Study On Equivalent Method For Dynamic Characteristics Of Fixed Joint Surface Based On Fractal Contact Theory

The machine joint surfaces exist in assembling the machinery parts. A microscopic or macroscopic vibration displacement occurs when joint surfaces under a complex dynamic load, which will increase friction and abrasion, wear of joint surface. Then vibration-friction phenomenon is stimulateed and the mechanical system presents complicated dynamical characteristic. The dynamical characteristic of joint surfaces (contact stiffness and contact damping) is not only necessary for system functions, but also an important factor in inducing the singular perturbation and failure of a mechanical system, which should be paid more attention during the mechanical design process.In this thesis, the joint surface of composite beams with bolt tightening is taken as the study subject. Fractal theory is taken as the research method. This thesis pays more attention on the influence of friction, the microscopic cottact stiffness and contact damping, the equivalent damping layer, the dynamic modeling and simulations and modal tests.The tasks of this thesis are completed on the below several aspect:(1) Reviewed the related literatures, summing up the equivalent method of dynamic characteristics for joint surface while considerning the influence of friction.(2) Deducing the fractal prediction model of the normal contact stiffness and normal contact damping of the joint surface reference to the damping loss factor considering the influence of friction. Research the impact of fractal parameters, friction coefficient, contact load and other factors on the normal stiffness and damping with numerical simulation.(3) Deducing the fractal prediction model of the tangential contact stiffness and tangential contact damping of the joint surface reference to dry friction equivalent viscous damping considering the influence of friction. Research the impact of fractal parameters, friction coefficient, contact load and other factors on the tangential stiffness and damping with numerical simulation.(4) Researching the impact of bolt tightening torque and friction coefficient on composite beams each mode with nonlinear prestressed modal analysis method while consider the influence of the nonlinear characteristics caused by surface friction contact.(5) Determining the material constants of equivalent damping layer based on the fractal contact stiffness and the strain energy equivalence principle. The first5order modal and modal loss factors of composite beams are acquired with modal strain energy method.(6) Acquiring the first5order modal parameters of composite beam by single-mode least-squares admittance circle fitting method after modal testing. And the scientific rationality of equivalent damping layer method of dynamic characteristics for fixed joint surface is verified.