Theoretical And Application Study Of Dynamic Mechanical Properties Of Viscoelastic Composite Structures

Benefitting from the excellent broadband vibration and noise reduction capability,composite structures with viscoelastic materials are widely used in aerospace and marine industries.With more advanced materials springing up,the capacity of viscoelastic composite structures becomes more excellent yet the mechanisms becomes more complex.This causes the difficulty on evaluating the dynamic mechanical properties(DMP)efficiently by engineering methods,which may fail to meet the research needs of vibroacoustic analysis in complex situations.On the other hand,the acquisition of the DMP of viscoelastic materials relies on experimental research.Herein proposing more convenient and reliable test methods will be of great significance.In this thesis,the theoretical study on the DMP of viscoelastic composite structures is carried out,aiming at establishing a general theoretical model for laminated structures to calculate the damping and bending stiffness efficiently.Furthermore,the proposed method is applied in testing technology of material performances,acoustic stealth technology of underwater vehicles and environment prediction of aerocraft.Firstly,the theoretical model is established for the DMP analysis of general free-layer damping treated(FLDT)structures.With each layer's tensile and bending deformation taken into account,the research scope is extended to FLDT structures that undergo significant damping in the base layer.On this basis,two modified calculation methods of the loss factor and bending stiffness are presented,employing the complex stiffness method and the strain energy method.Explicit solution formulas for loss factor and bending stiffness are given to facilitate engineering analysis,which provides a convenient tool for the characteristic analysis and optimization design of FLDT structures.Simulation results show that calculated results of the proposed method is consistent with those by the finite element method.Furthermore,using the modified calculation method,the loss factor conversion method between different FLDT structures is proposed,providing theoretical support for the damping conversion between FLDT large-scale cylindrical shells and equivalent plates.Theoretical and experimental studies have shown that the damping property of large-scale cylindrical shells can be predicted by measuring the damping properties of unconstrained plate.The theoretical model for DMP analysis of general viscoelastic sandwich structures is established.Different from the classical RUK theory for the constrained-layer damping treated(CLDT)structures,the base layer's damping is not neglected and the extending deformation of the core layer is considered.On this basis,the explicit solution formulas of the loss factor and bending stiffness of viscoelastic sandwich structures are given.The effectiveness of the proposed method is validated by comparing the loss factor results with the spectral finite element method(SFEM).Furthermore,the theory is applied to the material testing technology.The strict symmetry limitation for sandwich beam specimens of the vibration cantilever beam test method(VCBTM)is broken and a modified test method for the DMP of materials is proposed based on asymmetrical sandwich beam specimens.Theoretical and experimental studies show that the test method proposed in this thesis is universal and applicable for both asymmetrical specimens and traditional symmetrical specimens.Compared with the traditional test technology,the measuring method based on the asymmetrical sandwich specimens can greatly reduce the difficulty in specimen preparation and enrich the test data while ensuring the same test accuracy.A general theoretical model for the DMP analysis of multi-layered composite structures(MLCS)is established,which is applicable for composite structures with arbitrary homogeneous materials and sequence distribution.On this basis,the calculation method of loss factor and bending stiffness of multi-layered composite structures is given.Theoretical study shows that the DMP of MLCS is frequency-dependent and have asymptotic values in the low and high frequency domain.The structural loss factor firstly increase and then decrease with increasing frequency,which means that dominant frequency band of damping capacity exists.The proposed calculation method only needs to solve low-order linear equations,which avoids eigenvalue solving problems and thus is more stable and faster in calculation.It can be conveniently applied in the analysis and optimization of DMP.Test results of multilayered constrained-layer damped cantilever beams show that the theoretical results of modal frequencies and loss factors are in good agreement with the experimental results.It can also be observed that the proposed method can effectively evaluate the DMP of multilayered composite structures that consist viscoelastic materials that perform high damping capacity.Finally,the theory of DMP analysis of MLCS is applied to the engineering practice,with the design of multi-layered anechoic coating and vibration prediction of multi-layered composite aerocraft taken as examples.The multi-layered anechoic coating of high damping capacity is proposed with a stand-off layer and multiple CLDT.Subsquently the parameter affect analysis and verification experiment are carried out.Research results show that the damping property of the anechoic coating in low and middle frequency domain can be significantly improved by means of increasing the number of CLDT,adding the stand-off layer,increasing the thickness or loss factor of the damping layer as well as increasing the thickness or stiffness of the constraining layer.Test results show that constrained-layer damped plates with a stand-off layer have significantly high damping capacity in middle and high frequency domains.With the equivalent model of the multi-layered aerocraft established,the statistical energy analysis model of the aerocraft under turbulent boundary is constructed.The vibration environment of the aircraft under flight condition is predicted in whole frequency domain.The comparison between the predicted vibration and flight test data shows that the proposed method is effective to provide input parameters for coupling analysis of vibration,acoustic and fluid.