Multi-scale Multiaxial Fatigue Life Prediction Model Considering Non-proportional Additional Strengthening Effect

Multi-axial fatigue has become one of the most common fatigue failure behaviors in engineering.Under multiaxial non-proportional loading conditions,the fatigue deformation of materials is much more complex than under uniaxial and proportional cyclic loading.There will be significant non-proportional additional strengthening effect in the stress concentration area of the component.The physical mechanism of plastic deformation of the material under non-proportional additional strengthening effect is not clear enough.This makes many of the current fatigue damage assessment guidelines and life prediction methods based on macroscopic damage parameters and equivalent force-strain theory inapplicable.It is difficult to explore the strength potential of materials and ensure the safety of service components.Therefore,based on the material macroscopic and microscopic characterization parameters,this paper establishes a multi-axial fatigue damage theory and life prediction model,which can reflect the non-proportional additional strengthening effect of the material.The fatigue life prediction of metal structural components is also carried out for this model.The research contents are as follows:Based on the atomic thermal perturbation theory and Frank-Read source dislocation theory,the mechanism of non-proportional additional strengthening effect is explained from the micro-fine scale.The multi-axial non-proportional cyclic loading conditions have a facilitating effect on the microscopic atomic escape behavior of the material.The generated vacancies will have a pinned effect on the dislocation motion,resulting in continuous dislocation appreciation and further promoting the interaction of multi-system resident slip bands.Thus,the evolution of fatigue damage is accelerated.Based on the equivalent effect variable damage theory,the characterization coefficients of non-proportional damage at macroscopic and microscopic scales are coupled to define the fatigue damage characterization parameters of multiaxial non-proportional loading.The theory of microscopic non-proportional atomic escape rate is developed for the sensitivity of material response to non-proportional cyclic loading.Combined with the analysis of the influence of the load magnitude and the variation of the loading path on the additional strengthening effect,the load non-proportionality characterization parameter is introduced.Considering the influence of both load and material influences on the additional strengthening effect,the theory of multi-axial fatigue damage is developed.Based on the critical plane method and Mises yield criterion,non-proportional additional reinforcement damage parameters are introduced to construct a multi-axis fatigue life prediction model considering non-proportional additional reinforcement effects at multiple scales.The maximum shear strain plane is selected as the critical plane.The shear strain and positive strain on this plane are used as the basic parameters for fatigue damage evolution.A multi-axis fatigue life prediction model considering non-proportional additional reinforcement is established.The multi-axial fatigue test analysis of a typical thin-walled circular tube specimen is used as an example.Based on the developed multi-axial fatigue life prediction model,the multi-axial fatigue damage analysis and life prediction of metal specimens are carried out by combining Abaqus and Fe-safe software.The results show that the prediction effect of the developed model is significantly better than that of the traditional model.The fatigue life of metal components under uniaxial,multiaxial proportional and multiaxial non-proportional loading can be accurately predicted.And the prediction accuracy is higher under multi-axial loading conditions.