Low Cycle Fatigue Life Evaluation Of Notched Metal Specimens Under Multiaxial Loading

After many years of research,the problem of fatigue has not been completely solved,and accidents caused by fatigue damage still happen from time to time.In particular,the fatigue law of notched member is very different from that of smooth member because the deformation of the member at the notch is non-uniform and the strain gradient exists.In the previous studies on fatigue law of notched members,the influence of strain gradient is mainly reflected in the modification of characteristic strain or stress.It is rare to study the fatigue properties of materials based on the effect of strain gradient on the constitutive behavior of materials.Taking construction steel Q235 and powder superalloy FGH96 as the main research objects,the fatigue problems of metal notch specimens affected by strain gradient are studied by combining fatigue test and finite element numerical simulation.Firstly,in order to study the fatigue law of notched specimens and the influence of strain gradient under uniaxial loading,uniaxial tensile and compressive fatigue tests of Q235 smooth specimens and U-shaped notched specimens of four sizes were carried out.According to the fatigue test results of Q235 smooth specimen and notched specimen,it can be found that the fatigue life curves of the four sizes of notched specimens are significantly different from that of the smooth specimens.In order to study the fatigue law of notched solid specimen and the influence of strain gradient under multi-axial loading,the high-temperature uniaxial and multiaxial fatigue tests of FGH96 smooth solid and hollow specimens and the high-temperature multi-axial fatigue tests of two types of notched solid specimens were carried out.When FGH96 notched solid specimen is subjected to tension and torsion composite loading,the strain gradient distribution at the notched root is complex,and the fatigue life law of FGH96 notched solid specimen is very different from that of smooth specimen.In the high temperature fatigue test of FGH96 solid and hollow specimens,the crack source,crack propagation area and the final fracture area were observed.Through observation,it can be found that the crack source tends to form near the location of the segregation compound,and the final fracture zone is not uniformly distributed and has the characteristics of intergranular fracture,dimple,slip fracture,etc.,which are greatly correlated with the distance to the crack source.According to the fatigue test results,if the influence of strain gradient is not taken into account,there will be a large deviation in predicting or evaluating the fatigue life of notched specimens by the nominal stress strain method or local stress strain method.In this paper,a cyclic plastic constitutive model which can take the influence of strain gradient into account is proposed from the point of view that strain gradient affects the cyclic plastic constitutive model and then the fatigue property of the material.According to the theory of low-order strain gradient,the strain gradient is introduced into the cyclic plastic constitutive model to reflect its influence on the hardening property of materials.The user element subroutine of ABAQUS finite element analysis software for calculating strain gradient and the user material subroutine UMAT for calculating material constitutive relation are prepared.These two subroutines were used to analyze the behavior of the Chaboche cyclic plastic constitutive model considering the influence of strain gradient.The validity of the finite element method is tested by referring to the filament torsion test example studied by Fleck,and the simulation results prove the rationality of the method.Aiming at the calibration problem of related parameters of strain gradient in the model,a method of measuring the strain distribution on the surface of notch root of notched specimen by digital image processing technology was proposed.Finally,the new model is used to evaluate the fatigue life of specimens affected by strain gradient.The new model is used to simulate the fatigue tests of torsional solid specimen and notched specimen under their respective loading conditions,and the fatigue life of the specimen affected by strain gradient is evaluated according to the residual equivalent plastic strain at the hazard point obtained by simulation.With finite element method considering the influences of strain gradient and critical distance method,the results show that compared with the other two methods,this paper puts forward the consideration of the influence of strain gradient Chaboche models to predict the fatigue life of both for Q235 notch samples.The uniaxial fatigue test,the FGH96 solid single shaft torsional fatigue test specimens or FGH96 notch samples.The compound torsional fatigue test of the predicted results are more consistent.This method can be used in engineering practice to predict the fatigue life of specimens and components affected by strain gradient more conveniently and accurately.The fatigue life predicted by the proposed Chaboche model considering the influence of strain gradient is more consistent with the measured results of uniaxial fatigue test for Q235 notched specimen,uniaxial torsional fatigue test for FGH96 solid specimen,or composite tensile and torsion fatigue test for FGH96 notched specimen.This method can be used in engineering practice to predict the fatigue life of specimens and components affected by strain gradient more conveniently and accurately.