A New Theory Of Stress Field Intensity Based On Internal Damage Dissipation In Critical Domain

Fatigue failure,one of the most serious forms of material damage,endangers the safe operation of components and may cause huge economic losses.In the field of modern industry,more than 80% of structural strength damage is caused by fatigue,such as rail,aircraft,spacecraft,steam turbine and rolling mill transmission shaft are threatened by fatigue failure to varying degrees.Because most of the modern mechanical components need to meet the requirements of long life,high cycle fatigue damage has become one of the main damage forms of components.Underestimation of high-cycle fatigue strength of structural components can not bring its strength potential into full play,which is not conducive to improving economic benefits,while,overestimation threatens the service safety of structural components and may lead to safety accidents.Therefore,it has far-reaching engineering application value to carry out high-precision prediction of high-cycle fatigue strength of structural components,and the construction of corresponding theoretical model is the key scientific problem to solve the above problems.The critical point method,represented by stress invariant method,critical damage surface method,meso-integration method,energy method and damage mechanics method,aggregates stress and strain at dangerous points,constructs damage control parameters with stress and strain at dangerous points,and establishes high cycle fatigue failure criteria.The high cycle fatigue damage of structural components occurs in a specific region and is gradual,accompanied by crack initiation and propagation.Therefore,the critical point method does not coincide with the physical phenomena of high cycle fatigue damage,while the critical region method focuses on the fatigue damage process of a specific region,which coincides with the physical phenomena of fatigue damage.The critical region method represented by the traditional stress field intensity method has strong physical significance,but the failure stress function and its weight function in the field lack the corresponding mechanical mechanism and the foundation of failure mechanics.Based on the damage mechanics of continuum and its irreversible thermodynamic framework,combining with the idea of critical region theory modeling,a new stress field intensity theory is constructed in this paper,in which the dissipated energy of intrinsic damage of uniform critical region for the whole life is taken as the index of evaluating fatigue life.The effective stress tensor considering the tension-compression anisotropy is described by the plane normal stress with the maximum shear stress amplitude.This description describes the tension-compression anisotropy of the normal stress on the crack initiation plane,which has definite physical significance and overcomes the shortcomings of the principal stress method in describing the tension-compression anisotropy.Based on the continuum damage mechanics and its irreversible thermodynamic framework,D-type description of the intrinsic damage dissipation evolution model considering the tension-compression heterogeneity,is constructed.A new critical value of internal damage variable is proposed to characterize the cyclic damage driving force and material properties.These work lay a foundation for constructing a new stress field intensity theory based on intrinsic damage dissipation.Taking the damage dissipation of whole life in the uniform critical region as the equal-life condition,a new expression of stress field intensity is deduced for high-cycle fatigue under tension superposed mean stress and high-cycle fatigue under pure torsion superposed mean shear stress and high-cycle fatigue under combined bending and torsion,which takes into account tension-compression anisotropy.The suitability of the new stress field intensity under three loading forms for predicting fatigue strength(life)was demonstrated by comparing uniaxial tension of aluminium alloy(LC4 and LC9),pure torsion of metal materials(76S-T61,2A12-T4 and 34CrNiMo6)and metal materials(Hard Steel,Mild Steel,34Cr4,30NCD16,C20,EN-GJS800-2 and Ti-6Al-4V),respectively.The results show that the new stress field intensity method is superior to the existing models in predicting fatigue strength(life).Secondly,the new stress field intensity theory is extended to the condition of variable amplitude load,and the damage accumulation model for variable amplitude load path based on the new stress field intensity theory is established.The applicability of the new damage cumulative model for uniaxial and multiaxial variable amplitude loads is demonstrated by comparing the uniaxial variable amplitude fatigue test data of three common metal materials and variable amplitude fatigue test data under combined tensile torsion and under loading block of LY12 CZ aluminum alloy.The results show that the new model performs the best.Finally,the high-cycle fatigue strength of typical double-notched specimens is analyzed by using the new stress field intensity theory.The fatigue strength of two typical double-notched specimens is predicted theoretically through the development tools of UMAT and Excel-VBA provided by ABAQUS.The prediction results show that the new stress field intensity theory can accurately predict the fatigue fracture location of multi-notched parts,and the prediction effect is better than the traditional stress field intensity method,which is more suitable for the fatigue intensity prediction of notched parts.