| The sound stealth performance is crucial for the strategic deterrence of submarines and its sound stealth performance can be significantly improved by laying rubber-based underwater sound-absorbing materials on the surface of the hull.With the development of low-frequency sonar detection technology,it is of great significance for enhancing stealth performance of submarine by researching and developping high-performance underwater sound-absorbing materials.Considering the large submersible depth of submarines,it is crucial for designing high-performance underwater sound-absorbing structures by studying the deformation and changes in sound-absorbing properties of structures under hydrostatic pressure.Meanwhile,the sound-absorbing properties of underwater sound-absorbing materials is closely related to the thermal-mechanical properties of the rubber matrix,so,it is imperative to study the mechanism of their interaction in order to develop high-performance underwater sound-absorbing materials in a targeted manner.Regarding the issues mentioned above,we took revealing the sound-absorbing mechanism and improving the sound-absorbing properties of styrene-butadiene rubber(SBR)-based underwater sound-absorbing materials under hydrostatic pressure as the starting point,and the sound-absorbing mechanism of SBR-based underwater sound-absorbing materials as well as the thermo-mechanical properties of SBR were studied by means of numerical simulation,molecular dynamics(MD)simulation,theoretical calculation and experiment.The main research contents are as follows:An underwater sound-absorbing composite structure was designed based on the idea of coupling multiple sound-absorbing effects,and structural parameters were optimized by the optimization method combining mathematical model and intelligence algorithm.The sound-absorbing mechanisms of composite structures mainly included viscous loss,relaxation loss,local resonance,and waveform conversion,etc.The errors between the experimental and numerical simulation results were relatively small,and compared with the structures reported in the literature,the structure designed in this work had a wider absorbing bandwidth under the premise of sound-absorbing coefficients(α)>0.8.In addition,the newly proposed optimization method enbodied advantages in both efficiency and accuracy.Subsequently,a nonlinear constitutive model was employed to characterize the stress-strain relationship of SBR matrix under hydrostatic pressure,and the effect of hydrostatic pressure on the sound-absorbing properties of the composite structure were investigated.The accuracy of research results on the sound-absorbing properties of composite structure under hydrostatic pressure were improved obviously when using a nonlinear constitutive model.The effects of viscous loss,waveform conversion and local resonance of the composite structure were suppressed by hydrostatic pressure to varying degrees,which adversely affects the sound-absorbing properties of the structure.On the basis of the optimized composite structure,thin film metamaterials were introduced innovatively,aims to improve its underwater sound-absorbing properties at low frequency(0.1-1 kHz),and the impacts of the material,thickness of thin films,as well as the size and distribution of the clump weights on the sound-absorbing properties of the structure were explored.Results showed that the thin film metamaterials can significantly improve the underwater sound-absorbing properties of the composite structures at low frequency.The thin films made of silicone rubber(SR)with small Young’s modulus had lower first three natural frequencies,and thicker SR films had relatively higher elastic strain energy density,which is beneficial for improving the underwater sound-absorbing properties of structure.Adding clump weights had a positive effect on improving sound-absorbing properties of the structure at low frequency,and the first natural frequency of the thin film metamaterial will reduce when the surface density of a single distributed clump weight increased.Thin film metamaterials containing symmetrically distributed clump weights exhibitted superior underwater sound-absorbing properties due to their more complex resonance characteristics.The thin film undergoes significant deformation when subjected to hydrostatic pressure,resulting in increased tension and higher elastic strain energy,hence the underwater sound-absorbing properties of structure did not decreased significantly under hydrostatic pressure.Molecular dynamics(MD)models of SBR with different cross-linking degrees(Dc)were established and MD simulations were conducted to explore the impact of Dc on the thermal-mechanical properties of SBR and the underlying micromechanism,which provided data support for further research on the micromechanism of underwater sound-absorbing of SBR matrix.The"quasi-Payne effect" of vulcanizd SBR under different shear strains(γ0)was discovered,and the main reasons for the "quasi-Payne effect" were the fracture of cross-linking bond and the decrease in flexibility of molecular chain,and the movement of molecular chain during the shear process was mainly dominated by elongation of bonds and rotation of bond angles.The increase of the shear rate(γ)made the molecular chain more prone to orientation,leading to a decrease in dynamic viscosity(η),meanwhile,larger γ will enhance the intermolecular interaction force,restricting the movement of molecular chains and resulting in an increase in bulk viscosity(ηb).SBR with higher Dc exhibited higher thermal conductivity(κ)due to its larger low-frequency phonon density.At the same time,the higher Dc also restricted the movement of SBR molecular chains,leading to a decrease in heat absorption capacities,resulting in a decrease in specity heat capacities at constant pressure(Cp)and specific heat capacity at constant volume(Cv)as Dc increased.Based on the thermal-mechanical properties of SBR with different Dc,the effects of Dc and frequency on the sound-absorbing properties of SBR were investigated by means of theoretical calculation,experiment,numerical and MD simulation,and the micromechanism of underwater sound-absorbing was investigated.Due to the use of different thermal-mechanical properties parameters for calculation,the results obtained from different research methods showed significant differences.The acceleration of SBR molecular chain movement and the enhancement of intermolecular interaction were the main reasons for the increase of a with the increase of frequency.However,the a calculated by transfer matrix and numerical simulation decreased slightly as the frequency increased for considering the reflection and transmission of sound waves.The Dc mainly affectted underwater sound-absorbing properties of SBR matrix by changing its microstructure and movement of molecular chain.The micro-motions inside SBR under the action of sound waves were mainly dominated by the elongation of bonds and rotation of bond angles.Additionally,the reduction in the freedom of molecular chain movement and the decrease in free volume due to hydrostatic pressure were the main causes of the decrease in underwater sound-absorbing properties of SBR matrix. |
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