximized power bandwidth performance while incorporating given design requirements.

Evaluating the dynamic characteristics of microwave-casted metal matrix composite material by using experimental modal analysis

This paper proposes an experimental modal analysis technique to measure the dynamic characteristics of microwave-casted metal matrix composite (MMC). To fabricate MMC, silicon carbide (SiC) is reinforced into the SS202 matrix. The damping capacities of MMC at different reinforcements of SiC powder such as 0.05%, 0.1% and 0.2% are measured through experimental modal analysis. The main objective is to study the effect of reinforcement on the damping capacity of SS202. The domestic microwave oven with the specification of 2.45 GHz and 900 W power is used to cast the MMC material. The graphite split mold is used for in-situ microwave casting. The mechanical characterization in terms of micro-hardness and scanning electron microscopy (SEM) of MMC is also examined. From SEM imagining, complete diffusion of the SiC in SS matrix is observed for 0.05% of SiC reinforcement; however, incomplete diffusion is observed for 0.1% and 0.2% of SiC reinforcement. It is observed that the hardness of the SS202 is increasing with the decrease in the percentage of SiC powder. The experimental modal analysis is carried out by using SO Analyzer. The response model in terms of the frequency response function (FRF) curve is measured when the specimen is excited at impact excitations. The half power bandwidth method is used to calculate the damping factor of the material from the measured FRFs. It is observed that the damping capacity increases with the increase in percentage of SiC in SS202. SS202 with 0.05%, 0.1% and 0.2% SiC have the hardness value of 155.33 HV, 107.33 HV and 58.33 HV, respectively.

Recommendations and requirements for the wavelengths in Rayleigh equation anomaloscopes.

DIN 6160:2019 is a technical standard that sets requirements for Rayleigh equation anomaloscopes. Table 1 of the standard contains the limits for centroid wavelengths and spectral half power bandwidths (SHBW). The centroid limits are more restrictive than dominant wavelength recommendations. The SHBW limits have no known evidence base and are inconsistent between colors. The spectral characteristics of three commercial anomaloscopes brands were measured using a telespectroradiometer. Only the oculus instruments complied with DIN 6160 Table1, but all the anomaloscopes complied with published recommendations. All complied with the DIN 6160 bandwidth requirements. This highlights the need to provide an evidence base for such requirements.

A novel evaluation method for rolling energy losses of tacked vehicle road wheels using experimental modal analysis

A novel evaluation method for rolling energy losses of road wheels of tracked vehicle is proposed, in which damping of road wheel surface is identified based on single vibration excitation and single point picking up frequency response function using a simplified modal experiment with an acceleration sensor. Three road wheels of tracked vehicle with different tread rubber are utilized as specimens during the modal experiments. The half power bandwidth method is employed to identify the viscous damping parameters. The damping parameters of road wheels are ranked by their values, and then these values of every road wheels are compared with their corresponding rolling energy losses to validate their correlative relationship. Moreover, the nonlinear deformation and stiffness of road wheels are investigated about their correlation with the rolling energy losses through model development and experimental validation. The results prove that the damping ratios of road wheels are correlated well with the rolling energy losses for all the three road wheels. The proposed evaluation method could effectively evaluate the rolling energy losses of road wheels, which suggests a simplified and economical alternative over the conventional rolling energy losses experimental method of road wheels.

Design optimization of microfabricated coils for volume-limited miniaturized broadband electromagnetic vibration energy harvester

Broadband electromagnetic vibration energy harvesters (EMVEHs) can effectively convert mechanical energy into electricity from human motions with low and random frequencies. However, such applications require a small device footprint which limits the power density of the energy harvester, making it hard to generate sufficient energy for wearable electronics. Here, we present a design of a nonresonant broadband EMVEH based on a magnet array with alternating magnetic polarities and microfabricated planar coils. To enhance the power output and power density of the EMVEH, we propose a geometric optimization method for determining the coil geometry. First, the magnetic flux density (MFD) distribution and the power output under different types of excitation motions are studied by finite element analysis and numerical simulation, respectively. Based on these results, a design of a stackable microfabricated double-sided planar coil sheet consisting of 10 coil groups is chosen. Through the optimization based on the trade-off between induced voltage and coil resistance, four coil turns in each coil group is determined as the optimal design for maximizing the output power, which proved independent of the excitation conditions. Based on the optimized design, a prototype consisting of 20 layers of coil sheets connected in series on each side is fabricated, occupying only 1.73 cm3. The device is tested to have a wide half-power bandwidth of 8 Hz, a maximum output power of 346.7 μW, and a maximum power density of 200.4 μW/cm3, when connected to a matched load of 69.46 Ω under sinusoidal excitations of 2.0 g acceleration. Furthermore, we propose a preliminary optimization methodology to determine the optimal number of stacked coil layers.

Experimental and numerical analysis of different natural fiber polymer composite

ABSTRACT In this study, different combinations of Hemp, Bamboo and Coir fiber are considered as reinforcement materials, while AW106 epoxy resin and HV953 are used as hardeners. The composite lamina was prepared and tested using the hand layup technique with short and random fibers. The tensile and compressive test performed for the analysis of mechanical properties shows that 15% hemp fiber composite is better than bamboo and coir fiber composite. With the increase in fiber volume fraction mechanical properties increases up to a certain limit and a further increase in fiber volume ratio shows adverse effect. Numerical modeling of 15% hemp fiber composite, 15% bamboo fiber composite and 15% coir fiber composite has been done for the calculation of natural frequency and damping factor of natural fiber composite. Numerical modelling was performed using ANSYS 2021 R1 and the specimen was considered in the form of a cantilever beam with a size 200 mm × 20 mm × 4 mm. The damping factor was calculated with the help of the half-power bandwidth method. Numerical analysis reveals that natural frequency and damping values of hemp fiber composite are better than bamboo and coir fiber composite. The worst results were obtained for coir fiber composite. The nondimensional frequency response of hemp fiber composite is better than bamboo and coir fiber composite.

Analyses of Damping Sustainability of Additively Manufactured Nickel Alloy Components Subjected to High Strain Loading Cycles

Abstract Integrally bladed rotors are more susceptible to vibration and high cycle fatigue failure. In response, damping treatments such as viscoelastic layers, hard coatings, and particle dampers have been utilized to reduce vibration of integrally bladed rotors and promote longer service life. In recent experiments, nickel-based alloy 718 structural beams representative of turbine blades were manufactured via laser powder bed fusion (LPBF) with internal geometries containing unfused powder. Compared to fully fused beams, these beams with internal particle dampers have demonstrated a significant improvement in damping performance, suppressing vibrations by as much as 95%. However, the inherent damping of the particle dampers decreases as the strain amplitude increases when some of the unfused powder fuses to the walls of the pockets. This study investigates the effect of the location of the particle dampers within the beam on the sustainability of its damping performance. The LPBF nickel-based alloy 718 beams are subjected to successive resonance dwells with increasing strain amplitude. After each dwell, the damping performance is assessed via the half-power bandwidth method of the frequency response function. Results from this study indicate that while the location of the particle dampers within the specimen does influence when the damping begins to degrade, it is not the optimal parameter for damping sustainability. The aim of this and future work is to improve the long-term damping performance and viability of the internal particle damper so that it can be more effectively utilized to extend the lifetime of turbomachinery components.

Control system performance of a 7.3-meter radio telescope converted from telecommunication antenna to radio astronomy purposes in Malaysia

We present a crucial step in converting a commercial telecommunication antenna system into a radio astronomy telescope. This step involved the development of a newly installed and simple control system to manage the telescope movement with relatively good pointing accuracy. The control system was designed using a programmable logic controller (PLC) that is robust and affordable. Based on the performance testing, this control system achieved average accuracies of 0.95 and 1.083 arcmin in azimuth and elevation positioning respectively, and an RMS of 9 arcmin across the open sky. In addition, it reached average rotation speeds of about 5.4, 2.4, and 4.8 arcmin/s for azimuth, elevation, and tracking rate, respectively. The results also showed that the converted radio telescope has an acceptable tracking accuracy of errors below 10% of the antenna half-power bandwidth at a frequency of 1.4 GHz while generating movement speed suitable for basic radio astronomy observations. These results show that this converted small radio telescope has the basic requirements for tracking and imaging celestial radio objects, primarily via interferometry with larger radio telescopes at low observation frequencies.

MEMD-Based Hybrid Modal Identification for High-Rise Structures with Multi-Sensor Vibration Measurements

Although widely used in various fields due to its powerful capability of signal processing, empirical mode decomposition has to decompose signals separately, which limits its application for multivariate data such as the structural monitoring data recorded by multiple sensors. In order to avoid this shortcoming, a multivariate extension of empirical mode decomposition is proposed to deal with the multidimensional signals synchronously by employing a real-valued projection on hyperspheres. This study presents a hybrid modal identification method combining the multivariate empirical mode decomposition with stochastic subspace identification and fast Bayesian FFT methods to more conveniently and accurately identify structural dynamic parameters from multi-sensor vibration measurements. Deployed as a preprocessing tool, the multivariate signals are decomposed into several aligned intrinsic mode functions, which contain only a dominant component in the frequency domain. Then, the modal parameters can be identified by advanced fast Bayesian FFT and stochastic subspace identification directly. The combined method is first validated by a numerical illustration of a frame structure and then is applied in a shaking table test and a full-scale measurement under nonstationary earthquake excitation. Compared with the finite element method, the peak–pick, the half-power bandwidth methods, and Hilbert–Huang transform method, the results show that this hybrid method is more robust and reliable in the modal parameters identification. The main contribution of this paper is to develop a more effective integrated approach for accurate modal identification with the output-only multi-dimensional nonstationary signal.

An Experimental Approach for the Determination of the Mechanical Properties of Base-Excited Polymeric Specimens at Higher Frequency Modes

Structures made of the thermoplastic polymer polyether ether ketone (PEEK) are widely used in dynamically-loaded applications due to their high-temperature resistance and high mechanical properties. To design these dynamic applications, in addition to the well-known stiffness and strength properties the vibration-damping properties at the given frequencies are required. Depending on the application, frequencies from a few hertz to the ultrasonic range are of interest here. To characterize the frequency-dependent behavior, an experimental approach was chosen and applied to a sample polymer PEEK. The test setup consists of a piezoelectrically driven base excitation of the polymeric specimen and the non-contact measurement of the velocity as well as the surface temperature. The beam’s bending vibrations were analyzed by means of the Timoshenko theory to determine the polymer’s storage modulus. The mechanical loss factor was calculated using the half-power bandwidth method. For PEEK and a considered frequency range of 1 kHz to 16 kHz, a storage modulus between 3.9 GPa and 4.2 GPa and a loss factor between 9 × 10−3 and 17 × 10−3 were determined. For the used experimental parameters, the resulting mechanical properties were not essentially influenced by the amplitude of excitation, the duration of excitation, or thermal degrad.ation due to self-heating, but rather slightly by the clamping force within the fixation area.

Effect of Types of Microparticles on Vibration Reducibility of Cementitious Composites.

The vibration-reducing ability of construction materials is generally described by the damping ratio of the materials. Previously, many studies on the damping ratio of concrete have been done, such as the addition of rubber, polymer, fiber, and recycled aggregates in the concrete. However, the application of these materials in construction is limited due to their drawbacks. This paper investigated the effect of the replacement ratio and the size of the hollow glass microspheres (HGM), cenospheres (CS), and graphite flakes (GF) on the damping ratio of mortar. Furthermore, rubber particles (RP), aluminum powder (AP), and natural fiber (NF) were investigated to find if they have a combination effect with HGM. The half-power bandwidth method was conducted to obtain the damping ratio at 28 days of curing, and the compressive and flexural strength tests were also conducted to study the mechanical properties of mortar that contained HGM, CS, and GF. The results show that increases in the size of HGM and the replacement ratio of sand with HGM lead to an increase in the damping ratio. Moreover, RP and NF do not provide a combination effect with HGM on the damping ratio, whereas the application of AP results in a drastic compressive strength decrease even with an increase in damping ratio when incorporated with HGM. Besides, an increase in the replacement percentage of CS also leads to an improvement in the damping ratio, and a smaller size and higher replacement ratio of GFs can improve the damping ratio compared to other additives. As a result, CS and GF are more effective than HGM. 50% replacement ratio of CS slightly reduced the compressive strength by 6.4 MPa while improving the damping ratio by 15%, and 10% replacement ratio of samller GF can enhance the flexural strength by over 4.55% while increasing the damping ratio by 20.83%.

Adaptation of Multi Walled Carbon Nanotubes on Viscoelastic and Damping Behavior of Flax Fiber Composite

ABSTRACT Damping characteristics and viscoelastic behavior of flax/epoxy composites reinforced with carboxyl-functionalized multi-walled carbon nanotubes (COOH-MWCNT) were investigated in this study. The viscoelastic behavior was studied utilizing dynamic mechanical analysis (DMA), conducted on all the fabricated composite specimens and neat-resin (pure epoxy). The results revealed an improvement in glass transition temperature (T g) for flax epoxy composites with 0 wt.%, 0.5 wt.%, and 1 wt.% reinforcement of MWCNT when compared to the free-resin. From the obtained graphs, C-factor was estimated, adhesion factor and Cole–Cole plots were drawn. The frequency response function (FRF) plot, obtained from free vibration testing, unveils the natural frequencies at mode-I and mode-II vibration, and their respective damping ratios are estimated by half-power bandwidth method. The damping ratios and the natural frequencies increased with the addition of MWCNT. The flax/epoxy composite reinforced with 1 wt.% of MWCNT was found to prevail over the rest by exposing great vibration damping, viscoelastic and mechanical properties.