Multiaxial Low Cycle Fatigue Life Prediction Of Metal Materials

Fatigue has become one of the most common failure modes of engineering structures subjected to cyclic loads.Fatigue life related to the safe operation of engineering structure and the strength potential of materials.Therefore,fatigue life prediction has always been a major direction of strength research.In the field of simple uniaxial fatigue,researchers have done extensive research and achieved satisfactory fatigue life prediction accuracy.However,in the field of engineering structures,multiaxial fatigue loads are more likely to arise,there is no fatigue life prediction models cowidely accepted by all researchers at present,especially when multiaxial fatigue loads coupling with mean strain,non-proportional loading and other factors,accurate fatigue life prediction are more difficult.In this paper,the fatigue life prediction of plain specimens under symmetric non-proportional loading and mean strain loading is studied.The main research contents are as follows:Based on the study of the variation regularity of shear strain amplitude of different plane of thin-walled circular plain tube under tension-compression and torsion biaxial loading,a new definition method of path non-proportional factor is proposed,which can represents path non-proportional degree influence on fatigue life.The fatigue test data of 304 stainless steel are used to verify the validity and feasibility of the path factor defined in this paper.Its accuracy is compared with Itoh method was used to prove proposed model's accuracy.Based on the shortcomings of existing cyclic yield strength prediction models for some materials,a cyclic yield strength prediction model based on tensile strength is proposed.The effectiveness of the model is verified by the data collected from public paper,and its prediction ability is compared with other models.The existing additional hardening factor prediction model does not consider the influence of the equivalent strain amplitude of loading,which account for lower prediction accuracy.The materials are divided into two classes to study the relationship between the additional hardening factor and the equivalent strain amplitude of loading.It is found that the additional hardening factor of one class of materials increases with the increase of the equivalent strain amplitude of loading,while the additional hardening factor of the other class of materials don't change obviously with the change of the equivalent strain amplitude of loading.Based on this phenomenon,the additional hardening factors prediction models are proposed,one don't consider the influence of equivalent amplitude of loading and the other consider the influence of equivalent strain amplitude of loading.The collected experimental data are used to verify the prediction ability of proposed models,and compare with other models to prove its prediction accuracy.For that the existing non-proportional influence factor can't comprehensively consider the effects of path factors and material factors under non-proportional loading on fatigue life,a non-proportional influence factor that can comprehensively consider the effects of path factors and material factors on fatigue life is proposed in this paper,and a multiaxial low cycle fatigue life prediction model is proposed based on the non-proportional influence factor.The fatigue test data of 8 kinds of materials are used to verify the prediction ability and application range of proposed model,and the prediction ability is compared with FS model and MKBM model for that thses two models have higher prediction accuracy than other models under non-proportional loading.For the shortcomings of the existing considering mean strain effect of fatigue life prediction models,such as the complexity of calculation,can't consider of torsion mean strain loading,based on the interpolation method of MMCCM model,a fatigue life prediction model considering the effect of mean strain is proposed.In the proposed model,the effects of strain multiaxial and mean strain can be considered respectively.The model has form invariance and good expansibility.Combined with the non-proportional influence factor proposed before,we can extended this model into predict the fatigue life under non-proportional and asymmetric loading,and the prediction ability of the model is verified by comparing the collected material fatigue test data with other models.In this paper,the definition of path non-proportional factor,the prediction model of cyclic yield strength,the prediction model of non proportional additional hardening factor,the prediction formula of multiaxial fatigue life for a wider range of materials and the considering the effect of mean strain of fatigue life predict model are proposed,which provide some reference for the future research of fatigue life under complex loading.