Investigation On Hydrogen-induced Metal Fatigue Failure Analysis And Predicted Method Of Mesoscopic Model

Hydrogen atom is the smallest atom in diameter.It enters into metal materials and deteriorates their mechanical properties,mainly for reducing the service life,plastic deformation ability and strength of metal materials,resulting in premature failure of engineering structures or components,resulting in catastrophic accidents.Some researcher have proposed theories based on macroscopic and microscopic experimental observations and numerical simulation to explain hydrogen-induced damage phenomena,but the mechanism of interaction between hydrogen and the microstructure of materials and resulting in embrittlement of materials is still controversial.In this dissertation,the uniaxial tensile tests of Q235 low carbon steel,45CrNiMo VA low alloy steel and FGH96 powder metallurgy material without and with H-hydrogen were carried out.On this basis,strain-fatigue tests were further carried out on Q235 steel and45CrNiMo VA low alloy steel without and with hydrogen-charged and comparing the influence of hydrogen on cyclic characteristics of two materials,fatigue fracture was analyzed to discuss the mechanism of hydrogen-induced fatigue failure.Based on the experimental analysis,a crystal plastic constitutive model combined with the RVE(Representative Volume Element,RVE)polycrystalline aggregate model,which reflects microstructure characteristics of the polycrystalline materials,was used to simulate the low-cycle fatigue behavior of the material.The stress and strain of RVE model at grain scale during cycling were statistically analyzed.Fatigue damage of materials was measured by using statistical parameters as Fatigue Indicator Parameter(FIP),and predicted fatigue life of materials.The main research works and conclusions are summarized as follows:1.The uniaxial tensile testing of Q235 steel,45CrNiMo VA low alloy steel and FGH96powder superalloy with different microstructures were carried out.The hydrogen embrittlement sensitivity and fracture mode of three materials with different microstructures are different.(a)Yield strength,tensile strength and strain hardening effect of 235 steel are not significantly affected by hydrogen,but the ductility of 235 steel is greatly reduced.For45CrNiMo VA alloy steel,hydrogen can significantly reduced the ductility and strength,even brittle fracture occured when stress level was not reached yield strength.High concentration of hydrogen would reduce the strain hardening effect,tensile strength and ductility of FGH96powder metallurgy.(b)SEM images of tensile fractures of the three materials shows that fracture modes are strongly related to material microstructure and hydrogen-charged time:Crack initation sites of Q235 steel with hydrogen-charged originate from the inclusions inside the material,and shows a"fish-eye"fracture morphology around the inclusions,and the entire cross section was almost covered with the dimple and cleavage features.Crack initial sites of45CrNiMo VA alloy steel transfer from the center of the cross section to the surface with increase of hydrogen-charged time,and mixed fracture morphology of dimple,cleavage and intergranular characteristics can be observed in crack propagation region.Hydrogen mainly promotes surface crack initiation FGH96 alloy,but has no obvious effect on the fracture morphology of inner region.brittle cleavage fracture can be seen near the specimen surface,and the depth of the brittle zone increases with increase of hydrogen-charged time.2.Strain fatigue testing of Q235 steel and 45CrNiMo VA low alloy steel with and without hydrogen-charged were carried out,Tested results shows that:(a)Q235 steel without hydrogen-uncharged at all strain amplitude shows almost cyclic steady state and exhibits Masing characteristics,Hydrogen can increase the elastic range(isotropic stress)of the hysteresis loop of Q235 steel at low strain amplitude,which results in the increase of the peak stress level,but has little effect on the hysteresis behavior at high strain amplitude.Cyclic softening behavior of 45CrNiMo VA steel is closely related to the strain amplitude,and the non-Masing behavior was more obvious at low strain amplitude,moreover,non-Masing behavior is more obvious at low strain amplitude,Hysteresis behavior of 45CrNiMo VA steel was not significantly affected by hydrogen at all strain amplitudes.(b)The analysis of low cycle fatigue test data of two materials show that the reduction of fatigue life with hydrogen-charged specimens at high strain amplitude is much greater than that at low strain amplitude.Moreover,fatigue life curves of Coffin-Manson and Basquin were fitted by the test data and the elastic and plastic strain amplitudes respectively to evaluate their contribution to the damage in presence of hydrogen.It was found that the reduction of fatigue life of45CrNiMo VA alloy steel with little hydrogen-charged time was mainly due to the plastic strain amplitude,and the elastic strain amplitude have little effect on the reduction of fatigue life.However,both elastic and plastic strain amplitudes have obvious effects on the reduction of fatigue life of Q235 steel with hydrogen-charged.With increase of H-charged time,the effect of elastic strain amplitude on damage becomes more and more significant.However,both elastic and plastic strain amplitude have obvious effects on fatigue damage of Q235 steel with hydrogen-charged specimens.(c)The detailed analysis of the fatigue fracture for two materials show that hydrogen mainly promotes the crack initiation and accelerates the growth fatigue cracks,which reduce fatigue life.3.A physically-based crystal plasticty constitutive model and the RVE model reflecting the structural characteristics of polycrystalline materials were established,so that the cyclic plastic behavior of polycrystalline materials can be reasonably simulated,and the evolution of meso-stress and strain with number of cycles in RVE model can be shown clearly.The statistical standard deviation(?)_l(?)of the longitudinal strain in loading direction,the statistical standard deviation(?)₁ of the first principal strain and the maximum value(?)_1,maxof the first principal strain are selected as FIPs to evaluate fatigue life of polycrystalline materials,respectively.Finally,the rationality of the proposed three FIPs for life estimation was verified by tested fatigue life.It was proved that the proposed method can reasonably predict the low cycle fatigue life of Masing and non-Masing materials,the FIP critical value in this dissertation can be determined by tested fatigue life at a strain amplitude,Fatigue life under other strain amplitudes can be predicted according to the determined FIP critical values.Thus,the strain amplitude-life curve can be obtained.4.Based on the prediction of fatigue life without hydrogen-charged specimens,a modified FIP model considering the hydrogen-induced fatigue damage was proposed,which can describe the influence of different hydrogen-charged time on variation law of fatigue life of materials.