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Published Articles

The Volume 10, No 4, December 2005

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A Review on Vibration Damping in Sandwich Composite Structures

Zhuang Li and Malcolm J. Crocker


In applications where the use of lightweight structures is important, the introduction of a viscoelastic core layer, which has high inherent damping between two face sheets, can produce a sandwich structure with high damping. Composite sandwich structures have several advantages, such as their high strength-to-weight ratio, excellent thermal insulation, and good performance as water and vapour barriers. So in recent years, such structures have become used increasingly in transportation vehicles and other applications. Care must be taken in their design to ensure that their sound isolation capabilities are adequate because coincidence generally occurs at a lower frequency for sandwich panels than for typical metal panels. Passive damping properties of composite sandwich panels are important because the damping properties affect their sound transmission loss, especially in the critical frequency range, and also their vibration response to excitation. Research on damping in sandwich composite structures is reviewed in this paper. This review includes analytical approaches, finite element models, statistical energy analysis, and damping measurement techniques. Other mechanical properties of composite sandwich structures, for example, stiffness and damage tolerance, affect each other and in turn are affected by damping. The overall effects of damping and other factors on structural response and sound radiation of composite sandwich structures are reviewed.

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A New Application of the Fixed-Points Theory for the Control of Kinetic Energy of a Continuous Structure

Jedol Dayou


The fixed-points theory was originally used to determine the optimum tuning and damping ratios of a vibration neutraliser that may flatten the frequency response function (FRF) of a simple structure. Although the theory has also been used for the control of vibration of a continuous structure, applications have been limited only to point control. In this paper, a new application of the theory is discussed, which is for the global control of vibration of a continuous structure. The theory is used to determine the optimum tuning and damping ratios of the vibration neutraliser that flatten the global response of the structure. Kinetic energy is used as a measure of the structure?s global response, and the application is demonstrated on undamped and damped simply supported beams. It is found that by using these optimum values, it is possible to remove the effects of the dominant mode leaving only the effect of the residual modes in the global behaviour. This shows that the theory can also be used to reduce the vibration of a continuous structure at all points instead of at a particular point only, such as in the conventional application.

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Active Vibration Isolation in Ships: A Pre-Analysis of Sound and Vibration Problems

Mathias Winberg, Sven Johansson, Lars Ha?kansson, Ingvar Claesson, Thomas Lago


Engine-induced sound and vibration levels in boats for professional and leisure use are in many cases unacceptably high in terms of comfort and environmental disturbance. Classical methods for passive treatment are normally less efficient due to the low frequency content and often lead to a substantial increase in weight. The requirements for lower weight, which would increase the maximum speed of the boat as well as improve fuel economy, have to be considered. More efficient vibration damping methods must therefore be found. With, for example, active engine mounts, it is possible to achieve a decrease in the vibrations even for cases when the hull is not very stiff. This is especially important in marine applications, since the engines are usually mounted on flexible and light structures. The project Active Vibration Isolation in Ships (AVIIS) aims at investigating the effects of using a type of Active Noise and Vibration Control system (ANVC) in this particular marine application. This paper presents an analysis of sound and vibration problems in one particular leisure boat from an ANVC point of view. A very thorough investigation was carried out, and the main noise and vibration sources were established as well as the transmission paths of the noise into the boat. Answers were made after this investigation about where the actuators should be positioned, which kind of ANVC approach should be used and the expected interior noise reduction. This is the kind of pre-analysis that is needed for a complex structure, such as that found in a marine vessel, for the successful implementation of ANVC. An optimised passive engine mount, with a stiffness adapted to hull mobility and the engine vibration level, was also designed and evaluated, resulting in an A-weighted saloon sound pressure level reduction of 10 dB compared to the standard engine mounts. Additionally, with the optimised engine mounts, the vibration levels at the hull were also reduced by up to 15 dB at the main harmonic components. This paper also presents a feasible way to estimate the performance of a potential active control system based on feedforward narrowband control of engine and propeller harmonics. In this paper secondary sources (inertial mass actuators) are proposed; and for error sensors, accelerometers or microphones, or a combination of the two, are used. In the low frequency range, below 300 Hz, a further reduction of engine orders and propeller BPFs in the order of 5-10 dB are predicted.

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Vibration Analysis of a Cracked Beam Subjected to a Moving Mass

R.K. Behera, D.R.K. Parhi, S.K. Sahu


An analytical method is used to investigate the dynamic behaviour of a two-crack cantilever beam with a moving mass. In order to obtain the characteristic functions of a multi-cracked beam, the local stiffness matrices are taken into account. The Runge-Kutta numerical method has been used to solve the differential equations involved in analysing the dynamic deflection of a cracked cantilever beam with a moving mass. Comparisons are made between the dynamic deflection of a beam with a moving mass having no cracks and one with two-cracks, both of which are subjected to varying velocities and masses.

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