Multiaxial Fatigue Life Prediction Of Components Under Variable Amplitude Loadings Using Critical Plane Methods

Mechanical structural failures are caused by fatigue fracture in the field of equipment.For meeting the high performance and reliability service requirements for actual mechanical structures(e.g.aero-engine rotor components,motor train axles,etc.),it is urgent to clarify the physical mechanism behind fatigue failure.Owing to the sophisticated geometrical characteristics of these components(including inverted/rounded corners,holes,grooves,etc.)and uncertain service environments(such as high temperature and pressure,vibration,random/variable amplitude loads,etc.),all of these will lead to complex stress/strain responses of local hazard units in actual mechanical structures.This complex load path influence and the characteristics of load amplitude changing will bring tough problems to the fatigue assessment.Substantial results have been achieved considering various studies under multiaxial constantamplitude loads,of which the multiaxial fatigue method based on critical plane is the most prevalent.However,studies on multiaxial variable amplitude fatigue life prediction are still lacking.The purpose of this thesis is to promote the development of multiaxial fatigue from experimental research to practical engineering applications.Centering on the multiaxial loading path and variable amplitude characteristics,the existing multiaxial fatigue life models are summarized.Combined with experimental data and finite element calculation results,a general fatigue life prediction method considering the effect of multiaxial variable amplitude and loading path is established,as follows:(1)The research history of multiaxial fatigue is summarized,and two methods of multiaxial stress/strain analysis are introduced.The method combined with elastoplastic finite element analysis and plane transformation theory is more suitable for multiaxial fatigue analysis of mechanical structures.Three types of multiaxial fatigue model of the existing critical plane method are described respectively,and the existing models are evaluated based on the results of multiaxial fatigue test.The results show that the prediction ability is poor under multiaxial non-proportional and asymmetric loads,which provides a theoretical basis and research interests for the subsequent construction of damage parameters on the critical plane.(2)In view of the coupling critical plane theory and the concept of Pearson correlation coefficient,the proposed method to calculate the non-proportionality of loadings is established.Based on this,a new multiaxial fatigue life assessment model combining the critical plane method with the suggested non-proportional factor is proposed.Using fatigue experimental data of three metal materials under various load paths for verification,compared with the multiaxial model proposed by FS,Chen and Itoh,the data points computed by the proposed are more concentrated within the life scatter factor of 2.(3)Based on the research of critical plane method and energy model,the concept of maximum variance method is introduced,and a method of determining critical plane under multiaxial load based on the maximum variance method of virtual strain energy is proposed.At the same time,the multiaxial variable amplitude fatigue prediction framework and implementation process based on critical plane method are established by coupling multiaxial cycle counting method and damage accumulation theory.Combined with the multiaxial variable-amplitude loading test of two metal materials,this method can effectively predict the fatigue life of metal materials under multiaxial variable amplitude loading and the working life of compressor blade-disc,which can provide references for the fatigue design and evaluation of mechanical structures in service conditions.