Fatigue Crack Diagnosis And Life Evaluation Approach Of The Orthotropic Steel Decks In Service Environment

An orthotropic steel deck(OSD) has a complicated structure,and its fatigue life is mainly determined by various welding details.The fatigue crack of the deck-to-rib joints is harmful to the bridge and difficult to repair.Moreover,the coupling effect of welding residual stress and vehicle stress and the stress relaxation effect in the process are primary causes of fatigue damage at this kind of welding details.However,the distribution of initial residual stress itself is highly nonlinear,and the relaxation behavior of residual stress under external cyclic loading is more complex,which makes the present evaluation methods unable to consider the welding residual stress and its relaxation effect effectively.In this paper,a simplified method of stress time history at welding details is established based on the residual stress relaxation evolution law and the simplified coupling model of welding residual stress and vehicle-induced stress.Then,the fatigue life of deck-to-rib welded details are evaluated by a full-range S-N curve model of steel fatigue resistance integrating welding residual stress and its relaxation effects systematically and deeply.(1)To investigate the influence of welding residual stress on the fatigue performance of deck-to-rib welded details,the experiments of full-size OSD specimen were carried out by using the hole drilling strain-gauge method.On this basis,an elastic-plastic finite element model(FEM)was established by using the thermal-structure coupling analysis method,and the distribution of the welding residual stress of deck-to-rib welded details is obtained.Then,the influence rule of the design parameters of deck plate and rib variation on the residual stress is mainly analyzed,based on which the unified distribution model of residual stress in the deck plate is proposed for the fatigue analysis.This model can simplify the complicated residual stress analysis process and directly calculate the residual stress of key welded details.(2)To quantify the relaxation effect of welding residual stress in deck-to-rib welded details of OSDs under external cyclic loading,a multi-scale coupling stress analysis model that considers the relaxation effect of residual stress using cyclic plastic constitutive model is established.Furthermore,the relaxation behavior of residual stress on welded details under vehicle loading is analyzed and the calculation method of fatigue parameters,including the stress ratio and equivalent stress amplitude under the coupling action of welding residual stress and vehicle-induced stress are established.Based on the refined solid element sub-model of deck-to-rib,the combined influence of number of cycles,stress amplitude,and stress ratio on multi-axial residual stress relaxation effect under constant/variable amplitude cyclic loading is investigated.Finally,a release formula of welding residual stress relaxation coefficient is proposed based on the external loading stress amplitude,stress ratio,and material yield stress.This formula can simplify the complex coupling analysis process of welding residual stress and vehicle-induced stress,and calculate the value of residual stress release by the value of initial residual stress and the external loading stress quantitatively.(3)As for the difficult solution problems of combining the welding residual stress field and vehicle-induced stress field,the simplified coupling model of welding residual stress and vehicle-induced stress is advanced,based on the similar P-E method and the simplified nodal force structural stress method.Then,the peak value of stress time history curve at welded details is determined under external cyclic loading.On the basis of the release formula of welding residual stress relaxation coefficient,the valley value of stress time history curve at welded details is determined under external cyclic unloading.As a result,a simplified method of stress time history at welding details integrating welding residual stress and its relaxation effects is established.This method can simplify the time and computational efficiency problems caused by the complex multiple nonlinear coupling calculations in the traditional analysis of welding residual stress and it has been validated by the FEM data.The real stress time history curve of welding details considering welding residual stress and its relaxation effect can be calculated by the external loading stress and the structural stress under external cyclic loading.(4)In order to consider the influence of welding residual stress and its relaxation effects on the fatigue life of OSDs,a full-range S-N curve model of steel fatigue resistance integrating welding residual stress and its relaxation effects is proposed by adopting the theory of metal fatigue and the Soderberg formula.Then,the influence mechanism of welding residual stress,stress relaxation and mean stress on the fatigue life the deck-to-rib welded details is revealed.Moreover,the analysis of traffic loading effect is carried out with the engineering background of Jiangyin Yangtze River Bridge.By the way,the effects of the growth of vehicle weight and traffic flow on fatigue life of deck-to-rib joints have been analyzed and the accuracy of the proposed method is validated by comparing with the Eruocode3 and AASHTO standard.