Low Cycle Fatigue Life Prediction Based On Intrinsic Damage Dissipation

With the advancement of technology and the development of the national economy,people put forward more demanding requirements on the service life,safety performance and economic benefits of mechanical equipment.Fatigue failure,as one of the most serious forms of component failure,endangers its safe operation and may cause huge economic losses.Fatigue can be divided into low cycle fatigue and high cycle fatigue according to the number of failure cycles,among which low cycle fatigue widely exists in important engineering fields such as petroleum,chemical,nuclear power,etc.In these fields,once fatigue failure occurs,the consequences are unimaginable.Therefore,it has important engineering application value to carry out low cycle fatigue life prediction.According to the characteristics of low-cycle fatigue failure behavior,based on continuum mechanics and its irreversible thermodynamic framework,the intrinsic damage dissipation work was used as the fatigue life evaluation index.And the lowcycle fatigue life prediction model was constructed under various load conditions.Including low-cycle fatigue load condition with average strain,two-stage variable amplitude low cycle fatigue load conditions and multi-axis non-proportional low-cycle fatigue load conditions.Firstly,introduce the internal variables of damage and the effective stress under different damage states is described,and the constraint conditions are established according to the basic theories of continuum mechanics and thermodynamics.Under the constraint conditions,the T-type expressions of low-cycle fatigue damage evolution model is established considering only the isotropy of materials.The critical value of damage internal variable which is highly related to material properties and load properties is proposed.Secondly,considering the influence of average strain on fatigue life under low cycle fatigue load,based on continuum damage mechanics and its irreversible thermodynamic framework,the Ramberg-Osgood cyclic constitutive model is introduced to establish the D-type expression of intrinsic damage dissipation work.And the equivalent intrinsic damage dissipation work is taken as equal life condition.A low cycle fatigue life prediction model considering average strain is established.The lowcycle fatigue life prediction of 45 steel and 2124-T851 aluminum alloy is carried out by four kinds of prediction models considering the effect of average strain including intrinsic damage dissipation model,and the corresponding experimental results are compared and demonstrated.The comparison models include modified Ohji model,Sandor model and Wei-Wong model.The results show that the new intrinsic damage dissipation model is superior to the contrast model.Thirdly,according to the D-type expression of the intrinsic damage dissipation work,taking the equivalent intrinsic damage dissipation work as the damage conversion condition,a variable amplitude low cycle fatigue life prediction model considering the load sequence effect iss established.The uniaxial low-cycle fatigue experimental data of P355NL1 structural steel and Ti-6Al-4V titanium alloy under two-stage variable loads are used to compare and demonstrate the applicability of the intrinsic damage dissipation fatigue damage accumulation model under two-stage variable loads.The comparison models include Manson model,Kwofie model and Peng model.The results show that the intrinsic damage dissipation fatigue damage accumulation model has the best performance.Finally,the influence of loading path and strain amplitude on fatigue life are analyzed,and the cyclic stress-strain relationship is established under multiaxial nonproportional loading.Taking the equivalent intrinsic damage dissipated work as the equal life condition,a multiaxial low-cycle fatigue life prediction model is established considering non-proportional additional strengthening effect.The low-cycle fatigue experimental results of Q235 steel,SNCM630 steel and 45 steel thin-walled cylinder specimens under biaxial tension-compression-torsion load conditions are used to compare and demonstrate four models,including the intrinsic damage dissipation model.The comparison models include KBM model,FS model and Li model.The results show that the intrinsic damage dissipation model has the best performance.