Damping Effect of a Continuous Mass-Distributed Viscoelastic Layer.

Constrained layer damping treatments are widely used in mechanical structures to damp acoustic noise and mechanical vibrations. A viscoelastic layer is thereby applied to a structure and covered by a stiff constraining layer. When the structure vibrates in a bending mode, the viscoelastic layer is forced to deform in shear mode. Thus, the vibration energy is dissipated as low grade frictional heat. This paper documents the efficiency of passive constrained layer damping treatments for low frequency vibrations of cylindrical composite specimens made of glass fibre-reinforced plastics. Different cross section geometries with shear webs have been investigated in order to study a beneficial effect on the damping characteristics of the cylinder. The viscoelastic damping layers are placed at different locations within the composite cylinder e.g. circumferential and along the neutral plane to evaluate the location-dependent efficiency of constrained layer damping treatments. The results of the study provide a thorough understanding of constrained layer damping treatments and an improved damping design of the cylindrical composite structure. The highest damping is achieved when placing the damping layer in the neutral plane perpendicular to the bending load. The results are based on free decay tests of the composite structure.

This work aims to further promote the research and development of flexible damping technology for the seismic damage control of underground engineering structures. On the basis of the understanding of the application of viscoelastic damping theory to the field of earthquake resistance of bridge buildings, impact tests on different rock damping layer– concrete specimens are conducted using the improved split Hopkinson pressure bar test system to analyze the energy absorption characteristics of the viscoelastic damping layers. A comparative analysis of the test results is also performed to differentiate the laws of energy absorption of the viscoelastic damping layer structure using different damping layer materials (rubber and silicone) and different damping layer shapes (honeycomb, corrugated, and cylindrical). Given damping layers with the same material and structure, the energy absorption laws of the structures with different damping layer thicknesses are compared and analyzed. The optimal thickness of the damping layers is also discussed. Results show that the incident energy absorbed by the damping layers of different materials increases by more than 10% relative to the control group (rock–concrete specimen). Such an increase indicates that the structures have superior energy absorption characteristics and that the rubber material can absorb more incident energy than the high-damping silicone as the damping layer. As for the damping layer shapes, the composite damping layer structure with a honeycomb shape achieves the best energy absorption among all structures. The comparison and analysis of the energy absorption effects when the damping layer thicknesses are 10, 20, and 25 mm reveal that a 20 mm-thick damping layer shows the best energy absorption effect. The current findings can provide theoretical foundation and data support for the application of composite damping layers to the seismic design of underground engineering structures.

Dynamic responses of flexible linkage mechanisms with viscoelastic constrained layer damping treatment

This paper describes the formulation of a theory for the prediction of damping and natural frequencies of laminated composite beams with multiple viscoelastic damping layers. The damping layers are constrained (or sandwiched) by anisotropic laminates. The in-plane shear strains of the damping layers and the constraining layers are included in the model. Closed-form solutions for the resonance frequencies and modal loss factors of the composite beam system under simple supports are derived using the energy and Ritz method. A parametric study has been conducted to study the variation of dynamic stiffness and modal loss factor of the system with structural parameters (e.g., the ply orientations of laminas, thickness of the damping layers and the laminates), operating temperature, and damping material properties. The design of composite beams for maximizing the damping capacity is also presented in this paper which includes the determination of operating temperature range corresponding to given structural parameters and finding optimal structural parameters corresponding to given temperature range. Finally, some experimental results are compared with theory for the cases of single and double damping layer beam systems that show good agreement between predicted and measured natural frequencies. © 1993 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.

This paper concerns the dynamic analysis of composite beams containing elastic and viscoelastic (VE) layers. A method for determination of the dynamic characteristics of multilayered beams with VE layers (i.e., natural frequencies, non-dimensional damping ratios and modes of vibration) is presented. The Euler–Bernoulli beam theory and the Timoshenko theory are used to describe the elastic and VE layers, respectively. To describe the mechanical properties of a VE layer, the four-parameter rheological model with fractional derivative is applied. A number of particular rheological models are particular cases of this general model. The virtual work principle and the finite element method together with the Laplace transform are used to derive the equation of motion in the frequency domain. The dynamic characteristics of a beam with VE layers are obtained as the solution to a properly defined nonlinear eigenvalue problem. The continuation method is adopted for solving the nonlinear eigenproblem. Several conclusions concerning the accuracy of the method and variability of results are presented on the basis of numerical studies.

In this work, the effectiveness of a shear actuation-based hybrid active-passive damping treatment is investigated by incorporating the inclusion of graphite particles within the viscoelastic damping layer. The study is performed through the flexural vibration analysis of a sandwich plate-strip where the core is made of a laminate of active layers and graphite particle-filled viscoelastic layers in two different stacking sequences. The active layers are comprised of shear mode piezoelectric actuator patches that are activated according to a shear-based velocity feedback control strategy. The analysis is performed by deriving a closed-loop finite element model of the sandwich plate-strip, and it reveals that the hybrid damping is significantly dependent on the stacking sequence of active and passive damping layers at the core. The inclusion of graphite particles not only provides augmented passive damping but also causes enhanced transfer of shear actuation force from the active layers to other layers. As a result, a significantly improved shear actuation-based hybrid active-passive damping is achieved due to the inclusion. The effectiveness of this hybrid damping in attenuation of resonant displacement-amplitude is also presented by configuring the volume fraction of graphite particles and shear actuator patches in an optimal manner.

The acoustic black hole (ABH) has been developed in recent years as an effective, passive, and lightweight method for attenuating bending wave vibrations in beams and plates and reducing the sound radiation and structural-acoustic response of structures. The ABH effect utilizes a local change in the plate or beam thickness to reduce the bending wave speed and increase the transverse vibration amplitude. Attaching a viscoelastic damping layer to the ABH results in effective energy dissipation and vibration reduction. Surface-averaged mobility and radiated sound power measurements were performed on an aluminum plate containing an array of 20 two-dimensional ABHs with damping layers and compared to a similar uniform plate. Detailed laser vibrometer scans of an ABH cell (including the ABH and surrounding homogeneous plate) were also performed to analyze the vibratory characteristics of individual ABH cells and compared with mode shapes calculated using finite elements. The results showed that the surface-averaged mobility was reduced by up to 14 dB for the fully damped ABH plate compared to a uniform reference plate while also reducing the mass of the plate. The results demonstrated that the dynamics of plates with embedded ABHs can be characterized by low, mid, and high frequency ranges, with low-order local ABH modes contributing significantly to low frequency ABH performance. The effects of damping layer thickness and diameter were also investigated to assess ABH performance optimization. It was shown that the damping layer can have the added benefit of mass loading the ABH and enhancing low frequency performance. The results will be useful for designing the low frequency performance of future ABH systems and describing ABH performance in terms of design parameters.

This report is about response analysis using Statistical Energy Analysis (SEA) with FEM for two panels connected in L-type shape. We adopt two panels with viscoelastic damping layers and the panel connected in L-type shape as substructures. One of the panels has a curved surface and the other panel is flat. There are viscoelastic layers on the panels. Usually, modal damping is influenced by shape of panels and forms of the connection. Therefore, to consider coupling in damping between substructures, we use FEM and MSE (Modal Strain Energy) method to identify Internal Loss Factors (ILF). We get the Coupling Loss Factors (CLF) using the Element Energy and ILF. For us to verify whether a numerical value of the proposed method is right, we compare the results from this method and those from direct FEM. As a result, we show that this method is able to get SEA parameters (ILF and CLF) of the high precision for the damped panel having curved surface. This method shows that the precision of the response calculation in the low frequency range as well as in the high frequency range could be improved.

Bending vibration of flat plates is controlled using patches of active constrained layer damping (ACLD) treatments. Each ACLD patch consists of a visco-elastic damping layer which is sandwiched between two piezo-electric layers. The first layer is directly bonded to the plate to sense its vibration and the second layer acts as an actuator to actively control the shear deformation of the visco-elastic damping layer according to the plate response. With such active/passive control capabilities the energy dissipation mechanism of the visco-elastic layer is enhanced and its damping characteristics of the plate vibration is improved. A finite element model is developed to analyze the dynamics and control of flat plates which are partially treated with multi-patches of ACLD treatments. The model is validated experimentally using an aluminum plate which is 0.05 cm thick, 25.0 cm long, and 12.5 cm wide. The plate is treated with two ACLD patches. Each of which is made of SOUNDCOAT (Dyad 606) visco- elastic layer sandwiched between two layers of AMP/polyvinylidene fluoride (PVDF) piezo- electric films. The piezo-electric axes of the patches are set at zero degrees relative to the plate longitudinal axis to control the bending mode. The effect of the gain of a proportional control action on the system performance is presented. Comparisons between the theoretical predictions and the experimental results suggest the validity of the developed finite element model. Also, comparisons with the performance of conventional passive constrained layer damping clearly demonstrate the merits of the ACLD as an effective means for suppressing the vibration of flat plates.

Lightweight design, lower fuel consumption and easier recyclability are the current concerns of the automotive industry as a response to the increasing demand for energy saving and better environmental impact of the transportation section. This framework was the opportunity to develop multi-layer materials, as an alternative to the high-density homogeneous metal sheets, maintaining the structural properties of the component. The multi-layer materials, known as composite sandwich or hybrid materials, are composed of outer metal sheets with reduced thickness and a polymeric matrix core (low density), in which high specific strengths are achieved. However, the lack of studies and information about the behavior of these non-homogeneous materials posed new challenges, either with regard to their use in the sheet metal forming processes, but also when performing material mechanical characterization. This paper presents the problems faced with bending test and springback analysis of a sandwich metal polymer sheet, compared with a high strength low alloy steel (HSLA420). Experimental tests and corresponding numerical simulations were performed using the press brake air bending process and a Numisheet benchmark test, unconstrained bending test, at room temperature, using the same experimental conditions for both materials. The numerical simulations were performed using finite element modeling. The results show that the sandwich material have problems on the structural integrity for a lower punch radius, on the outer surface (tension side). Additionally, the comparison of springback behavior showed that there is no significant difference between the materials in the study, although the non-homogeneous sandwich material presented a lower springback.

Viscoelastic material can significantly reduce the vibration energy and radiated noise of a structure, so it is widely used in lightweight sandwich structures. The accurate and efficient determination of the frequency-dependent complex modulus of viscoelastic material is the basis for the correct analysis of the vibro-acoustic behavior of sandwich structures. Based on the behavior of a sandwich beam whose core is a viscoelastic layer, a combined theoretical and experimental study is proposed to characterize the properties of the viscoelastic layer constituting the core. In this method, the viscoelastic layer is bonded between two constraining layers. Then, a genetic algorithm is used to fit the analytical solution of the frequency¬ response function of the free–free constrained beam to the measured result, and then the frequency-dependent complex modulus is estimated for the viscoelastic layer. Moreover, by varying the length of the beams, it is possible to characterize the frequency-dependent complex modulus of the viscoelastic material over a wide frequency range. Finally, the characterized frequency-dependent complex modulus is imported into a finite element model to compute the complex natural frequencies of a sandwich beam, and a comparison of the simulated and measured results displays that the errors in the real parts are within 2.33% and the errors in the imaginary parts are within 3.31%. It is confirmed that the proposed method is feasible, accurate, and reliable. This provides essential technical support for improving the acoustic vibration characteristics of sandwich panels by introducing viscoelastic materials.

This paper describes the design and evaluation of a noise suppressing hydrophone that is robust to external noise without sacrificing its performance as a receiver. To increase robustness of the receiver to the external noise, first, effects of location of the external noise on its performance are analyzed with the finite element method (FEM). Based on the results, geometrical variations are implemented on the structure with additional air pockets and damping layers that work as acoustic shields or scatterers of the noise, and fourteen trial models are developed for the noise suppressing hydrophone structures. Also acoustic impedance and damping coefficients of the material inside the acoustic walls are varied and the effects of the material property on the hydrophone response to the external noise are analyzed. The results show that the effect of the external noise is most significant when it is applied to near the mid-side surface of the hydrophone housing. The external noise is isolated most efficiently when two thin damping layers combined with five air pockets are inserted to the circumference of the hydrophone housing. The damping coefficients are not so influential in de-coupling the structural vibration from the sensing element. The optimum acoustic impedance of the damping layer is estimated to be smaller than 1 Mrayl or larger than 4 Mrayl. These properties can not be readily obtained from conventional polymers alone, and hence development of new composite materials, i.e. ceramic-polymer composite or metal-ceramic composites, is required for realization of effective damping layers. Overall, of the fourteen structural variations of the hydrophone, the best one shows about 87% reduction in the response of the original structure to external noise.

Active vibration control of thin-plate structures with partial SCLD treatment

The aim of this research is to determine the optimal shape of a constrained viscoelastic damping layer on an elastic beam by means of topology optimization. The optimization objective is to maximize the system loss factor for the first resonance frequency of the base beam. All previous optimal design studies on viscoelastic lamina have been size or shape optimization studies, assuming a certain topology for the damping treatment. In this study, this assumption is relaxed, allowing an optimal topology to emerge. The loss factor is computed using the Modal Strain Energy method in the optimization process. Loss factor results are validated by using the half-power bandwidth method, which requires obtaining the forced response of the structure. The ABAQUS finite element code is used to model the structure with two-dimensional continuum elements. The optimization code uses a Sequential Quadratic Programming algorithm. Results show that significant improvements in damping performance, on the order of 100% to 300%, are obtained by optimizing the constrained damping layer topology. A novel topology for the constraining layer emerges through the optimization process.

Free vibration characteristics of periodically stiffened panels with damped stringers