| To solve the severe vibration and noise problem of high-speed moving aircraft and vehicle in their running process, my research attempts to design a lightweight and cheap damping encapsulating material, which can be poured into the steel pipes to enhance the structural damping characteristics but do not change the pipes' original structure. In this dissertation, two types of hollow spheres (fly ash (FA) and Al2O3 hollow sphere(HS)) are filled into the modified epoxy to prepare the low cost, low density (0.580.93g/cm3) and high damping porous composites. By means of scanning electron microscope(SEM), thermogravimetric analysis(TG), and dynamic mechanical analyzer(DMA), the microstructure, thermal properties as well as damping properties of porous composites are investigated systematically. The modal damping characteristics and vibration damping characteristics of encapsulating pipes are measured by using the strike method and shaking table method for the first time.The particles size of the fly ash and Al2O3 hollow sphere used in my paper are 74206μm and 0.54mm. The matrix is 90% epoxy resin-10% polyurethane (90EP-10PU). Research shows that the densities of the porous composites decrease linearly with the increasing Vf and particles size of cenosphere FA. When Vf is 5070vol.%, the density of fly ash/(90EP-10PU) can reduce to 0.800.93g/cm3. For the Al2O3 hollow spheres/(90EP-10PU) porous composites, their densities are 0.580.62g/cm3 with the Vf of 88vol.%.The interface behavior between FA and the matrix is not a"free interface", but a"weak bonding interface"when the FA is surface-modified by the silane coupling agent. When Vf is 3050vol.%, the FA spheres are dispersed homogeneously in the matrix with farthing holes, and the fracture of the porous composites is caused by the rupture of FA spheres. When Vf>50vol.%, the defects are brought by the discontinuity of the matrix and the accumulation of FA spheres.Thermogravimetric (TG) test shows that the initial decompositions temperatures and the temperatures of 10% thermal weight loss for porous composites both enhance with increasing Vf and particles size of FA. The oxidative decomposition activation energies increase gradually, which indicates the heat resistance of the FA/(90EP-10PU) porous composites is improved.The damping properties of the FA/(90EP-10PU) porous composites in the temperature range from -40 to 150oC and in the frequency range from 10 to 800Hz are studied systematically. The peak values of loss factor(tanδ) are 0.6280.892, the glass transition temperatures(Tg) shift to low temperature, and the high damping temperature region(ΔT0.5) widen. In tanδf curves, the attenuation of tanδis more slowly with the increase of frequency, which indicate that the damping properties of porous composites can keep stable.The research on the change of tanδwith the volume fraction (Vf), particles size and kinds of FA microspheres shows that there is a peak in the tanδVf curves. When the Vf is 30vol.%, the peak values of tanδare 0.892. The variation of peak values of tanδis less than 0.1 with the change of particles size and kinds of FA microspheres. This indicates that the effect of fly ash Vf on the damping properties of porous composites is more important.The studies on modal damping characteristics of hollow spheres/(90EP-10PU) encapsulating Q235 pipes show that the structural damping factor(η) of the encapsulating pipes are (59)×10-2, which is (612) times higher than that of Q235 pipe. The effect of impulsive force on the modal damping characteristics is not significant. The thicker of the composites damping layers, the better of the modal damping characteristics. In addition, the effects of Vf, particles size and types of hollow spheres on modal damping characteristics are analyzed. Results reveal that theηincreases at first then decreases with increasing Vf of FA. When the Vf is 3050vol.%, theηis (89)×10-2.ηhas no obvious change(Δη<0.01) with the variation of particles size and types. The Modal Strain Energy(MSE) method belonging to the finite element software is applied to calculate the structural damping factors of the hollow spheres/(90EP-10PU) encapsulation pipes firstly. The results show that the calculation values are basically consistent with the experimental data by the establishment of shell-solid element model and selection of fixed-free boundary conditions.The studies on vibration damping characteristics of hollow spheres/(90EP-10PU) encapsulating Q235 pipes indicate that the first order natural frequency (f1) of Q235 pipe decreases by 1032Hz with the addition of fly ash and Al2O3 hollow spheres. The natural frequencies of hollow spheres/(90EP-10PU) encapsulation pipes enhance with the increase of the Vf of hollow spheres, but which is changed little with the variation of the particles size; the natural frequencies of hollow spheres/(90EP-10PU) encapsulation pipes decrease with the increasing thickness of damping composites. The studies on the relationship between the vibration damping characteristics and the vibration frequency show that the vibration transmissibilities (T) of each encapsulating pipes are more than 1 when the frequency is less than 21/2 f1, which indicates that the encapsulating pipes have no damping effect in this frequency region. At the frequency of 2502000Hz, the T of each encapsulating pipes are 0.150.64, which reveals that the T of each encapsulating pipes increase slowly, and the vibration transmission decrease 4.50.5 times.The structural damping characteristics of encapsulating pipes depend on the damping properties of hollow spheres/(90EP-10PU) porous composites. Researches show that the fly ash/(90EP-10PU) porous composites and their encapsulating pipes both have the superior damping properties when the fly ash volume fraction is 30vol.% and particles size is 120μm.
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