| The research about bolted joint interface has always been a hot content.Since a large number of bolted joint interfaces exist on machine tools,and many mechanical parts assembly requires bolt connection,thus the study of dynamic characteristics of bolted joint interface is necessary.Bolted joint interfaces of different structures have different mechanical characteristics,and they will be affected by many factors,such as bolt torque and external load etc.,and also are characterized with nonlinear stiffness and damping characteristics.At present,the theoretical research of bolted joint interface modeling method is a key technology in the evaluation of the dynamic characteristics of the system and also an important development direction.Therefore,establishing a reasonable bolted joint interface's simulation model and system dynamics model,and analyzing and studying in depth the nonlinear dynamic characteristics of bolted joint structure,have very important theoretical value and practical significance for building the accurate overall model and evaluating sub structures and system's dynamic response characteristics.In this paper,the bolted joint interface was analyzed in details,and its nonlinear modeling method and nonlinear stiffness and damping characteristics,and nonlinear dynamic characteristics of bolt connection structure were studied by various methods:finite element simulation analysis,theoretical modeling and experimental study.Some useful conclusions were obtained.The results of this study have important theoretical significance and practical value for the whole structural dynamic response analysis.The main contents and innovativeachievements in this paper are as follows:1.Taking a single bolted joint structure as an example,the nonlinear contact finite element model was established.Through the static analysis,the influence of bolt preload on contact stiffness of interface was studied.It was found that under the action of a reasonable range's preload the interface area that produced contact stress could be described as an annular region around bolt hole,whose outer radius had increased by 85%compared with radius of bolt hole.Then the contact stiffness model of interface is obtained.On this basis,the nonlinear finite element model with annular contact pairs for the multi-bolted joint structure was built,and then after the bolts were tightening,free modal analysis was done.Results show that the natural frequencies were lower than those in the analysis result of model without interface,and the simulation model was reliable.2.The steady-state harmonic response simulation analysis of multi-bolted joint structure was carried out.By defining the contact pairs and setting the boundary conditions,the dynamic finite element model based on the nonlinear contact algorithm was established in the ABAQUS software,and the frequency response characteristics of structure under different preloads were discussed;meanwhile,the harmonic excitation test was built to investigate the effects of different torques on the amplitude and resonance frequency of the system,and also verified the accuracy of the finite element model.Furthermore,the transient excitation test of a more complex double shear multi-bolted joint structure was carried out,and the effects of different preloads and excitation levels on results were discussed.It indicated the nonlinear behavior of the system,which laid the foundation for further study.3.For bolted joint interface with composite damping,a single degree of freedom composite damping model was established to identify the interface's relative parameters.Under the premise of only considering the linear stiffness,the balance energy method was used to build the relationship between the parameters and solve the dynamic equation.The expressions of the dry friction parameters and the viscous damping factor were obtained.Also,the harmonic excitation experiments were carried out.Combined with the obtained expressions,the composite damping parameters of interface were identified by changing the excitation levels,and the dry friction parameter curves under different bolt preloads were fitted.Thereby,the composite damping parameter values were applied into the finite element simulation analysis.The system's nonlinear behaviors under different dry friction parameters were compared,and the accuracy of the simulation model was verified.4.For a kind of common double shear lap joint structure,system dynamics analysis and research were carried out.Two degree of freedom nonlinear analytical model of its equivalent simple bolted joint beam structure was established,and the fourth-order Runge-Kutta method was employed to solve dynamic response of the system under the transient shear excitation.The frequency response curves obtained under various stiffness parameters were compared.The simulation results showed that when the torque was increased,the cubic spring stiffness had dependency on the variation of preload only in a certain range,and only changing the cubic stiffness parameters in the nonlinear stiffness expression,the natural frequency would not change significantly;cubic stiffness term is more suitable for modeling of nonlinear system of a wider frequency range.5.A series Iwan model containing cubic stiffness term was established.Under the interaction of nonlinear stiffness,viscous damping and Coulomb damping,the dynamic characteristics of simple bolted joint beams under shear vibration were described.Considering respectively two different of motion states,namely the stick-slip and macroslip motion,nonlinear dynamics equation of system was established under transient shear excitation to analyze the dynamic response under different parameters.Also,when the bolt torque and the excitation amplitude changed,the system's FRF curves showed that nonlinear characteristics of the overall structure were more obvious in case of low preload.6.The new extended Iwan series model was applied into the finite element analysis.Combining double characteristic of nonlinear stiffness and damping,based on the contact algorithm,nonlinear finite element models of two kinds of double shear lap beams with different number of bolts were established under transient shear excitation.Also,the transient excitation experiment were proposed to describe the influence of different preloads and excitation amplitudes on the dynamic behavior of system.By qualitative discussion and analysis,it could be found that the variation of frequency response amplitude and natural frequency were consistent with the simulation datum of the simple bolted beam structure;under action of the same torque,the transfer function curves of two kinds of double shear lap joint structures with different numbers of bolts respectively kept good agreement with experiment datum,which verified the accuracy of the simulation model.7.The nonlinear modeling method and dynamic response analysis of a double shear lap joint structure with viscoelastic layer were presented.Based on the simplified structure of the composite beam model,a four degree of freedom dynamic model based on the phenomenological model in the shear direction was established.The vibration characteristics of the system under various Coulomb frictions and harmonic shear excitation amplitude were analyzed.The results showed that the viscoelastic interlayer material played a certain role in reducing the vibration of the structure,and reduced the energy dissipation of the system;When the bolt preload changed,affected by the viscoelastic layer,Coulomb friction of bolted joint interface was changed,and so did stiffness and damping characteristics of interface,which resulted in the system's dynamic response and total dissipation exhibiting complex nonlinear behavious.8.The nonlinear finite element model of double shear lap joint structure with viscoelastic layer was established.Combined with the non-linear properties of the viscoelastic material,the nonlinear stiffness including cubic stiffness term was employed in the tangential direction,and based on the contact algorithm,a nonlinear stiffness model considering both normal and tangential stiffness of bolted joint interface was established.Also,the proposed harmonic excitation experiments with laser vibrometer in tangential direction are carried out to investigate the nonlinear dynamic response characteristics of system,as well as the influence of bolt preload and viscoelastic material on dynamic characteristics of the bolted joint beam.The results showed that the viscoelastic layer helped reduce vibration at certain extent,especially in the high frequency region of vibration,and some peak frequencies of system could be changed through the viscoelastic layer so that the global stiffness was different.Simultaneously,the simulation curve agreed with the experimental one,which showed that the model could describe the vibration characteristics of the system well.9.The influence of bolted joint interface on different types of the machine tool's sub structures and dynamic characteristic of the whole structural system was investigated.Main key bolted joint interface's models on machine tool were established,and the modal analysis results of spindle box and lathe bed were compared.Also,the harmonic excitation experiments in the normal and tangential directions of the front of spindle box were carried out.The stiffness and damping characteristics in different directions of the bolted joint interfaces were analyzed,and the results showed that the experimental data was in good agreement with the simulation data.Furthermore,the finite element model of overall machine tools were built.The results of modal analysis before and after bolted joint interfaces'treatment were compared.It showed that the high order mode of the machine tool was greatly affected by the bolted joint interface,and the natural frequencies of the latter were lower;the key bolted joint interfaces had greater influence on the stiffness characteristics of the complex whole structural system. |
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