Low Damping Properties Research Articles

Experimental investigation of wind-induced vibration of multi-cross-section steel tube lightning rods in substation

The multi-cross-section steel tube lightning rod (MSTLR) in the substation are susceptible to along-wind buffeting and vortex-induced vibration due to their inherent flexibility, low damping properties and multi-circular-section. This paper conducted a wind tunnel experiment on a 1:5 aeroelastic model of MSTLR under three wind fields with different turbulence intensity (uniform, 4.0%, and 7.0%). The amplitude, PSD, time-frequency curves, frequency components, and trajectory of wind-induced vibration are analyzed by measuring the acceleration responses at different heights and bottom reaction responses in the along-wind and cross-wind directions. A comparison was made between the relevant wind load parameters and those specified in the Eurocode. The results indicate that the distribution of the key participating modes exhibits different patterns with the variance of wind speed and turbulence intensity. Multi-order vibrations and structural coupling in orthogonal directions are observed with total response higher than the Eurocode. With the increase of turbulence intensity from 4.0% to 7.0%, the contribution of turbulence to along-wind acceleration response reaches saturation and has the opposite effects. When the speeds are 21.55 m/s and 27.33 m/s, respectively, the vortex-shedding frequencies of section 2 and section 3 coincide with the second-order natural frequency, and the vortex-induced resonance occurs. Those two vortex-induced resonances lead to a sudden increase in the standard deviation of the cross-wind response.

Programming viscoelastic properties in a complexation gel composite by utilizing entropy-driven topologically frustrated dynamical state

Hydrogel composites in an aqueous media with viscoelastic properties and elastic modulus that can be precisely tailored are desirable to mimic many biological tissues ranging from mucus, vitreous humor, and nucleus pulposus as well as build up biosensors. Without altering the chemistry, tuning the physical interactions and structures to govern the viscoelastic properties of the hydrogels is indispensable for their applications but quite limited. Here we design a complexation gel composite and utilize the physical principle of topologically frustrated dynamical state to tune the correlated structures between the guest polycation chains and negatively charged host gels. We precisely quantify the mesh size of the host gel and guest chain size. By designing various topologically correlated structures, a viscoelastic moduli map can be built up, ranging from tough to ultrasoft, and from elastic-like with low damping properties to viscous-like with high damping properties. We also tune the swelling ratio by using entropy effect and discover an Entropy-driven Topologically Isovolumetric Point. Our findings provide essential physics to understand the relationship between entropy-driven correlated structures and their viscoelastic properties of the complexation hydrogel composites and will have diverse applications in tissue engineering and soft biomaterials.

Preparation and characterization of high damping poly(epichlorohydrin) and poly(epichlorohydrin-co-ethylene oxide-co-allyl glycidyl ether) elastomers: I. Effect of curing system and blending on the damping properties

In this study, the effect of curing system on the rheological, network structure, mechanical and damping properties of poly(epichlorohydrin) (CO) and poly (epichlorohydrin-co-ethylene oxide-co-allyl glycidyl ether) (GECO) elastomers and also blend of CO and GECO were investigated. To identify the curing system's effect on elastomers damping behavior, ethylene thiourea (ETU) and 2,4,6-Trimercapto-s-triazine (TMT) were used as curing agents. To determine the impact of blending on the damping properties of epichlorohydrin-based elastomers, the homopolymer of epichlorohydrin (CO-DP5245) and the terpolymer of epichlorohydrin (GECO-T3108) was mixed in equal proportions. It has been observed that with the use of TMT instead of ETU for both CO and GECO elastomers, the mechanical properties can be significantly increased without changing the damping behavior. CO-DP5245, which has a much higher long chain branching value than GECO-T3108, shows effective damping (tanδ > 0.3) at particularly high temperatures (between −1.7oC to +70oC), while GECO-T3108 has been found to show effective damping at low temperatures (between −41.6oC and − 8.3oC). By blending the homopolymer DP5245 and terpolymer T3108 of epichlorohydrin, it has been proven that elastomers with high damping properties can be easily prepared for moderate temperatures where these elastomers have relatively low damping properties. As a result of these studies, it has been proven that the mechanical, dynamic-mechanical, damping, and energy dissipation properties of ECH-based elastomers can be changed in a controlled manner by changing the type of curing agent by preparing compounds in the presence or absence of resin or by using high damping ECH homo and terpolymer blends.

Research on damping properties and microscopic mechanism of polyurethane cement-based composites

The normal concrete is widely used in bridge, high speed railways and high-rise buildings. However, the low damping properties cannot provide enough energy dissipation. Therefore, it is of great significance to improve the damping properties of normal concrete. Polyurethane has the characteristics of high damping and high strength, which maybe a useful method to improve the damping properties of concrete. In this paper, eleven groups of polyurethane-based concrete (PUCC) mix proportions were designed. The different contents of basalt aggregate (BA), polyethylene fiber (PF) and rubber powder (RP) were added. The mechanical properties of PUCC were tested by universal testing machine. Moreover, the damping properties of PUCC were investigated by data acquisition & signal processing (DASP) vibration and control test system on the base of free vibration attenuation method and INV damping meter method. The mechanism of the different admixtures on PUCC damping were analyzed. The results show that the compressive strength of reference group (RF) reaches 43.3 MPa and the tensile strength reaches 6.89 MPa. Moreover, the damping ratio of PUCC reaches 3.168 %. Compared with RF, the damping ratio of PUCC after adding basalt (30 %) was increased by 51 %. A huge number of interfaces were introduced into the matrix after adding the basalt. The damping was increased because of friction slip between basalt and matrix. The damping ratio of PUCC after adding polyethylene fiber was only slightly improved. Although the interface transition zone between the polyethylene fiber and the matrix is large, which is prone to slip friction. The surface of polyethylene fiber is smooth, the energy dissipation efficiency will be reduced. The damping ratio of PUCC after adding rubber powder (40 %) was significantly increased, which was 262 % higher than that of RF. Rubber powder has strong deformation ability. When subjected to external force, the rubber powder dissipates energy through its own contraction and expansion. In addition, a model is proposed to describe the acceleration-time curve of polyurethane cement-based composites. The constitutive model has a good correlation with the test results, and the simulation is accurate.

Analysis of acoustic radiation of rectangular wooden panels made of spruce, maple and cherry wood

Wood is a main component of the forest ecosystem and also a versatile raw material with enormous potential for application. Its application ranges from construction material to an important source of energy. A balanced stand of hardwoods and conifers can be assumed in the future due to the increased establishment of mixed forests. The usage of wood as an environmentally friendly material combines an optimised room aesthetics with mechanical and/or acoustic functionality. Instrument makers have long been aware of the special acoustic properties of wood. This awareness is moving into the interest of transportation. It has an increasing social acceptance. Ultimately, wood is experiencing a renaissance. Cellular materials like wood play an important role in applications where vibration amplitudes and noise need to be reduced. In general, wood is considered to have low damping properties due to its molecular structure and its relatively high modulus of elasticity. Nevertheless, further studies into its acoustic properties are essential in order to be established as an alternative substitute material. In this work, plates made of spruce, maple and cherry wood were mechanically excited in the centre on the back. The acoustic intensity of the panel on the other side was then measured as a spatial function using an acoustic sensor. On the one hand, the results showed a different vibration pattern between the softwood spruce and the hardwoods maple and cherry. On the other hand, the intensity of the acoustic radiation was different. These results were discussed on basis of the structure-dependent damping of wood.

A Practical Approach for the Mitigation of Seismic-Induced Vibrations in Slender Metallic Structures through Magnetorheological Fluid Dampers

The mitigation of seismic-induced vibrations is essential for the effective protection of buildings and occupants during earthquakes. This especially applies to slender buildings with metallic frames; in this case, the structure’s geometrical layout and relatively low damping properties favor an excessive and potentially catastrophic oscillatory response to a seismic event. Semiactive systems for energy dissipation are among the most commonly used strategies to control this oscillatory response. They offer the right balance between the reliability of passive devices and the versatility and adaptability of fully active systems. In this work, a vibration-suppression system based on dissipative bracings that integrate commercial magnetorheological fluid dampers (MRDs) was designed and validated through experimental tests on a true-scale structural model that was representative of a five-story slender building with a metallic frame. A practical and robust approach was proposed for: (1) The definition of the MRD type in compliance with a predefined mitigation target for seismic-induced accelerations on each floor of the structure; (2) The modeling of the MRDs, contribute to the dynamic response of the structural system. The approach involves a linearized formulation of the characteristic damping curves of the MRDs at different values of the activating current. By relying upon this linearization, a rapidly converging iterative process was set up to simulate the seismic response of the structure in the case of activated or deactivated dampers. The reference structure and the vibration-suppression system were then manufactured and tested on a sliding table, which provided realistic seismic excitation. The good correlation levels between the numerical predictions and the experimental measurements proved the effectiveness of the conceived system and of the approaches that were used for its design and simulation.

Impact protection of offshore structures via high specific stiffness sandwich braces with a multistable transition mechanism

ABSTRACT Offshore structures are impacted by environmental loads, and the low damping properties of metal structures make them prone to lasting vibrations. A sandwich metal brace containing an composite column with a multistable transition mechanism, which exhibits a high specific stiffness and enhanced damping, is designed. Performing a hybrid analysis of sandwich lamination and multi-stable buckling and noted: the sandwich structure conducts the effective cross section of bending stiffness of the composite column, which control critical buckling load; the eccentric end caps conduct the loading axis of the composite column, which controls the displacement increment in the postbuckling. So, the force-displacement hysteretic relation of the composite column can be effectually adjusted by the appropriate physical parameters of the asymmetric composite structure of the novel sandwich brace. The novel braces are utilized to stiffen a offshore structure, which effectively mitigates the slamming response in the finite element simulation.

The Effect of the Decorative Surface Layer on the Dynamic Properties of a Symmetric Concrete Slab

In the past few years, the immense advances in building materials and construction techniques have inspired the development of large span, light, and flexible structures with low damping. The low frequency and low damping properties of the mentioned structures result in the problem of serviceability caused by human-induced vibrations. An evaluation of the serviceability of a structure requires obtaining the modes and natural frequencies of the structure via the finite element method (FEM). In the design stage, the structural model considers the contribution of involved elements made to the stiffness of the whole structure, such as beams, slabs, and columns, while the decorative surface layer above the floor is often regarded as an additional mass, regardless of its contribution to the stiffness of the floor slab. In this study, the dynamic properties of a symmetric concrete slab were tested with an ambient excitation method to obtain the dynamic properties of the original empty structure and the structure decorated with a tiled surface, a marble surface, and a terrazzo surface, respectively. The results show that the first-order natural frequencies of floor slabs decorated with tile, marble, and terrazzo finishes are decreased compared to the original empty structure, while the second- and third-order ones are increased, which indicates that it is improper to treat decorative finishes purely as an additional mass. By equating the decorative layer to a certain thickness of additional concrete layer in the finite element model, it is found that, if the decorative surface layer is equated to a 29–31 mm thick additional layer and the weight of the equivalent additional layer is the same as that of the actual decorative surface, the simulation results will be in good agreement with the measured results. Moreover, the test results indicate that the first-order shape function of the structure is symmetric and its second- and third-order shape functions are antisymmetric, which is consistent with the results of simulations under FEM method. This provides a basis for designers to evaluate the contribution of the additional layers in structural serviceability analysis.

Optimal Vibration Control of Membrane Structures with In-Plane Polyvinylidene Fluoride Actuators

Different kinds of membrane structures have been proposed for future space exploration and earth observation. However, due to the low stiffness, high flexibility, and low damping properties, membrane structures are likely to generate large-amplitude (compared to the thickness) vibrations, which may lead to the degradation of their working performance. In this work, the governing equations are established at first, taking into account the modal control force induced by the polyvinylidene fluoride (PVDF) actuator. The optimal vibration control of the membrane structure is explored subsequently. A square PVDF actuator is attached on the membrane to achieve the vibration suppression. The influence of actuator position and control gains on the vibration control performance are studied. The optimal criteria for actuator placement and energy allocation are developed. Several case studies are numerically simulated to demonstrate the validity of the proposed optimization criteria. The analytical results in this study can serve as guidelines for optimal vibration control of membrane structures. Additionally, the proposed optimization criteria can be applied to active control of different flexible structures.

Shear thickening fluid damper and its application to vibration mitigation of stay cable

Stay cables are vital structure elements of cable-stayed bridges, which feature large span, high flexibility and low damping properties. The vibration control of stay cables is of very importance to ensure the serviceability and integrity of cable-stayed bridges. This paper concerns the design of a novel self-adaptive full-scale damper with shear thickening fluid (STFD) and its application to the mitigation of undesirable cable vibration. Dynamic experiments are performed to study the properties of the STFD under a series of sine wave at different frequencies but a constant amplitude. The output damping force of the STFD significantly increases as the velocity increases and a phenomenological model is proposed to fit the observed performance of the STFD. A general differential equation of the cable attached with a STFD is developed to solve the eigenvalue problem of the cable-STFD system by the finite difference method. This method also provides a tool for accurate determination of decay process and damping force. The results show that, for a cable-STFD system, the STFD produces a combined performance of friction damper and viscous damper. The features of either friction damper or viscous damper predominates at different stages of the decay process. The comparison also shows that, in general, the STFD is more effective on the mitigation of the stay cable vibration by comparing to the cable attached a viscous damper.

Experimental Investigations on Vibration Properties of Aluminium Matrix Composites Reinforced with Iron Oxide Particles

Aluminium matrix composites offer improved damping properties than other metals and its alloy. Generally pure metals and its alloys may have fairly good mechanical properties but falls short in damping properties. Aluminium matrix composites are becoming important in aerospace automobile and marine applications due to its god damping properties. The present investigation is concerned with the damping capacity of iron oxide (Fe2O3) reinforced aluminium matrix composite. The composites were fabricated with 2%, 4% and 6%, by weight of iron oxide with varied particle of size 40 μm and 500 nm in equal proportions using stir casting process. From the results obtained the 500 nm size with 4 wt% of iron oxide showed improved dynamic properties. The iron oxides reinforced with aluminum matrix are found to be new substitutes for the existing materials with low damping properties.

Active vibration control of a polyvinylidene fluoride laminated membrane plate mirror

Lightweight optical mirrors usually play key roles in aerospace and optical structural systems applied to space telescopes, radars, solar collectors, communication antennas, etc. Due to their high flexibility and low damping properties, external excitations such as orbital maneuver may induce unexpected oscillations and thus reduce their working performance. Active vibration control is therefore essential for the lightweight optical mirror systems. In this spirit, a lightweight mirror structronic system with a linear quadratic optimal controller is presented. The mirror is modeled as a membrane plate with pretension and distributed polyvinylidene fluoride sensors and actuators. The sensing sensitivity of the piezoelectric (PVDF) sensors and the modal actuation factor of the PVDF actuators are derived. The state-space equations are established and the feedback control gains between sensing and control signals are obtained. Sensor and actuator of different shape, size, and position are employed to actively control the first four natural modes of the mirror. The influences of mode order, pretension, and the two weighting factors Q and R on the control performance are also investigated. Analytical results in this paper could guide the design and layout of the PZT sensor and actuator on lightweight membrane plate mirrors.

Heavy‐Metal‐Free, Low‐Damping, and Non‐Interface Perpendicular Fe16N2 Thin Film and Magnetoresistance Device

Realization of sub‐10 nm spin‐based logic and memory devices relies on the development of magnetic materials with perpendicular magnetic anisotropy that can provide low switching current and large thermal stability simultaneously. In this work, the authors report on one promising candidate, Fe16N2, a heavy‐metal‐free, non‐interface perpendicular magnetic material and demonstrate a perpendicularly magnetized current‐perpendicular‐to‐plane (CPP) giant magnetoresistance (GMR) device based on Fe16N2. The crystalline‐based perpendicular anisotropy of Fe16N2 in the CPP GMR device is measured to be about 1.9 × 106 J m−3 (1.9 × 107 erg cm−3), which is sufficient to maintain the thermal stability of sub‐10 nm devices. A first principle calculation is performed to support this large magnitude of the perpendicular anisotropy. Moreover, the Gilbert damping constant of the Fe16N2 thin film (α ≈0.01) measured by ferromagnetic resonance (FMR) is lower than for most existing materials with crystalline perpendicular magnetic anisotropy. The non‐interface perpendicular anisotropy and low damping properties of Fe16N2 may offer a pathway for future spintronics logic and memory devices.

Distributed microscopic actuation analysis of deformable plate membrane mirrors

Abstract To further reduce the areal density of optical mirrors used in space telescopes and other space-borne optical structures, the concept of flexible membrane deformable mirror has been proposed. Because of their high flexibility, poor stiffness and low damping properties, environmental excitations such as orbital maneuver, path changing, and non-uniform heating may induce unexpected vibrations and thus reduce working performance. Therefore, active vibration control is essential for these membrane mirrors. In this paper, two different mirror models, i.e., the plate membrane model and pure membrane model, are studied respectively. In order to investigate the modal vibration characteristics of the mirror, a piezoelectric layer is fully laminated on its non-reflective side to serve as actuators. Dynamic equations of the mirror laminated with piezoelectric actuators are presented first. Then, the actuator induced modal control force is defined. When the actuator area shrinks to infinitesimal, the expressions of microscopic local modal control force and its two components are obtained to predict the spatial microscopic actuation behavior of the mirror. Different membrane pretension forces are also applied to reveal the tension effects on the actuation of the mirror. Analyses indicate that the spatial distribution of modal micro-control forces is exactly the same with the sensing signals distribution of the mirror, which provides crucial guidelines for optimal actuator placement of membrane deformable mirrors.

Experimental Study on Passive Negative Stiffness Damper for Cable Vibration Mitigation

AbstractStay cables are vulnerable to excessive vibration because of their inherently low damping properties. Described in this paper is an experimental investigation of the vibration control perfo...

Improvement of structures vibroacoustics by widespread embodiment of viscoelastic materials

Thin walled structures, e.g. aircraft and helicopter fuselages, but also domestic electrical machines, are characterized by poor acoustic behavior. This is mainly due to the very low damping properties of the high efficiency structural elements and materials, prone to transmitting bending vibrations. Such unsatisfactory behavior leads to the need for acoustic treatments, which engender installation costs, troubleshooting problems and weight penalties, especially undesirable in aircraft applications.A possible solution to improve acoustic performance is to dissipate the vibrational energy during its passage through the structure, before coupling with the air of the cabin and becoming noise. The metal–polymer–metal sandwich technique, also adopted with multiple layering, could be exploited to efficiently dissipate bending waves by viscoelastic layers placed through the thickness. Dynamic and acoustic experimental campaigns have been carried out on specimens and panels made of either aluminum alloy or carbon composite layers with viscoelastic inclusions made with styrene–butadiene rubber, testifying the correctness of the idea. The benefits of such approach are tested also in thick cases where multilayered stacking is very efficient. The approach can be exploited irrespectively to the nature of the structural material (composite or metallic) and related technological problems seem to be solved.

Control of vibrations induced by people walking on large span composite floor decks

The low damping properties of lightweight large span floor decks composed of a reinforced concrete slab on top of a steel space frame structure may lead to undesirable dynamic responses, even to ordinary human actions such as walking. This problem was investigated through laboratory tests performed on a 1:1 scale prototype of a composite floor deck structure. Experimental measurements were taken for the structure subjected to several dynamic human loads, especially those produced by the random walking of people. To compensate for the lack of damping, a passive control system was designed and installed in the composite structure prototype. The performance of the mechanical control devices was evaluated by means of straight comparisons between the experimental acceleration amplitudes obtained for the controlled and uncontrolled structure subjected to similar dynamic forces produced by one or more persons walking. The most relevant results for both time and frequency responses are presented and used to argue that small and low cost passive control devices can already be included in the design stage of a smart structure as effective accessories to substantially reduce vibrations induced by people in low damped large span composite floor decks.

Cantilever Dynamic Vibration Absorber for Reducing Optical Disk Drive Vibration

This research explores the design of a dynamic vibration absorber (DVA) using a cantilever beam with tip mass. Compared to a conventional DVA using a rubber bobbin, the proposed DVA isolates vibrations more effectively due to the low damping properties of its structure, which are the principal reason for its excellent anti-vibration performance near the anti-resonance frequency. This low damping decreases the harmonic response at the rotational frequency of the disk. To design the proposed DVA, the dynamic characteristics of the optical disk drive were represented with lumped parameter and finite element models. The dimensions of the beam were tuned to reduce the vibration in multiple axes based on this model. Tolerance analysis were performed so the drive would be robust against normal dimension variance due to the manufacturing process, and the variation in the dynamic characteristics with respect to the pickup position was investigated. The anti-vibration performance of the DVA was also measured experimentally.

Wave Propagation in Channel with Side Porous Caves

A three-dimensional numerical solution was developed to determine the wave field in a narrow channel with side porous caves. The solution was achieved by applying the boundary element method. The model was applied to determine the effect of the geometry of caves and the physical and hydraulic properties of the porous material in the caves on the wave field in the channel. The results show that the cave geometry and the physical and hydraulic properties of porous material in the cave have a significant effect on the wave field in a channel. Wave transmission down the channel usually decreases with increasing cave length and cave width and decreases with increasing porosity. Moreover, wave transmission usually decreases with increasing wave-damping properties for porous material of low damping properties and increases with increasing wave-damping properties for porous material of high damping properties. A reduction of wave transmission down the channel, often substantial, can usually be achieved by increasing the porosity and increasing the wave-damping properties of the porous material. The results and additional analysis show that a reasonable wave transmission level can be achieved even for fairly short porous caves and that the model can be applied to select an optimal cave system for any prespecified conditions. Theoretical results are found to be in reasonable agreement with experimental data.

Measurement of a high electrical quality factor in a niobium resonator for a gravitational radiation detector

The mechanical and electrical quality factors of a 10-g niobium resonator were measured at 4.4 K and were found to be 8.1×106, and 3.8×106, respectively. The value for the electrical quality factor is high enough for a system operating at 50 mK at a sensitivity level of one phonon. The resonator’s low damping properties make it suitable for use as a transducer for a cryogenic three-mode gravitational radiation detector. A practical design is given for the mounting of the resonator on a 2400-kg aluminum-bar detector. Projections are made for the sensitivity of a 2400-kg bar instrumented as a three-mode system with this resonator inductively coupled to a SQUID.