Study On High Temperature Creep And Fatigue Life For Typically Threaded Connections Of Aero-engine

Low cycle fatigue of bolt is the main failure mode in the structure of high temperature threaded connections,meanwhile,the creep effect of threaded connections could not be ignored.Therefore,it is of great significance to study high-temperature creep and low cycle fatigue life of bolt.In this paper,high temperature creep and low cycle fatigue life of typically threaded connections of aero-engine were studied.The main contents include the relationship between tightening torque and preload and influencing factors;the study on sealing performance of threaded flange connections;and the research on creep damage and the prediction method for fatigue life of threaded connections.The specific work is summarized as follows: 1.In this study,a detailed three-dimensional finite element model of bolt and nut was established with consideration of helix angle,pitch,and other details for the threaded connections tightened by torque method.The process of pre-tightening of threaded connections was simulated to study the relationship between torque and preload.The curves for the relationship between torque and preload obtained from finite element analysis were compared with those calculated from theoretical equations.Then,the range of tightening torque coefficient was determined,and the influence of friction coefficient,pitch,elastic modulus,assembly clearance,and strain-hardening exponent on the relationship was studied.2.The sealing performance of threaded flange connections was researched using finite element method.The contact stress distribution on contact surfaces of threaded flanges in radial and circumferential direction under preload and work load was analyzed.The results provide a basis for sealing performance evaluation of threaded flange connections.3.In this study,Kachanov-Rabotnov model and stress relaxation damage constitutive equations deduced from Kachanov-Rabotnov model were applied to analyze the creep damage and predict life for threaded connection structure at high temperature with finite element method.The parameters of Kachanov-Rabotnov model were obtained by fitting the results of creep experiments for titanium alloy at 650?.Based on the experimental and finite element analysis results for standard specimen,a creep failure criterion was established.Then the influences of the external tensile load on the creep damage and life,as well as the stress relaxation on the initial preload were studied.The analysis of stress relaxation for bolt shows that the stress relaxation has a remarkable effect on the bolt preload.The preloads decrease to a determined value with creep time and remain almost unchanged later.When the determined value is less than the required preload acting on the bolt,the structure will fail due to insufficient preload caused by stress relaxation.4.High temperature low cycle fatigue of bolted connections under the action of axial tensile cyclic load was studied through finite element method and experiment.A new low cycle fatigue life prediction model was proposed,the low cycle fatigue life of bolt fatigue life calculated by the proposed model were compared with test results.The results shown that the fatigue life obtained through the proposed model can agree well with the experimental results.