Experimental Study On Combined High And Low Cycle Fatigue Of Structural Steel And Combined Fatigue Damage Evaluation Of Steel Bridges

Since the presence of the ultra-low cycle fatigue failure of steel structures in Northridge earthquake and Kobe earthquake,a considerable number of researches have been done on the low cycle fatigue failure mechanism of steel materials and steel joints,and many different low-cycle fatigue damage evaluation methods have been proposed.However,these evaluation methods are effective only for steel materials or structures without any initial damages.Before the outbreak of the seismic event,a certain extent of damage during service life has been accumulated on the steel bridges as well as the main components.At present,the effect of the aforementioned initial damage on the low cycle fatigue damage induced by an earthquake is still unknown.Therefore,with the support of the National Natural Science Foundation of China（No.51708485）,this paper conducted two types of fatigue tests on Q345 structural steel specimens,and proposed practical evaluation formulae of combined high-low cycle fatigue damage of steel and steel piers with consideration of the initial damage.On this basis,the combined fatigue damages of in-service steel piers and a steel arch bridge under strong earthquakeswere analyzed and evaluated.Firstly,the low cycle fatigue and combined high-low cycle fatigue behavior of Q345 structural steel were experimentally studied.Starting from the three aspects of cyclic response characteristics,cyclic hysteretic curve and cyclic stress-strain relationship,the low cycle fatigue performance and combined high-low cycle fatigue performance of this type of steel were analyzed,and the corresponding fatigue life prediction formulae were fitted respectively.The results show that in the process of low cycle fatigue loading,the cyclic peak stress of steel decreases with the increase of strain amplitude,and the hysteretic energy dissipation coefficient increases with the increase of strain amplitude,which reflects a superior hysteretic energy dissipation capacity.The cyclic stress-strain relationship is in good agreement with Ramberg-Osgood formula,which verifies the applicability of Ramberg-Osgood formula.In the process of combined high and low cycle fatigue loading,the cyclic softening index decreases with the increase of initial high cycle fatigue damage DH,and the cyclic stress amplitude decreases significantly when DH reaches 0.7.With the accumulation of initial high cycle fatigue damage,the hysteretic energy dissipation coefficient is almostly unchanged.Comparing the stress amplitude curve predicted by the Ramberg-Osgood formula with the test results,it is found that the average value of steel stress amplitude decreases significantly after DH value reaching 0.7.From the perspective of fatigue life,the pure low cycle fatigue life decreases with the increase of the applied strain amplitude,while the remaining low cycle fatigue life with initial high cycle fatigue damage decreases with the increase of initial damage.In order to study the combined fatigue damage characteristics of in-service steel piers under repeated loads,both the circular-and rectangular-section steel piers with different structural parameters were numerically analyzed.The combined fatigue damage level of steel piers with different initial high cycle fatigue was obtained.Subsequently,the influence of structural parameters on the damage evolution was studied,and the empirical formula for the combined fatigue damage evaluation of in-service steel piers was proposed.The results show that the combined fatigue damage level of steel piers with circular-or rectangularsection increases with the increase of initial high cycle fatigue damage level.Under the same loading conditions,the combined fatigue damage of steel bridge pier is greatly affected by the diameter-thickness ratio,the width-thickness ratio and the axial-compression ratio.When the diameter-thickness ratio or width-thickness ratio are relatively small,the corresponding combined fatigue damage increases sharply with the increase of the initial damage level.To study the combined fatigue damage characteristics of in-service steel piers under strong earthquake,the elastic-plastic seismic response analysis of circular and rectangular steel piers with different structural parameters was carried out.The combined fatigue damage index was obtained,and the applicability of the empirical formula for combined fatigue damage evaluation of in-service steel piers was further verified.The results show that the steel piers with initial fatigue damage are more prone to combined fatigue failure than those without initial damage under strong earthquake.The combined fatigue damage index value of in-service steel bridge piers under strong earthquake is consistent with the prediction of the empirical formula,and the variation with structural parameters is also consistent with each other,which further verifies the effectiveness of the proposed formula.Finally,a deck through steel arch bridge was taken as the object,the nonlinear time history response of the structure under earthquake was analyzed using a fine shell element FE model.The combined fatigue damage index value of the joints on the in service bridge was obtained,and the combined fatigue damage law was analyzed.The results show that the combined fatigue damage value of the steel arch bridge joints in service increases with the increase of high cycle fatigue damage in the early stage under strong earthquake.Under the action of Niigata earthquake,the evolution of the combined fatigue damage index is fast first and then becomes slow,while under the action of Chi-chi earthquake,the evolution of the index is slow first and then becomes fast,indicating that the ground motions have a great influence on the evolution of structural combined fatigue damage.