Research On Low-cycle Fatigue Behavior And Damage Mechanism Of DP600 Steel Sheet Under Shear Path

Thin-walled metal parts are often used as key load-bearing mechanical component in many products,such as aircraft and automobiles.However,these components are often subjected to complex forces in practice,which makes the fatigue performance for thin-walled component very difficult to predict.Based on existing researches,the fatigue behavior of thin-walled metal parts mainly focuses on the uniaxial tension-tension and tension-compression loads.However,as one of the main loads of thin-walled parts,the effect of shear on the initiation and evolution mechanism of fatigue crack defects is not clear.In addition,metal thin-walled structures can still play a supporting role after buckling under shear loads,and the plastic deformation and buckling fatigue caused by shear loads will inevitably affect the service performance of thin-walled parts,which has gradually attracted the attention of many researchers in recent years.At present,the experimental methods and theoretical systems for the study of the fatigue behavior of metal sheets under the cyclic shear path and the composite path need to be further enriched and improved.Therefore,this study first developed a cyclic shear fatigue testing machine and a composite path fatigue testing machine.On this basis,the DP600 sheet was subjected to strain-and stress-controlled low-cycle fatigue tests,and the cyclic shear path and tensile-shear fatigue tests were developed.The research on the soft/hardening behavior,fatigue life,micro-deformation,ratchet effect,texture development of DP600 under the composite path,the main research contents and results are as follows:Devices for fatigue tests under cyclic shear and tensile-shear composite paths are constructed in this thesis.Based on the analysis of existing specimen form and fatigue test characteristics,the overall structure of the device is designed,the clearance of transmission system is shielded and the influence of elastic deformation of fixture parts is weakened,in result,the high-precision and long-term stable acquisition of shear deformation is realized.Meanwhile,the structure avoids the reaction torque in the shear path and reduces the mutual interference between tensile and shear loads in the composite path.From the observation of the tests,the variation trend of stress-strain curve is continuously smooth,which has a high response accuracy to the loading conditions,and can clearly describe the mechanical change of the material during the fatigue loading process.The experimental results show that the data of the devices have high reliability and good repeatability.The low cycle fatigue behavior of DP600 under strain control has been studied and discussed in this thesis.The effect of strain ratio on low cycle fatigue characteristics was analyzed,the development trend of softening and hardening behavior,cyclic stress-strain curve,plastic hysteresis energy density,and fatigue life under different strain ratios are analyzed and compared.The crack propagation process is observed,the damage mechanism is analyzed,and the effect of strain ratio on the damage development process,micro-deformation,and texture change is revealed.The results show that a larger strain ratio is beneficial to the development of y-fiber texture,and it will improve the fluidity and the micro-deformation inhomogeneity of DP600,which is beneficial for fatigue life improving.At a larger strain ratio,the softening rate of the stable section with the same loading amplitude is smaller,and the plastic strain energy density is smaller.The effect of pre-strain on cyclic shear properties has also been studied.The tensile pre-strain of the material is carried out on the mechanical testing device with a special fixture,and then the original specimens and pre-strain specimen s in different directions are selected for the fatigue test under strain control.The results show that the fatigue life in the TD direction is longer than that in the RD direction,and the difference between fatigue life in different directions decreases after pre-deformation.Large pre-strain will reduce the fatigue life under the shearing path.The ratcheting effect under the shearing path is studied and the effect of ratcheting effect on fatigue is analyzed.Firstly,the effects of stress amplitude and mean stress on ratcheting strain accumulation are studied.Then,the cyclic loading test with variable parameters is carried out to reveal that material hardening is the internal influencing factor of ratcheting suppression.The influence of ratchet loading on fatigue cycle strength and fatigue life is studied.The ratchet loading produces high dislocation density and local dislocation density concentration,and leads to grain refinement during fatigue loading,which will reduce the dislocation density and the risk of micro-cracks,improve the cycle strength and fatigue life.This study also conducted the relationship between tensile load and cyclic shear load.Meanwhile,the non-unloading conditions,the effects of pre-stretching on the yield point,strengthening rate and elongation of uniaxial shear are studied.The interaction of stretching loads on shear fatigue behavior and ratcheting effect has been analyzed.Based on experimental results and theoretical analysis,the unidirectional load under different paths softens the other path.Pre-stretching deformation increases the initial ratcheting strain but has no effect on the accumulation trend and speed of the ratcheting strain.Ratchet loading improves the strengthening rate of the tensile path more than strain-controlled fatigue loading,but with reduce of elongation which is little affected by fatigue loading.