Fatigue Life Prediction Of Aviation Structure Under Multiaxial Loading

Fatigue failure is one of the main causes of mechanical parts failure according to the rearch,especially multiaxial fatigue failure.Statistically,about 80% of all strength problems occurred in aircraft are caused by fatigue damage.Compared to uniaxial loading,the direction and magnitude of principal stresses vary with time under multiaxial fatigue loading.Owing to that the critical plane concept is based on the physical mechanism,the critical plane method has been widely used in the prediction of multiaxial fatigue life.The method to predict multiaxial fatigue life in this article is also based on this concept.The following works have been done in this thesis:The methods of calculating shear stress/strain amplitude and mean stress/strain on material plane under multiaxial nonproportional loading are systematic analysed,including Longest Projection(LP)method,Longest Chord(LC)method,Minimum Circumscribed Circle(MCC)method,Minimum Circumscribed Ellipse(MCE)method,Maximum Rectangular Hull(MRH)method and Maximum Variance Method(MVM)method.A new model,Maximum Nonproportional Factor Method(MNFM)is proposed based on the idea of discribing the intensity of non-proportionality of the strain history proposed by Itoh.The experimental data of six steels under multiaxial loading with mean stress is used to verify the MNFM method.The results show that there is little difference between MNFM and other methods for loading without mean stress,while MNFM gives a more satisfactory prediction for loading with tensile and torsional mean stresses.Life prediction models combined with MNFM are utilized to analyse the fatigue lifetime of BT1-0,BT9,TC4,TC21,ZTC4,S460 N and Inconel 718.Three models are chosen to analyse the fatigue life of titanium alloys under loading without mean stress,i.e.,Fatemi-Socie(FS)model,CHEN energy model and Wu–Hu–Song(WHS)model.All the results are within a 30% deviation band.The Fatemi-Socie(FS)model is used to analyse the fatigue life of S460N?TC4 and Inconel 718 which are under loading with mean stress.All the results are within a 20% deviation band.Based on MNFM,a new modified FS method which contains the ratio of the maximum value of the normal stress to shear stress amplitude is proposed.It is capable of modelling not only the presence of superimposed static stresses,but also the degree of multiaxiality and non-proportionality of the applied load history.The new modified FS method is used to predict multiaxial fatigue life of five kinds of titanium alloys and the other five steels,i.e.,S355J2,SNCM630,304,S45 C and 1050 N.All the results are within a 30% deviation band.The fatigue lifetime of landing gear of one certain aircraft under multiaxial loading is analysed.The critical points of strut cylinder which have stress concentration is assessed through finite element analysis.The MVM method,combined with Zhang-Yao model,modified FS model,rain-flow counting method and the Miner's linear rule is used to accomplish life prediction under multiaxial loading.The result shows a satisfactory life prediction for strut cylinder.This research can be used as a reference for actual structure design.