Local Stress-Strain Analysis And Fatigue Life Prediction Of Notched Components Under Multiaxial Loading

With the technical advance,higher performance and reliability index for engineering critical components are highly expected.Catering to the requirement of assembling and different functional needs,engineering components with complex configurations or geometric discontinuities have been utilized nowadays.Subjected to complex cyclic loadings,these geometric discontinuities(notches)cause significant stress concentrations,then mechanical components fail at low stress levels due to irreversible microplastic deformation followed by crack growth and propagation as well as fracture and final failure.In fatigue analyses of structures with notches,evaluating the fatigue life of the whole structure by employing the stress-strain data at the root of the notch only generally yields conservative predictions.Whereas domains near the surface quickly reach yield strength and are therefore plastically strained,neighbouring material which less stressed will support the highly stressed area thus shielding it and retard the process of fatigue crack initiation and fatigue failure;thus,traditional methods which only consider critical points no longer serve notch effect.Therefore,when performing fatigue analysis on notched components,to consider the influence of notch support effect,stress-strain states in regions near the notch root should be included.Besides,another important influencing factor cannot be ignored is the multiaxial stress state.Affected by irregular geometry,even under uniaxial loading,a complex multiaxial stress field appears near the notch root region or geometrical discontinuities.If subjected to multiaxial loading,its stress-strain states will be more complicated.With the study extends,various models for multiaxial fatigue analysis have been developed;however,valid methods considering both notch effects and multiaxial loads in fatigue design of engineering components are still lacking and highly desirable.According to this,coupling with FE analysis,this thesis focuses on the following works:(1)Summarize the developments on multiaxial fatigue and notch effect analysis,be familiar with the development of related theories and their application.(2)A new damage parameter based on stress gradient in the boundary element is proposed,which accounts for the notch effect on fatigue life and strength.Moreover,by coupling with Fatemi-Socie(FS)damage criterion,a computational framework for multiaxial fatigue analyses is established for notched components.In addition,structural strength evaluation of a compressor blade-disc attachment is performed by employing the proposed analyze framework.(3)A modified Ellyin's model is elaborated based on total strain energy density under multiaxial loadings.By defining a proper weight function and the effective damage zone,a concept of energy field intensity is proposed,which inherits the merits of energy criteria as their unifying microscopic and macroscopic experimental evidences,and meanwhile considers the interactive influence of critical regions.The combination of energy dissipation and the concept of field makes this energy field intensity approach innovative,which could be a promising approach to characterize the notch effect of arbitrary geometry features under multiaxial loadings.(4)The feasibility of coupling the critical plane method(widely used in multiaxial fatigue evaluation)and the theory of critical distance(widely used in notch effect analysis)for fatigue analysis of notched parts under multiaxial loading is discussed.Besides,the selection of point method(PM)or line method(LM)in critical distance theory in coupling analysis and the rationality of regarding the critical distance as a material constant or as a function related to fatigue life are also studied.Finally,the optimal coupling sequence of the critical plane method and the theory of critical distance is determined.