J.A. Rongong

Advances in damping materials and technology

Abstract In the continual search for better damping materials and technologies, significant advances have been made of late. Functionally gradient materials, liquid crystal polymers, magnetostrictive materials and plasma deposited damping coatings are some of the novel materials and technologies being investigated in the Dynamics Research Group at the University of Sheffield. This article presents an overview of the work being carried out in these areas.

Modelling of a Hybrid Constrained Layer/Piezoceramic Approach to Active Damping

It has been shown that significant reductions in structural vibration levels can be achieved using a hybrid system involving constrained layer damping and active control with piezoceramics. In this paper, mathematical models based on the Rayleigh Ritz approach, are developed to describe the longitudinal and flexural vibration behaviour of a cantilevered beam when excited using piezoceramic patches bonded to a constrained layer damping treatment. Predictions of static and steady state dynamic behaviour, obtained using the models are validated by comparison with results from finite element analysis and laboratory experiments. The models are then used in open loop and closed loop velocity feedback control simulations to demonstrate the improvements in stability and performance achieved using this method over that achieved using conventional active control.

Active constrained-layer damping: A state-of-the-art review

Abstract In this paper the authors discuss the progress that has been made over the past decade in active constrained-layer damping (ACLD). ACLD treatments combine the best features of passive and active control of structural vibrations. By way of introduction the paper describes well-established techniques for passive control of structural vibrations and noise. A concise discussion of the development of so-called ‘smart’ (or ‘intelligent’) actuators and sensors and the emergence of suitable control algorithms show how passive techniques were extended to produce ACLD. A comprehensive literature review follows. It is shown how the passive and active components of ACLD complement each other to enable control of both high and low frequency modes of vibration. The active elements allow structures to adapt to suit a changing environment while the passive elements provide a fail-safe mechanism. Because of the available technology, these benefits are available without significant penalties in terms of cost, weight and complexity.

Amplitude Dependent Behaviour in the Application of Particle Dampers to Vibrating Structures

This paper presents results of experimental studies aimed at understandin g the effects of various design parameters on the amplitude -dependent behaviour of particle dampers. The dampers considered here contain several thousand hard particles with diameters in the range 0.1 to 1.6 mm and operate best around the transition betwee n solid -like and liquid -like behaviour. A test rig with one principal resonance is used to estimate the effectiveness of each damper at dissipating kinetic energy. Parameters considered include damper geometry, particle size, and particle material. Results show t he importance of the inter -particle contact condition s and the static pressure field. Measured amplitude -dependent energy dissipation properties, taken from a simple test rig, are then used to predict modal loss factors achieved by attaching several small particle dampers to a large, continuous structure . Comparison of predicted and measured behaviour shows that the approach used is reasonably accurate over a range of frequency and amplitude conditions. Poor correlation in higher modes is attributed to damper frequency -dependence .

Hysteretic properties of metal rubber particles

The hysteresis behaviour of metal rubber particles has been characterised. A quasi-static experiment was conducted to measure the damping behaviour of metal rubber particles. The effects of imposed displacement and filling density were examined. A mathematic model has been established to characterise the damping behaviour of metal rubber particles. A single-helix spring model was used to represent the microelement of the metal rubber material. This was composed of three contact states: open, slipping and sticking. The stiffness properties for the three cases were derived. The arrangement and spatial distribution of the microsprings were assumed to be uniform and periodic inside the metal rubber particles component. The test results were compared with the simulation results which showed a good correspondence.

Control of Particle Damper Nonlinearity

A particle damper comprises granular material enclosed in a container that is attached to or within a vibrating structure. Vibration is suppressed by the particle damper by a combination of friction, inelastic collisions, and kinetic energy storage. The principal challenges are that their performance is highly nonlinear and is dependent on many parameters. Although significant efforts have been made to characterize and model particle dampers, there is very little evidence of efforts made to tune or control their nonlinear behavior effectively. The research work carried out in this project is based on the supposition that the granular state within the damper can be altered in a predictable way to ensure that one operates in a zone where damping is high. This work shows that this can be achieved by altering the static effective pressure in the granular system. A variety of physical embodiments were considered (including magnetic fields and airbags): the design features sought were simplicity, uniform pressure distribution, and ease of automation. The final configuration selected was a thick layer of polyurethane foam that could be pressed down onto the particles with various pressures, depending on the tightness of the screw-top lid. A second set of energy loss estimates is also obtained by running the discrete element modeling prediction for the damper using appropriate interactions models. Results are finally compared with those obtained from physical experiments.

Passive and active constrained layer damping of ring-type structures

The performance of constrained layer damping treatments can be enhanced by optimizing the segment length or through active control by inducing strains in the constraining layer. This paper investigates the effect of these methods on the flexural and extensional modes of a ring over a wide frequency range. Finite element models are first verified experimentally and then used in parametric studies. It is shown that segmentation of the constraining layer does not increase the maximum damping obtainable for a particular configuration but alters the mode number at which the maximum occurs. It is also shown that the optimum viscoelastic layer stiffness for active constrained layer damping is higher than that for the passive case.

Suppression of ring vibration modes of high nodal diameter using constrained layer damping methods

Constrained layer damping treatments utilize the energy dissipative properties of viscoelastic materials to increase the passive damping in structures. This paper investigates the performance of constrained layer damping treatments applied to a ring. An analytical model is developed to predict the behaviour of such a ring over a large number of modes. Relative motion between the ring and constraining layer in both the circumferential and radial directions are considered. Predictions of steady state dynamic behaviour obtained using the model are validated by comparison with results from experiments and finite element analysis. The model is then used to evaluate the effect of various parameters on the damping achieved. It is shown that for any particular configuration a plot of damping against mode number has two maxima. These peaks occur when optimum values for the deformation of the viscoelastic layer are achieved firstly for shear and secondly for radial extension and compression.

Plasma deposition of constrained layer damping coatings

Abstract Plasma techniques are used to generate constrained layer damping (CLD) coatings on metallic substrates. The process involves the deposition of relatively thick, hard ceramic layers on to soft polymeric damping materials while maintaining the integrity of both layers. Reactive plasma sputter-deposition from an aluminium alloy target is used to deposit alumina layers, with Youngs modulus in the range 77–220GPa and thickness up to 335 μ, on top of a silicone film. This methodology is also used to deposit a 40μ alumina layer on a conventional viscoelastic damping film to produce an integral damping coating. Plasma CLD systems are shown to give at least 50 per cent more damping than equivalent metal-foil-based treatments. Numerical methods for rapid prediction of the performance of such coatings are discussed and validated by comparison with experimental results.

Improving the modal strain energy method for damped structures using a dyadic matrix perturbation approach

Abstract A dyadic matrix based perturbation method has been developed to evaluate the natural frequencies and modal loss factors of viscoelastically damped structures. This approach improves the accuracy of the prediction given by the modal strain energy (MSE) method by making use of the modal basis of the undamped system. Examples involving lumped parameter systems and a constrained layer viscoelastic sandwich beam are presented. In each case, the solutions given by exact numerical methods, MSE and the perturbation approach are compared. Results are also correlated with performance indices to assess the damping level and type in the system. A solution involving a reduced modal basis of the original structure using the perturbation approach is also presented. This approach also gives a significant improvement over the original MSE prediction.