Research On Ultrasonic Measurement Method For Assembly Stress Of Aero-engine Rotor System

Aero-engines are known as “flowers of industry”.Due to the complexity of technology,the difficulty of research and development,and the blockade of foreign technology,it has become a bottleneck problem in China’s high-end equipment manufacturing field.China has comprehensively launched the “two major special projects” of aero-engine and gas turbine,and upgraded them into a national strategy.Assembly is the final link of aero-engine manufacturing,and it is also a difficult and comprehensive integration technology involving many disciplines,which determines the final performance of aero-engine to a great extent.Rotor system is the core component of aero-engine.The uniformity of assembly stress distribution on its circumference is an important factor affecting the assembly accuracy and performance stability of the system.Accurate measurement of assembly stress is the key means to obtain the uniformity of stress distribution in the rotor system.Inaccurate measurement will not be able to accurately evaluate and regulate the distribution of assembly stress.Uneven distribution will cause problems such as aero-engine vibration exceeding the standard,and even disc rupture.Therefore,there is an urgent need to carry out the research on the assembly stress measurement method of rotor system,so as to provide strong theoretical and technical support for improving the assembly accuracy and comprehensive performance of aero-engine in our country.This subject “Research on ultrasonic measurement method for assembly stress of aero-engine rotor system” is facing the urgent requirements of assembly stress measurement,studies the measurement methods of the pretightening stress of the fastening components,the three-dimensional stress of the assembly components,and the contact stress of the faying surface of aero-engine rotor system.The main research contents of the paper are detailed as follows:Aiming at the problem that the acoustic time difference(ultrasonic propagation time difference)in the coupling layer affects the pretightening stress measurement accuracy of fastening components,a pretightening stress measurement method of aero-engine fastening components based on the separation of acoustic time difference in the coupling layer is proposed.This method reveals the influence of acoustic time difference of coupling layer on the measurement accuracy of pretightening stress based on theoretical analysis.By building a three-layer medium ultrasonic propagation model of transition layer-coupling layer-stress layer,the first echo containing coupling layer thickness information and the second echo containing coupling layer thickness and pretightening stress information are generated.The first echo propagation time difference before and after fastening is used to calculate the acoustic time difference of the coupling layer.The second echo before and after fastening is used to calculate the acoustic time difference caused by the coupling layer and the pretightening stress.The acoustic time difference of the coupling layer is separated by subtracting the two acoustic time differences,so as to build a more accurate functional relationship between the acoustic time difference and the pretightening stress of the fastening component,and improve the measurement accuracy of the pretightening stress of the aero-engine fastening component.Aiming at the problem that the coupling mechanism of stress in different directions on ultrasonic propagation is not clear,which makes it difficult to measure the three-dimensional stress of aero-engine assembly components,a three-dimensional stress measurement method based on spatial orthogonal stress decoupling and coupling layer acoustic time difference separation is proposed.Firstly,in order to realize the three-dimensional stress measurement of the assembled components at different depths,the variation of the propagation depth of the critical refracted longitudinal wave(LCR wave)with the frequency of the excitation signal is studied;Then,a three-dimensional stress multi transducer measurement scheme integrating LCR wave transmitting and receiving transducers and feedback transducers is innovatively designed.The acoustic time difference of LCR wave coupling layer is separated by solving the acoustic time difference of the received signal of the feedback transducer;Finally,the x,y,z space orthogonal stresses are converted into stress components parallel to and perpendicular to the propagation direction of LCR wave,and based on the function model of stress in different dimensions and the propagation velocity of LCR wave,an acoustic elastic equation containing space orthogonal stress parameters is established.The propagation direction of LCR wave is changed by rotating the transducer group,and the acoustic elastic equations in the three propagation directions are constructed.The acoustic elastic equations are solved simultaneously to decouple the space orthogonal stresses,and realizes the three-dimensional stress measurement of aero-engine assembly components.Aiming at the problem of low spatial resolution of faying surface contact stress measurement by non water immersion ultrasonic method,a faying surface contact stress measurement method of aero-engine assembly component based on Fresnel lens focused acoustic power spectrum density peak characterization is proposed.Firstly,Fresnel lens with multi ring coplanar structure is designed to overcome the dependence of contact stress focusing ultrasonic measurement on immersion environment;Secondly,the influence of lens and piezoelectric parameters on focusing characteristics is studied,and Fresnel lens parameters are optimized to maximize the focus sound intensity and improve the signal-to-noise ratio of focused ultrasonic signal;Finally,the residual reflected wave of the faying surface is obtained through the time-domain subtraction algorithm,and the peak value of the power spectrum density is used to characterize the residual reflected wave energy.According to the correlation between the residual reflected wave energy and the contact stress of the faying surface,a function model of the peak value of the power spectrum density and the contact stress of the faying surface is established,so as to realize the focused ultrasonic measurement of the contact stress of the faying surface of the aero-engine assembly components under non submerged conditions.On the basis of the above theoretical research,an experimental system for measuring the pretightening stress of the fastening components,the three-dimensional stress of the assembly components and the contact stress of the faying surface of the aero-engine rotor system was built and the measurement experiments were carried out to verify the effectiveness of the assembly stress measurement method proposed in this paper.The experimental results show that:(1)The measurement method based on the separation of acoustic time difference of the coupling layer improves the measurement accuracy of the pretightening stress of the fastening component,and the relative error of the measurement of the pretightening stress of the fastening component is reduced from 10.33% to 5.43%;(2)The three-dimensional stress measurement of assembly components is realized.The measurement errors of three-dimensional stress in x,y and z directions based on spatial orthogonal stress decoupling and coupling layer acoustic time difference separation are 6.772 MPa,3.365 MPa and-5.329 MPa respectively;(3)Focused ultrasonic measurement of the contact stress of the faying surface under non immersion conditions is realized.The maximum measurement error of the contact stress of the faying surface characterized by the peak value of the Fresnel lens focused ultrasonic power spectrum density is 0.719 MPa,which overcomes the dependence of the existing focused ultrasonic measurement on the immersion environment,and provides a new technical way for the contact stress measurement of the faying surface of aero-engine assembly components.