Vibration Damping Characteristics Of Lightweight Composite Innovative Sandwich Structures

As a new generation of advanced ultr-strong lightweight structural materials,composite grid and lattice sandwich structure have received extensive attention in acadimic and engineering field.An abundant amount of research about such novel sandwich structures focuses on the manufacturing techniques and mechanical properties is carried out.However,it needs to be pointed out that,the research on the vibration and damping characteristics of such sandwich structures has just started,which is a new topic.The development of damping composite materials and structures with both high mechanical properties and high damping is one of the key issues in the study of vibration and noise reduction of lightweight structures.In this dissertation,based on the structural damping integration technique,a new type of lightweight hybrid composite lattice and corrugated sandwich shell structure with excellent structural load-bearing and damping capacities are designed and prepared.The vibration damping characteristics,energy dissipation mechanism and defect sensitivity are investigated by means of numerical simulations,theoretical analyses and experiments.Then the performance parameters of the present hybrid composite sandwich structures are classified into Lakes' modulus-loss factor map,which improve the map and provide reference for the design of lightweight structural damping materials and structures.The main contents are as follows:Firstly,the hybrid composite lattice sandwich panels with viscoelastic layers embedded in the face sheets are fabricated by a hot press moulding integrated method.The vibration damping characteristics are studied by modal tests and finite element modal strain energy method.The damping energy dissipation mechanism is revealed by analyzing the variation of damping contribution factors.The influences of different fiber orientations and thickness of viscoelastic layers on the vibration properties are discussed.The results show that the hybrid composite lattice sandwich structure can significantly improve the damping loss factors of the structure and reduce the resonant amplitudes without significantly changing the natural frequencies compared to the composite lattice sandwich structure.Secondly,the lattice flat sandwich panel is extended to the lattice curved sandwich structure.The composite pyramid lattice sandwich cylindrical shells are prepared by a hot press moulding post-forming method.Three types of different fiber configurations including all carbon fiber,all-glass fiber and hybrid fiber are carried out and their vibration and damping characteristics under free-free boundary condition have been investigated using the Reissner-Mindlin first-order shear theory,numerical method and modal test.The predictions for the modal properties show good agreement with the experimental tests.The influences of fiber ply angles on the natural frequency and damping loss factor are investigated.Compared with the lattice sandwich panels,the lattice sandwich cylindrical shells show a distinct difference between the inherent vibration damping characteristics and variation tendency.Furthermore,considering the effect of the direction of core reinforcement on the vibration damping of the structures,a combined experimental and numerical method is conducted to investigate vibration characteristics of such composite corrugated sandwich cylindrical shells with free-free boundary condition.The influence of the corrugated inclination angle,sandwich core thickness and different topological corrugated cores on the structural vibration and damping performances are thoroughly investigated.The results show that the natural frequency and the damping loss factor of the circular corrugated cylindrical shell are higher than those of the axial corrugated sandwich cylindrical shell.The former is more sensitive to the change of geometric parameters than the latter.Several results and conlusions might be helpful to guide the manufacturing and dynamic analysis of lightweight cylindrical shells in engineering.Then,considering the influence of the structure quality,the vibration damping characteristics of the composite sandwich panels with multiple damping configurations are studied systematically by experiments and numerical methods.The stiffness efficiency,damping efficiency and overall efficiency of the composite structure are considered.Meanwhile the vibration damping of curved sandwich corrugated cylindrical panels are also investigated,and the corresponding performance parameters are also calssifid into Lakes' modulus-loss factor map.Results show that such structures combined with damping materials would significantly improve the damping loss efficiency but simultaneously decrease the stiffness efficiency in varying degrees compared with the bare hybrid sandwich panels.Among the alternative damping configurations,the bare sandwich panels filled with polyurethane foam can yield the best performance.It is also shown that multiple patch damping treatments based on the FE-MSE approach are suitable and effective to further improve the total damping efficiency.Finally,defects including debonding between face sheets and truss cores,truss missing,face sheet wrinkling and gap reinforcing are introduced into the intact composite pyramidal truss-like core sandwich cylindrical panels artificially.Modal testing and numerical models are conducted to study their dynamic behavior under free-free boundary conditions.The effects of defect extents,locations and forms on the modal parameters of the present sandwich cylindrical panels are discussed.Results indicate that the natural frequencies of the specimen with defects of would decrease in varying degrees compared with the intact specimen and the fundamental frequency is much more sensitive than other order frequencies.In addition,damping loss factors are much more sensitive than their corresponding frequencies.It can be found that the vibration amplitudes of the structures generally decrease by the introduction of defects,which implies higher damping.Some conclusions and essential mechanisms are summarized,which lay the foundation for the non-destructive evaluation of such kind of composite sandwich structures.