Acoustic insulation characteristics of sandwich composite shell systems with double curvature: The effect of nature of viscoelastic core

A viscoelastic model is proposed in this approach to determine the sound transmission loss coefficient of a sandwich shell system with double curvature. The structure is composed of a double-walled composite shell subjected to a viscoelastic core. Investigating the efficient impresses of rotary inertia and shear deformation, vibration equations of both outer and inner shells are extracted within the framework of shear deformation shallow shell theory. Besides, the Zener mathematical model is used for viscoelastic material, which is based on a spring connected in series with a parallel mixture of spring and dashpot. This model presents the dynamic response in the whole frequency domain at which shear modulus and bulk complex modulus are frequency dependent. Since the performed studies on the sound transmission loss of this kind of structures are insignificant, the outcomes of plate models with a viscoelastic core are used to provide a reliable sound transmission loss comparison. The results show that the applied strategy can improve the acoustic characteristics of the system at high frequencies compared to that of a single-layer one with the same mass. This issue is more highlighted while the thickness of the viscoelastic layer enhances, which confirms the positive performance of the viscoelastic materials in this range of frequency, particularly in the resonant frequency. In addition to the curvature effect on acoustic features, the vibration response of the system is configured based on various frequencies and materials.

Interacting inclined strike-slip faults in a layered medium

Two inclined, interacting, strike-slip faults, both buried, situated in a viscoelastic layer, resting on and in welded contact with a viscoelastic half space, representing the lithosphere-asthenosphere system, is considered. Solutions are obtained for the displacements, stresses and strains, using a technique involving the use of Green’s functions and integral transforms, for three possible cases - the case when no fault is slipping, the case when one fault is slipping and the other is locked and the case when both the faults are slipping. The effect of sudden movement across one fault on the shear stress near the fault itself and near the other faults has been investigated. Some situations are identified where a sudden movement across one fault results in the release of shear stress near the other fault, reducing the possibility of seismic movements across it. Other situations are also identified where a sudden movement across one fault increases the possibility of seismic fault movements. A detail study may lead to an estimation of the time span between two consecutive seismic events near the mid points of the faults. It is expected that such studies may be useful in understanding the mechanism of earthquake processes and may be identified as an earthquake precursor.

Construction of mathematical models of the stressed-strained state of a material with a porous water-saturated base under dynamic load

Materials of beams, plates, slabs, strips have been commonly applied in various fields of industry and agriculture as flat elements in the structures for machinery and construction. They are associated with the design of numerous engineering structures and facilities, such as the foundations of various buildings, airfield and road surfaces, floodgates, including underground structures. This paper reports a study into the interaction of the material (of beams, plates, slabs, strips) with the deformable base as a three-dimensional body and in the exact statement of a three-dimensional problem of mathematical physics under dynamic loads. The tasks of studying the interaction of a material (beams, plates, slabs, strips) with a deformable base have been set. A material lying on a porous water-saturated viscoelastic base is considered as a viscoelastic layer of the same geometry. It is assumed that the lower surface of the layer is flat while the upper surface, in a general case, is not flat and is given by some equation. Classical approximate theories of the interaction of a layer with a deformable base, based on the Kirchhoff hypothesis, have been considered. Using the well-known hypothesis by Timoshenko and others, the general three-dimensional problem is reduced to a two-dimensional one relative to the displacement of points of the median plane of the layer, which imposes restrictions on external efforts. In the examined problem, there is no median plane. Therefore, as the desired values, displacements and deformations of the points in the plane have been considered, which, under certain conditions, pass into the median plane of the layer. It is not possible to find a closed analytical solution for most problems while experimental studies often turn out to be time-consuming and dangerous processes

Influence of the Poisson's ratio on the efficiency of viscoelastic damping treatments

An efficient way of mitigating noise and vibration is to embed viscoelastic patches into the host structure. Viscoelastic properties are of significant importance in determining the performance of the passive damping treatment. The behaviour of homogeneous isotropic materials is described by two elastic constants (generally the Young modulus and the Poisson ratio, or the shear and bulk moduli), which are frequency- and temperature-dependent in the case of viscoelastic materials. In practice, the Poisson's ratio is often considered as independent of temperature and frequency. One goal of this work is to numerically evaluate the validity of this assumption and its limitations (frequency range, thickness of the viscoelastic layer). To this end, a thermo-mechanical characterization of a viscoelastic material is carried out by dynamic measurements of the complex shear and bulk moduli, allowing the indirect measurement of the frequency- and temperature-dependent Poisson's ratio. Moreover, the measurements of the Poisson's ratio (direct or indirect) can lead to considerable uncertainties. For instance, large discrepancies have been observed when characterizing the Poisson's ratio of polymer foams. Another goal of this work is to investigate the influence of those uncertainties on the dynamic response of a damped structure.

Effects of annular acoustic black holes on sound radiated by cylindrical shells

While most acoustic black hole (ABH) designs are intended to reduce vibrations in beams and plates, annular ABHs have been recently proposed for cylindrical shells. The key to achieve the ABH effect in a structure consists in embedding an indentation on it such that it slows down incident waves and concentrates their energy at the center of the ABH. There, it can be typically dissipated by means of a viscoelastic layer. Many studies exist on the vibration of structures with ABH indentations but only a few address the topic of sound radiation. In this work, we evaluate the impact that an annular ABH has on the sound radiated by a baffled cylindrical shell. The vibration of the cylinder is computed using Gaussian basis functions in the Rayleigh-Ritz method. Once determined the surface velocity of the ABH cylinder, a Green's function approach is employed to obtain its surface acoustic pressure and then the sound power level, radiation efficiency and supersonic intensity. The dependency of the latter on the ranges determined by the ring and critical frequencies is analyzed for the case of a thick acoustic shell. Beyond the critical frequency, supersonic flexural waves entering the ABH become subsonic, substantially reducing the radiation efficiency and therefore, the emitted sound. Further reduction is achieved once passed the ring frequency.