Research On The Nonlinear Dynamic Characteristics Of The Rotational Blade With A Dovetail Joint

As an significant part of aero-engine and gas turbine compressor,the blade with a dovetail joint is widely used in aviation,electric power,navigation and other fields.The blade with axial insertion is usually loosely fitted with the disc to form a jointed structure,which can suppress vibration and achieve low maintenance cost.The dovetail joint is compressed by mortise with the help of centrifugal load,so the surfaces on the mortise and joint contact with each other and allow a certain degree of relative movement between them.The existence of dovetail jointed structure not only changes the blade’s boundary conditions,but introduces nonlinear friction at the blade root,which makes the vibration characteristics of the blade become a problem including variable boundary conditions,variable contact stiffness and damping.Therefore,it is essential to investigate the nonlinear dynamic behavior of the blade with a dovetail joint.In this paper,the blade with a dovetail joint is taken as the research object,and the friction model on joint interfaces is developed.With that,the dynamics model of the blade is established.The contact behaviors on friction interfaces and the nonlinear responses of the blade are studied combining theoretical research with experimental test.The following four aspects are realized:(1)A time-domain algorithm and its linearization method for calculating the nonlinear friction force are developed,the effect of high-order harmonic components is considered.The dynamic differential equations of the blade with a dovetail joint are obtained conveniently by means of Chebyshev polynomials theory.Based on the developed model,the effect of the harmonic number on calculations is discussed by incremental harmonic balance method.Moreover,the influences of friction coefficient,excitation amplitude,and rotational speed on the amplitude-frequency response of the system are analyzed.The variation of amplitudefrequency curves is well explained from the perspective of contact stiffness and contact damping.(2)A novel micro-slip friction modeling approach based on the Iwan model is presented here,where the normal pressure distribution on the joint interfaces is considered.The normal load and tangential stiffness are reassigned reasonably to obtain the tangential force-displacement expression,and the relationship between the asymmetry of contact pressure distribution and the Iwan density function is established.Based on the model,a simplified non-linear dynamics model of the rotating blade with a dovetail joint is developed.The effects of pressure distribution coefficient and asymmetry coefficient on the blade’s forced response are discussed,and the variation of dissipated energy and contact behavior on the interfaces are investigated.(3)A testing apparatus for the blade with a dovetail joint is designed and built.The parameter identification experiment on the joint interfaces is carried out on a fatigue testing machine.The relationship between the tangential friction force and displacement is measured.An identification method of contact parameters is given,and then the friction coefficient,tangential stiffness and residual stiffness are obtained.The experimental results are substituted into the proposed micro-slip model,and the corrected hysteresis relationship is obtained.Compared with the experimental results,the effectiveness of the friction modeling process is verified.In addition,the blade’s vibration test system is put up,the vibration signals are measured by means of a non-contact exciter and a acceleration sensor.The effects of excitation level and pre-load on the vibration response of the blade are mainly discussed.(4)The finite element model of the pre-twisted blade with a dovetail joint is established,and the contact pressures at the each node of joint interfaces under different rotational speeds are obtained.The effect of the blade’s setting angle is involved.The numerical integration method is employed to reduce the dimension of the obtained pressure,and then the distribution functions of the contact pressure on the contact interfaces are acquired by fitting.Then,the relationship between the tangential friction force and displacement is derived through forge links between the original Iwan model and the distribution functions.Based on these,the nonlinear dynamic model of the blade is developed.The effects of the setting angle,pre-twisted angle,excitation level and rotating speed on the vibration response and the contact behavior on the joint interfaces are discussed.