Mechanism Research,Material Modification And Experimental Verification For A Damping And Decoupling Composite Acoustic Coating

As an effective noise reduction approach for submarine,the addition of a composite acoustic coating consisting of both damping and decoupling layers was proposed herein as a new method to reduce the vibrations and radiated noise generated in the structure.Such a composite acoustic coating combines the high vibration suppression performance of a damping layer and the superior vibration isolation performance of a decoupling layer,which can dramatically reduce vibrations and radiated noise with negligible increase in the thickness or weight of the acoustic coating.However,the mechanism of vibration and noise reduction,the improvement of vibration suppression performance in low frequency range,the development of matching acoustic materials,and the experimental verification of mechanism need to be further studied.Therefore,the objective of this paper is to study the vibration reduction and denoising mechanism of the composite coating,the modification method of the acoustic material and the experimental verification of the theoretical model.First,an exact analytical model for a fluid-loaded,simply supported rectangular plate covered by a damping and decoupling composite acoustic coating was proposed.The vibration reduction and denoising mechanism of the composite acoustic coating were analysed by compared with single damping or decoupling acoustic coating.The numerical results showed that the effect of radiated noise reduction of the composite coating was superior to that of single-type coatings,which was attributed to the fact that the composite coating combines the merits of both the high vibration suppression performance of the damping layer and the superior vibration isolation performance of the decoupling layer.Second,a bending model for a damping structure with a spacing layer was proposed.Compared with the traditional model,the model considered the effects of the shear deformation of the spacing layer on the tension or compression deformation of the damping layer and the total bending stiffness of the structure.The results showed that the structural loss factor was decreased with the increasing frequency as considering these effects.When the system reached a certain critical frequency,the loss factor was less favorable than that of a free-damping structure.While the structural loss factor of the traditional model was shown to be independent of frequency.In this paper,the calculation formula of the critical frequency was deduced,and the influences of the geometric parameters and material parameters on the structural loss factor and the critical frequency were numerically analyzed.Finally,the reliability of the theoretical model was verified by experiments.Thirdly,a series of organic hybrid damping materials were prepared by the introduction of two kinds of hindered amine antioxidants in the rubber substrate,and the quantitative design of the material loss factor and working temperature domain were realized by controlling the amount of antioxidant.The mechanism studies have shown that the increases(or regulation)of the loss factor and the glass transition temperature were due to the formation of a reversible hydrogen bond network between the hindered amine antioxidant and the polymer matrix.Polymer blending and stacking weakly constrained structures were studied for broaden the damping temperature field of the damping materials.Fourth,a series of para-phenylene terephthalamide(PPTA)pulp modified damping materials were prepared.The dynamic mechanical properties,differential scanning calorimetry,vibration damping properties,vulcanization property,tensile strengths as well as SEM micrographs of the damping materials were studied theoretically and experimentally.The dynamic mechanical properties of PPTA pulp modified damping materials were also compared with aramid short cut fiber,E-glass staple fiber and carbon fiber powder modified damping materials.The results showed that PPTA pulp modified damping materials exhibited the best damping property and highest modulus in comparison with the other types of fibers.The storage modulus,loss modulus and tensile strength of the materials were all increased significantly with increasing pulp content,and this trend was significantly greater in machine direction rather than in cross-machine direction.Excellent damping property was mainly attributed to the extremely high interfacial contact area which significantly improved the efficiency of energy dissipation of internal friction,interfacial sliding and dislocation motion between PPTA pulps and NBR chains.Fifth,a series of Graphene nanoplatelets(GnPs)modified Butadiene-acrylonitrile rubber-based(NBR-based)underwater acoustic absorbing materials were prepared.The dynamic mechanical properties,underwater sound absorption properties,differential scanning calorimetry,vulcanization property,mechanical properties of GnPs/NBR nanocomposites were studied theoretically and experimentally.The results indicated that GnPs modified NBR-based underwater acoustic absorbing materials exhibited excellent damping and underwater sound absorption properties.When the GnPs content was 10 phr,the average value of the sound absorption coefficient was increased by nearly 1 time.The notable improvement in the sound absorption coefficient was owing to the marked increases in damping properties and thermal conductivity of GnPs/NBR nanocomposites.Finally,the vibration suppression performance and noise reduction performance of the damping and decoupling composite coating were studied by using the "acoustic coating laser vibration test system" designed by the research group.The validity and limitation of the theoretical model were verified by comparison between the theoretical and experimental results.The results showed that the theoretical and experimental results were consistent with the vibration characteristics of the substrate and the coating.For the prediction of the sound radiation field,the theoretical model better reflected the law and trend of the radiation field of the same type of coating.But for the comparison between different types of coatings,the theoretical and experimental results were very different,the difference mainly due to the theoretical model ignoring the difference of the radiation efficiency between the different types of materials.Further research found that the noise reduction mechanism of a decoupling coating with low stiffness was not only by suppressing or isolating the substrate vibration transmiss to its wet surface;more importantly,because of its low radiation efficiency,thus blocking the wet surface vibration converting into radiation energy.