Study On Multiaxial Low Cycle Fatigue Life Prediction Method Based On Damage Mechanics

In the practical engineering,most parts are under multiaxial loading condition and lots of fatigue failure is occurred under complex stress state.As the fatigue design method is of gradually enriched and actual operation environment is more and more complex,multiaxial fatigue research field is also extended to all kinds of different load level from the high cycle fatigue.Especially the low cycle fatigue,due to its larger load level,damage is more serious,and it is widely exists in large engineering,such as large pressure vessel,aerospace equipment,which is related to national lifeline.Therefore,the research of multiaxial low cycle fatigue is of great realistic significance.Considering developing multiaxial low cycle fatigue life prediction method as the main line,this text,based on the method of modifying cyclic strength coefficient,a new formula of equivalent cyclic stress and strain relation is established.Based on this,a low cycle fatigue crack propagation formula under the proportional load is extended to the non-proportional range.Combining equivalent cyclic stress strain relation and damage evolution relation,a new multiaxial low cycle fatigue life prediction model is obtained.And the model is validated by the existing experimental data.Firstly,based on the theory of thermodynamics and continuum damage mechanics,export multiaxial low cycle fatigue damage evolution and set up its constraint in the frame of the thermodynamics.Secondly,analyzed the difference of description method of cyclic stress-strain relationship between non proportional loading and proportional loading.By defining the geometry of loading path and the method of data statistics,the cyclic strength coefficient is modified from two angles of load path and strain amplitude.And the equivalent cyclic stress-strain relationship is obtained under non proportional loadingThirdly,based on the method of modifying cyclic strength coefficient,a low cycle fatigue crack propagation formula under the proportional load is extended to the non-proportional range by recalculating the total deformation energy density.Finally,based on the damage evolution equation in continuum damage mechanics,which is used to analyses the fatigue damage problem,and new multiaxial cyclic stress strain relationship,the multiaxial low cycle fatigue life prediction formula under non-proportional loading is established.With the formula,the parameters calculation and life prediction of Q235 B steel and HRB335 steel thin walled tube specimens under biaxial tension torsion loading are carried out.The most of the predicted results are within 2 times the error band.