Study On The Modeling Method Of Structrue Interface Stiffness With Interference Fit Based On Fractal Theory

The structure of the aero-engine rotor is complex,and there are a large number of matching joint interfaces.With the existence of machining and assembly errors,the aeroengine has a problem of large vibration dispersion and difficult control in the development process.Practice has shown that the contribution of the dynamic stiffness in the joint face of the rotor never can be neglected.In the past,it is difficult to accurately predict the dynamic response characteristics of the rotor by simply fitting the interface to the rigid connection.The interference fit of the joint is widely used in the rotor structure of aero-engine.In this paper,the fractal theory is applied to focus on the analysis and analysis method of the interfacial stiffness of the interference fit,and the interference and microstructure of the joint are considered.An accurate model of the appearance parameters provides theoretical support for engine design and manufacturing.The main research work of the thesis is as follows:Aiming at the problem that the joint surface contact is simply treated as a single rough surface and the plane contact leads to low prediction accuracy,a reasonable modeling method for considering the contact area distribution of the two rough surfaces is proposed.Based on the MB fractal contact model,the W-M function is used to describe the surface topography of the rough surface.The intersection of the curve and the line is used to simulate the contact between the rough surface and the plane,and the contact area distribution characteristics are obtained to obtain the contact area distribution function.Furthermore,the fractal curve is used to simulate the two rough contacts,and the equivalent method of the fractal dimension of the two rough surfaces is obtained according to the variation law of the area distribution.The joint surface contact stiffness model is established according to the contact area function.The influences of fractal dimension,feature scale parameter and interference amount on the stiffness are analyzed.It is found that increasing the interference and the surface precision grade of the fitting part is beneficial to improve the contact stiffness.According to the contact stiffness between the original function of the contact area and the new function,the finite element model of the interference fit considering the contact stiffness is established by the equivalent spring unit.In the finite element analysis,the equivalent modeling scheme of the joint contact stiffness of the specific structure is obtained by comparison,and the simulation analysis of the modal characteristics of the assembly considering the contact state of the joint surface is realized.Substituting the calculated stiffness value into the finite element model simulation to obtain the natural frequency of the assembly,it is found that the natural frequency increases with the increase of the interference and the accuracy of the contact surface,and does not change when the threshold of the rigid connection is reached.The modal test of the assembly was designed and carried out.The influence of structural parameters on the modal frequency was studied and the theoretical model was verified.The simulated test pieces with typical stop interference fit were designed and processed,and the shape parameters of each test piece were measured and calculated.By analyzing the modal test of multiple sets of different structural parameters,the fractal dimension,feature scale parameters and The influence of the amount of surplus on the natural frequency;the comparison between the experimental results and the single rough surface and the double rough surface model established in this paper shows that the proposed two rough surface models have higher analytical precision and are better than single rough surface.The model also verifies the accuracy of the theoretical analysis of the influence of structural parameters on the natural frequency of the assembly.The stochastic finite element model of the assembly is established to study the influence of parameter uncertainty on the natural frequency of the assembly and the sensitivity of the parameters.Considering the uncertainty of each parameter,the Montecarlo method is used to randomly simulate the sensitivity of contact stiffness to various parameters.It is found that the contact stiffness exhibits a positive skew distribution and the most sensitive to fractal dimension,followed by the structural parameter.It is the excess amount.The stochastic finite element method is used to analyze the distribution characteristics of natural frequencies.It is found that the natural frequency is negatively skewed,and the dispersion is small when the stiffness is close to the threshold.The uncertainty of the natural frequency is negligible,but it is uncertain when the stiffness is low.The random distribution of natural frequencies caused by sex cannot be ignored.It can provide theoretical support for structural coordination design and effective control of vibration dispersion.