| As the most basic test equipment for aerodynamic research and aircraft development,wind tunnel plays an important role in aircraft development and training.The vibration of the wind tunnel pipelines will affect the flow of the vehicle model,which can cause a strong disturbance to the test result,and may even cause the dynamic test to fail.The wind tunnel piping system is a continuum structure with distributed parameters,and its large-scale thin-walled pipe is susceptible to its changing vibration source,so it has multiple resonance points under working conditions.As a consequence,the complex vibration characteristic of wind tunnel pipeline has brought great challenge to its vibration reduction.Magnetorheological elastomer(MRE)is a kind of magnetic-control intelligent material,which is prepared by dispersing micron magnetic particles into polymer elastomers.It not only possesses excellent mechanical properties of particle-reinforced composites,but also has the characteristics of rapid,continuous and reversible magnetic control stiffness.According to the vibration characteristics of the wind tunnel pipelines,the devices based on MRE can adjust their parameters in real time according to the variation of vibration excitation.Therefore,the distributed MRE device is a more effective method to reduce the vibration of wind tunnel pipelines.Currently,the researches on MRE are mainly focused on three aspects,such as the mechanism study,application research,preparation and characterization.Firstly,as the most important properties of MRE,the magnetorheological(MR)effect and mechanical strength of MRE are contradictory to some extent.As a consequence,the MRE materials cannot achieve the high MR effect and mechanical strength simultaneously,which limit the broad engineering application of MRE in the field of vibration reduction.Secondly,different MRE devices require the various MRE with different material performances,so developing MRE with specific properties is the key to improve the performance of MRE devices.In addition,the property research of MRE is mainly focused on the dynamic mechanical properties,and the studies on the static viscoelastic behavior are still less.The static viscoelastic behavior is sensitive to the microstructure of MRE,and the microscopic mechanism of MRE can be further studied by systematically studying the static viscoelastic behavior of MRE.Therefore,the research of MRE static viscoelastic behavior has great theoretical and engineering application value.For the potential engineering application of MRE in wind tunnel vibration reduction,the material preparation characterization,mechanism research and device application research of MRE have been studied systematically in this dissertation.The main contents of this thesis are summarized as follows:As the carrier of magnetic particles,the matrix is very important for the performance of MRE.Firstly,the MRE based on the polyurethane/epoxy interpenetrating network matrix has been prepared,and its mechanical properties,magnetic-control characteristics,damping property and heat resistance are comprehensively evaluated.The results show that the mechanical strength,heat resistance and damping properties of MRE have been greatly improved by the interpenetrating network structure,and the MR effect can also be maintained at a high level.On the other hand,another MRE based on the platinum-catalyzed silicone rubber with high MR effect and low damping was also prepared.The MRE has the advantages of low damping and excellent mechanical properties,and its MR effect can reach 691%.These works could provide a reference for the development of high performance MRE,and lay a foundation for the application of MRE in the vibration absorber and isolator.Interface,as a bridge between the particle and the matrix,is the key factor affecting the mechanical properties of MRE.The interfacial modification can improve the dispersion of carbonyl iron powder in the matrix and reduce particle agglomeration.In this chapter,the carbonyl iron powders coated with polyaniline were synthesized,which enhanced the magneto-induced modulus and reduced loss factor of the isotropous MRE.In addition,the Payne effect of MRE has been studied by using large amplitude oscillation shearing tests,and the influences of interface and magnetic field on Payne effect has been also discussed.By using the Kraus model and the modified Maier model,the experimental results are numerically fitted,and the coupling effect of particles-particle and particle-matrix on the Payne effect is discussed.This work provides a method for the interfacial modification of MRE,and the studies of Payne effect can provide theoretical support for the engineering application of MRE.The magnetic particle is the key to obtain the magnetic-control characteristic of MRE,and the magnetic MRE was prepared by substituting the hard magnetic particle NdFeB for carbonyl iron powder in this chapter.Firstly,the microstructure and magnetic properties of the particles and MRE were characterized,then the dynamic mechanical properties of the MRE under different magnetic fields were tested detailedly,and the corresponding physical mechanism was also proposed.Based on the demagnetization effect of hard magnetic particles,the interaction force between hard magnetic particles will increase or decrease under the change of magnetic field.Therefore,the modulus of the MRE will show an increasing or decreasing tendency under the changing magnetic field,that is,the bi-directional magnetic-control modulus of MRE.In addition,based on the large interfacial damping and hysteresis damping caused by the rotation of NdFeB particles and their magnetic domain,the MRE shows high damping characteristics.Creep recovery and stress relaxation are two important static viscoelastic behaviors.In this chapter,the Paar MCR301 rheometer was used to test the creep recovery and stress relaxation behaviors of MRE under quasi-static shear mode.The influence factors such as matrix,magnetic field and temperature are synthetically discussed,and the related mechanisms were proposed.In addition,a variety of viscoelastic models are used to explore the constitutive relation of the static viscoelastic behaviors of MRE.The creep recovery and stress relaxation behaviors of the MRE are simulated by using the Findly model,Burgers model,Weibull distribution equation,Kohlrausch stretched-exponential equation.It is found that the IPN structure can improve the stability of the molecule chain structure by restricting the movement of the chain segment,thus improve the dimensional stability of the MRE.The magnetic field can enhance the interaction between the particles and the matrix,thus affecting the physical state of the non-magnetic matrix,including restricting the movement of the molecule chain and improving the overall cross-linking degree of the matrix.For the application of MRE in the vibration reduction of wind tunnel pipeline,the experimental researches on the MRE vibration absorber and vibration isolator were carried out.In this chapter,a sleeve-shearing MRE vibration absorber is designed,which has the advantages of compact structure,high efficiency of magnetic circuit and large bearing capacity.Based on the platinum-catalyzed silicone rubber based MRE,the designed vibration absorber has been assembled.The mechanical and frequency-shift performance of the vibration absorber were evaluated by the electromagnetic vibration table,and the results have shown that the natural frequencies are increased from 41.2Hz to 88.1Hz when the excitation current is increased from 0A to 3A,with a relative increase of 113.8%.The damping ratio of MRE vibration absorber is lower than 0.122,and its transmissibility peak value is higher than 4.25.It is advantageous for the vibration absorber to move the vibrations from the pipelines to the mass block,which can ensure the effectiveness of of vibration absorption.In addition,the polyurethane/epoxy IPN structure based MRE is applied to the vibration isolator,which effectively reduces the resonant peak value.Experimental study on MRE absorbers and isolators has a valuable reference for MRE engineering applications,which is of great significance for solving the vibration reduction problem of wind tunnel pipeline. |
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