Nonlinear Characteristics Analysis And Dynamic Response Of Bolted Joint Surface

The bolted connection structure is easy to assemble and disassemble,the preloading force is easy to control,the cost is low,and the connection is reliable,therefore,it is widely used in various assembly structures.However,the bolted joint structure also lead to discontinuity in structure and materials,introduces contact nonlinearity,and the complex contact mechanism of the joint interface leads to the nonlinear characteristics of the bolted interface.The contact finite element algorithm is used to establish the assembly contact model,which has many degrees of freedom,low computational efficiency,difficulty in convergence,large memory space,and high time-consuming calculation of dynamic response of the contact model.The linear part of the complex assembly can be simplified by the modal synthesis method,and the method of simplifying the contact part is not mature.In this paper,the following work is carried out on the characterization of the nonlinear characteristics of the bolted joint surface,the hysteresis curve discretization method,the contact nonlinear simplified equivalent model establishment,the static and dynamic response,and the simplified equivalent model for building complex structures.:Based on Weierstrass-Mandbrot function,the micro-morphology of rough surfaces in contact is simulated,and the influence of fractal dimension and characteristic scale on the surface morphology of the interface is studied,the relationship between restoring force and displacement of the interface is deduced according to the surface morphology,deformation and stick-slip friction behavior,which is qualitatively revealed that the interface increases with tangential load,the stiffness softening phenomenon and hysteretic characteristics of restoring force and displacement are also discussed.The contact finite element model of overlap structure is established,the hysteretic curve of finite element simulation shows that the pre-tightening force,friction coefficient and displacement amplitude increase,the area surrounded by hysteretic curve increases,and the energy dissipation increases.At the same time,the residual stiffness of the joint surface exists,and the residual stiffness is mainly affected by friction coefficient,and MTS fatigue test machine is used and bolt overlap structure were tested,and the hysteretic curves of the contact model and the hysteretic curves of the contact model coincided,which indicated the validity of the contact model.The continuous Iwan model(C-Iwan model)has a hysteresis characteristic of restoring force and displacement,and the relationship between hysteresis characteristics and displacement amplitude is consistent with the bolted joint interface.But the C-Iwan model lacks residual stiffness,so paralleling a constant-coefficient spring in the C-Iwan model constitutes a modified continuous Iwan model(MC-Iwan model).Since the MC-Iwan model can only reveal the hysteresis characteristics and energy dissipation of the joint interface,it is not suitable for solving the dynamics response of the joint interface.Therefore,using the idea of turning the curve into a straight line,a modified discrete Iwan model(MD-Iwan model)composed of several Jenkins elements and a constant coefficient spring is used instead of the MC-Iwan model to solve piecewise linear analytical solution of the MD-Iwan model dynamic equation.The dynamic response is obtained by the analytical solution,and the dynamic response obtained by solving the MD-Iwan model with the COMBIN40,COMBIN14,and MASS21 elements in ANSYS is consistent with the analytical solution.The hysteresis characteristics of the MD-Iwan model and the bolted joint interface are consistent,based on the MD-Iwan model,a hysteresis curve discretization method is proposed.The hysteretic curve and the experimental hysteresis curve of the lap joint contact model are respectively discrete.The MD-Iwan parameters are obtained,and simplified equivalent model of overlap structure is established in ANSYS to solve the dynamic response,which is consistent with the static hysteresis characteristics and dynamic response of the contact model,and the computational efficiency is greatly improved,indicating that the discretization method is established and the equivalent model is valid.Finally,for the complex bolted joint structure,the contact model is firstly used to obtain the normal tension and compression bilinear and tangential hysteresis characteristics of the structure;the non-contact part is fixed by the fixed interface modal synthesis method.Perform modal polycondensation to establish a modal synthesis model of complex structures;for the complex structure of the tension and compression bilinear COMBIN40 with a gap and a COMBIN14 are used to replace,tangentially using the discretization method,coupling in three directions,to establish an equivalent model of complex structures.The axial and tangential characteristics of the equivalent model are verified,and the results are in good agreement.The modal analysis is carried out on the three models,the modal comprehensive model and the contact model are completely consistent,the equivalent model agrees well and only there are some difference in the mode of axial torsion.The tangential and normal dynamic harmonic excitation and impact response calculations are performed for the three models,the results show that the modal comprehensive model and the contact model are completely consistent,and the equivalent model tangential and normal harmonic excitation response and the tangential and normal shock response agree well,it shows that the simplified model is effective,and equivalent model improves computational efficiency and occupies less memory space,the equivalent model scalability is good.