Study On Torsional Fatigue And Effect Of Hydrogen Of High Strength Steel Based On The Evolution Of Deformation Inhomogeneity

During the process of use,fatigue failure of steel construct will occur due to cyclic torsion load.For the current fatigue theory,the torsional fatigue life law of steel is usually described by the fitting curve of a series of test fatigue data.The results obtained by empirical method cannot reflect the mesoscopic mechanism of torsion fatigue failure.In some service environments,hydrogen enters into steel by means of diffusion or other ways,which leading to serious degradation of mechanical properties such as strength and fatigue resistance.However,most of the studies on hydrogen-induced failure of steel mainly focus on the damage under axial(monotonic or reciprocating)loading for the limitation of test method and equipment.There are few reports on hydrogen on torsional fatigue.Therefore,it is necessary to explore the low cycle fatigue mechanism of steel,the corresponding life prediction method and the effect of hydrogen on the torsion fatigue properties of steel.30CrMnSiNi2A is high strength steel,which is often used in important structure.The evolution law of meso-deformation inhomogeneity,fatigue index parameters and life prediction methods of 30 Cr Mn Si Ni2 A steel with hydrogenated/unhydrogented are researched under the torsional cycle.The following results are obtained:(1)The results show that the torsional failure resistance and fatigue life of30 Cr Mn Si Ni2 A are greatly reduced with the increase of hydrogen concentration and the steel failure mode is changed from shear failure to tensile failure.Under the same condition of hydrogen charging,the larger the shear strain amplitude,the larger the decrease of the torsional fatigue life caused by hydrogen,but the effect of hydrogen does not significantly change the hysteretic behavior of the material.(2)The polycrystalline structure and deformation mode of the material are described by the representative volume element(RVE)of the voronoi polycrystalline set combined with the plastic constitutive model and the shear cycle process of 30 Cr Mn Si Ni2 A steel is simulated numerically.A calibration method of crystal plasticity model parameters considering non-Masing properties is proposed.The comparison between the numerical simulation and the test hysteresis loops shows that the non-Masing characteristics of materials can be described reasonably and accurately by the method.(3)It is found that there is a correlation between the evolution of meso-deformation inhomogeneity and the torsion fatigue life of 30 Cr Mn Si Ni2 A steel.Two fatigue indicator parameters(FIP)are studied: standard deviation of cyclic peak shear strain and standard deviation of cyclic peak shear strain weighted by the ratio of peak stress to yield stress.They are all used to characterize the evolution of inhomogeneous deformation with cycle number,and the torsional fatigue life curve of materials are predicted by their corresponding critical value,respectively.Both of them can give reasonable prediction,but the prediction error of the latter is smaller than the deviation of test.It is proved that the fatigue failure mechanism of materials can be described by the method because the torsional fatigue life of the other strain amplitudes can be predicted just only using the fatigue test data of one strain amplitude.(4)The results of experiments and numerical simulation with crystal plasticity mode of 30 Cr Mn Si Ni2 A with hydrogen-charged show that the shear stress has a great influence on the relationship between the evolution of meso-deformation inhomogeneity and the torsion fatigue life of the material.The higher the shear stress,the more serious the life decline.A method for describing the torsional fatigue failure law of hydrogen-charged is established based on the evolution of meso-deformation inhomogeneity.The applicability of the method in describing the failure law of hydrogen-induced torsional fatigue is verified by the torsional fatigue test of 30 Cr Mn Si Ni2 A with hydrogen charged for 1h.