Analysis Of Linear And Nonlinear Vibration Characteristics Of Fiber Reinforced Composite Thin Cylindrical Shells Under Partial Bolt Looseness Boundary

Fiber-reinforced thin cylindrical shells have excellent properties such as high specific strength,large specific modulus,and strong corrosion resistance.They are often used to make important parts of high-tech equipment,such as the space shuttle’s launch drum,wing fan case,torpedo and missile head.These composite thin-walled cylindrical shells usually work in harsh environments,such as high temperature,high pressure and vibration for a long time,and most components are connected by bolts.Bolted parts are usually weak link in the structural system and play a very important role in the safe operation of the mechanical structure.For example,a rocket developed in the former Soviet Union in the 1960s caused 29 of the 30 engines to stop working due to a loose bolt,resulting in an explosion that killed more than 100 people.Therefore,studying the vibration characteristics,stability and fatigue failure of composite thin shells under partial bolt looseness boundary of thin-walled is particularly important to improve the dynamic performance of the structure and prevent the occurrence of vibration hazards.Vibration problems of composite thin shells with loose bolts include:(1)The inherent properties of fiber reinforced composite thin shells(FCTS)were studied under partial bolt loosening boundaries.Firstly,the fiber-reinforced composite thin shell is taken as the research object,based on the Love’s shell theory,the eigen equation of the composite thin shell is constructed.According to the artificial spring technique,the theoretical model of the partial bolt loosening of the composite thin shell boundary is established,and the Fourier series method is applied to establish mode function of composite thin shell.Then,the natural frequency and mode shape of the theoretical model are solved by the Lagrangian energy method and the Rayleigh-Ritz method.Finally,the test platform for testing the inherent characteristics of fiber-reinforced composite thin shells was built,and experiments analysis were carried out on TC300 carbon fiber/epoxy-based composite shells to verify the correctness of the theoretical model.(2)The effect of fiber-reinforced composite thin shell boundary on modal response and modal damping under fundamental excitation is studied based on complex modulus method.Firstly,the theoretical model of the composite thin-shell boundary change is established,and the basic excitation is equivalent to the uniform inertial force.Then,the modal strain energy method is used to calculate the modal damping ratio of the fiber reinforced composite thin shell,and the modal response of the fiber reinforced composite thin shell under the fundamental excitation is solved by the modal superposition method and the space-time variable separation method.Finally,the test platform for testing the vibration characteristics of fiber reinforced composite thin shell was built.The TC300 carbon fiber/epoxy resin-based fiber reinforced composite thin shell was taken as the test object.The test data were obtained by fixed frequency and frequency sweep experiment respectively.And the experimental and theoretical predictions results were compared and verified(3)The concept of boundary damping is introduced because of considering the boundary conditions of the actual bolted joint.Firstly,the Jones-Nelson nonlinear theory and complex modulus method are used to express the elastic modulus of the composite thin shell and the nonlinear model of the fiber reinforced composite thin shell under the partial loosening boundary is established.Then,the nonlinear intrinsic characteristic of the composite thin shell are solved by Rayleigh-Ritz method and iterative technique,and the vibration response of the composite thin shell is obtained by Newton-Raphson iterative technique.Finally,an experimental platform was built to test the nonlinear vibration parameters of fiber-reinforced composite thin shells under different excitation amplitudes to verify the correctness of the theoretical model.In this paper,two partial bolt loosening models are established from shallow to deep,and the influence of partial bolt loosening boundary on the vibration characteristics of composite thin shell is discussed in depth.The two models plays an important role for accurately simulating the bolted joints,and provides important technical support for studying the linear and nonlinear vibration phenomena of composite thin shells under arbitrary boundaries.