A numerical vibroacoustic model that can manage multilayered plates locally covered with damping patches is presented. All the layers can have an on-axis orthotropic viscoelastic behavior. Continuity of displacements and transverse shear stresses at each interface is enforced, which permits to write the entire displacement field in function of the displacements of the--common--first layer, leading to a two-dimensional plate model. The problem is then discretized by Rayleigh-Ritz's method using a trigonometric basis that includes both sine and cosine functions in order to treat various boundary conditions. The excitation can be of mechanical kind (concentrated or distributed forces) or of acoustic kind (plane wave of any incidence, diffuse field, etc.). The model permits to compute different vibroacoustic indicators: the mean square velocity of the plate, the radiation efficiency, and the transmission loss. Comparisons between the present model and numerical results from literature or finite element computations show that the model gives good results in both mechanical and acoustical aspects. Then, a comparison of the effects of different distributions of patches is presented. The role of the surface covering rate is first discussed, followed by a study involving different geometries for the same surface covering rate.
This study proposes an experimental-numeric method to identify the viscoelastic properties of flax fibres reinforced composite laminate (flax/epoxide). The used method consists in identifying the evolutions of both loss factor and stiffness when vibrational frequency changes. In this way, several free-free symmetrically guided beams are excited on a dynamic range of 10 to 4000 Hz with sweep sine excitation focused around the 4-first’s modes. Fractional derivative Zener model is used to identify the on-axis ply complex moduli and describe the laminate dissipative linear behavior with the classical laminate theory. Results obtained on a quasi-isotropic laminate show that this model adequately predicts the vibrational behavior of the tested laminates.
The control of the vibratory and sound level inside the planes is a significant commercial argument which has direct consequences on comfort, health, and safety of the users. Viscoelastic multilayered patches, called systems passive constrained layer damping (PCLD), based on the damping brought by viscoelastic materials are passive systems used to reduce the vibratory levels. The objective of this presentation is to show, using numerical and experimental studies, that this damping applied to the structure has an influence on the sound transmission (outside/inside), which makes it possible to increase the noise reduction. The experimental study is carried out on a model made up of a cylindrical shell coupled to a cavity and excited by a white noise. The cylindrical shell is equipped with a damping device implying viscoelastic materials which work in shearing. Parallel to this, a numerical study was carried out to (1) predict the vibroacoustic behavior of the stiffened cylindrical shell coupled with the cavity and (2)validate the vibration attenuation brought by the PCLD technology in terms of acoustic performances. The high modal density and the big vibroacoustic size of the model are two problems for numerical modeling. These studies show that numerical and experimental results are similar.
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