Research On Multiaxial Fatigue Life Prediction Model Based On Critical Plane Theory

With the rapid development of various heavy machinery and construction machinery in production applications,higher requirements have been placed on the strength,rigidity and fatigue life of their key structures.Fatigue research under uniaxial loading has been quite mature,but in multiaxial fatigue research,especially under non-proportional loads,a unified multiaxial fatigue failure criterion that can be applied to different materials,structures and load conditions has not been found.Therefore,the study of multiaxial fatigue damage theory has important theoretical significance and application value for solving the fatigue strength problem of engineering components.In this paper,the shaft components of Q345 steel are taken as the research object,and the fatigue damage mechanism and fatigue life of the shaft members under multiaxial proportional and non-proportional loading are analyzed through theoretical methods,and finally verified by the multiaxial fatigue test results.The main research contents are as follows:(1)Multiaxial fatigue test research on Q345 steel.By changing the test load conditions and the shape and size of the test specimen,the effects of phase difference,load amplitude and geometric parameters on the multiaxi al fatigue life were studied.The loading method of multiaxial load is tensile and compression-torsion composite loading.A series of uniaxial and multiaxial fatigue tests are carried out on the smooth and notched specimens of Q345 steel material,which provides test basis for multiaxial fatigue theory.(2)A method for determining the critical plane of the notched specimen is proposed.In this method,the finite element software is used to obtain the stress and strain tensors at dangerous points under different wo rking conditions.The coordinate transformation matrix is used to obtain the stress and strain components on any plane.The plane where the maximum shear strain is defined is the critical plane.(3)A new non-proportional additional hardening factor related to material constants and phase differences is defined,which considers the effect of different phase differences and material properties on the additional hardening.Application of this factor can improve the applicability of the proposed life predictio n model to different loading methods and different materials.(4)Based on the critical plane method a multiaxial fatigue life prediction model of metal materials was established,which considers the effects of phase difference,material properties and mean strain.The model compared with the calculation results and fatigue test results,the FS model and SWT model.The results show that the multiaxial equivalent linear fatigue life prediction model is superior to the other two models and has a higher life prediction accuracy.