Theoretical And Experimental Study On Dynamic Properties Of Viscoelastic Isolators

Although viscoelastic isolators have been widely used in aerospace and other engineering fields, due to the high nonlinear properties of viscoelastic materials, it remains difficult in some aspects about dynamic performance, such as modeling, design and calculation, etc. Therefore, in this thesis, the involved key problems of viscoelastic isolators'dynamic performance have been investigated by comprehensive theoretical and experimental study, including viscoelastic temperature spectrum model, time-temperature superposition method, vibration isolators'support structure design and dynamics performance calculating method, temperature rise calculation.Viscoelastic materials are one of the most important elements of viscoelastic isolators. Based on the Five-Parameter Fractional Derivative Model of frequency spectrum and mathematic temperature-frequency mirror relationship, Six-Parameter Fractional Derivative Model of the temperature spectrum, hereinafter referred to as SPFDM, has been proposed. SPFDM can directly use the experimental results of dynamic mechanical analysis, is applicable to loss modulus and the loss factor with both symmetry and asymmetry. SPFDM' parameters have clear physical meaning, whose initial values can be deteminated by some deduced formulas. Parameter identification steps are also given.Viscoelastic materials'dynamic performance depends not only on temperature, but also on frequency, is commonly characterized with the master curve obtained by the time-temperature superposition. To obtain more accurate Time-Temperature shift, a Generalized Maxwell Model Based Shifting method, hereinafter referred to as GMMBS, has been put forward. Compared with those traditional methods based on the overlapping window, GMMBS doesn't need first judgment of the size and existence of the overlapping window, can use all the experimental data which contains useful information to calculate the shift factor.Based on viscoelastic materials' research, the equal strength mechanical model and new mode strain energy method have been established for viscoelastic isolators'bracket and whole dynamic parameters, respectively. Based on the assumption of small deformation, Mohr's law, equal strength theory, the equal strength mechanical model is suggested for isolators'bracket. The model can directly predict isolators'performance parameters, and can help to design isolators'bracket with the needed safety factor. Based on the analysis of the principle and the correction of existed modal strain energy methods, Zhang Modal Strain Energy, hereinafter referred to as ZMSE, have been proposed, of which the correction factor depends on the amplitude of the loss factor. Taking the four-parameter prototype system, which is equivalent to viscoelastic laminated beam/board, and a new type isolator as an example, ZMSE's accuracy is discussed through the comparison with the existing methods.In vibration process, viscoelastic isolators'temperature will rise due to energy dissipation, especially when the viscoelastic materials have high damping characteristics. For the general input, the thermal dynamic coupling differential equations are established based on the Five-Parameter Fractional Derivative Model, and the corresponding difference form is introduced too. For the steady-state harmonic input, which is very common in engineering application, the established differential equations can be significantly simplified by Fourier transformation. The calculation method of the temperature rise has also been deduced for the steady-state case.The proposed theories and methods above for viscoelastic isolators'dynamic performance still need to be fully verified by experiments. With the condition of fixed frequency and temperature, experiments have verified the accuracy of the SPFDM, and reveal that the SPFDM's error may be relatively big when there is a subprime transition region. With the condition of variable frequency and temperature, experiments have proven the accuracy of the GMMBS, and reveal that there may be both vertical shift and horizontal shift for the dynamic performance of viscoelastic materials. Sine and random experiments show that the ZMSE can calculate the dynamic parameters of the isolators with reasonable accuracy. The isolator with desired stiffness and damping can effectively restrain the vibration energy transfer in the wide bandwidth. Experiments of the temperature rise show that the thermal dynamic coupling calculation method is basically accurate. For those high damping viscoelastic isolators, the amplitude of the temperature rise is notable, which is necessary to be considered in the design of isolators in order to maximize the performance of vibration isolation.