Predicting Extremely Low-Cycle Fatigue Crack Initiation Life Of Steel Bridge Piers

Strong earthquakes have occurred frequently in recent years,which caused great damage to the traffic facilities in earthquake stricken areas,especially to bridge piers.The traditional reinforced concrete bridge piers are seriously damaged due to large self-weight and poor ductility,while the steel bridge piers have higher strength and better seismic performance.In addition,the steel bridges piers have the advantages of small occupation of land,short construction period,so it obtains a wide range of applications in Japan and other developed countries.Steel bridge pier dissipates seismic energy by large inelastic reciprocating deformation,which results in large plastic strain that is several times of yield strain at the pier and foundation connection.It may cause extremely low-cycle fatigue destruction in few cycles.Therefore,it has very strong practical significance to study the extremely low-cycle fatigue crack initiation life of the steel bridge piers.By using software package ABAQUS,three-dimensional finite element models for steel bridge piers subjected to a constant axial load and different lateral displacement cycles are established.The best method for predicting extremely low-cycle fatigue crack initiation life is proposed,and the factors to influence the extremely low-cycle fatigue crack initiation life are also studied.The main research results are as follows:(1)15 three-dimensional elastic-plastic finite element models are established to predict the lateral load-lateral displacement hysteretic curves of steel bridge piers based on the test results.The analytical results are in good agreement with the test results before the initiation of extremely lowcycle fatigue crack.(2)Both local and non-local damage methods in Ge model are employed to predict extremely low-cycle fatigue crack initiation life of steel bridge piers under various load patterns,assuming the grid density to be 2×2mm,1×1mm and 0.5×0.5mm,respectively.It is found that the non-local damage method in Ge model with the grid density of 2 × 2mm is suitable for extremely low-cycle fatigue crack initiation life prediction of steel bridge piers.For steel bridge piers with Q345 grade,the results obtained by Ge model are conservative compared with the test results,while the results obtained by Ge model are slightly larger than the test results for steel bridge piers with Q235 grade.(3)By using three dimensional solid elements at the base of steel bridge piers with Q345 grade,CVGM is employed to predict extremely low-cycle fatigue crack initiation life,assuming the grid density to be 2 × 2mm,1×1mm,respectively.It is found that the extremely low-cycle fatigue crack initiation life obtained by CVGM model with grid density of 1×1mm is more reliable than that with grid density of 2×2mm,and the non-local damage method in Ge model is more applicable in predicting extremely lowcycle fatigue crack initiation life of steel bridge piers.(4)Using non-local damage method in Ge model,extremely low-cycle fatigue crack initiation life of 72 steel bridge piers under various load patterns with the grid density of 2×2mm are predicted,and the effect of axial load ratio is also studied.The main conclusions are as follows:(a)With the same width-to-thickness ratio,extremely low-cycle crack initiation life remains unchanged with the increase of the slenderness ratio.(b)In the case of small width-to-thickness ratio,extremely low-cycle crack initiation life decreases with the increase of the width-to-thickness ratio;In the case of large slenderness ratio,the number of half cycles corresponding to local buckling is less than or equal to extremely low-cycle crack initiation life.On the basis of parametric analytical results,empirical formulas for predicting the crack initiation life of steel bridge piers are proposed.(c)When the displacement amplitude is consistent,the axial load ratio has few effects on the extremely low-cycle crack initiation life.When the displacement amplitude is equal to the yield value corresponding to different axial load ratio,the extremely low-cycle crack initiation life increases with the increase of axial load ratio.