Fatigue Failure Mode Analysis Of Riveted Structural Details Based On Structural Stress Method

As a very important and typical early steel bridge,the riveted steel bridge occupies a very important position in the traffic lines all over the world.Most of the early riveted steel bridges have been in service for more than 50 years.The mechanical properties of the materials used are poor,the loads they bear during the service period are much higher than they were originally designed and the key components and joints have different degrees of corrosion.The combined action of load and corrosion leads to corrosion fatigue failure of steel structure bridges,which seriously affects the structural safety during their service life.Therefore,in-depth study of the influence of corrosion development and evolution on early riveted steel bridges,and exploration of the deterioration process of fatigue performance of riveted steel bridges,so as to formulate targeted management methods.These studies are of great significance to this kind of early riveted steel bridges with important historical and artistic value.Based on the typical riveted steel truss girder bridge-Niujiaotuo Bridge,this thesis has carried out a series of researches on corrosion fatigue problems.The contents are as follows:（1）Fatigue tests were carried out on twelve groups of corroded specimen which come from Niujiaotuo bridge and six groups of uncorroded riveted butt joint specimen.The results showed that corrosion significantly reduced the fatigue resistance of the structure.Through further analysis of fatigue crack section,the law of evolution and development of fatigue crack is obtained.Through reverse modeling,the law and critical value of the migration of fatigue failure mode of riveted butt joint specimen caused by corrosion are studied.（2）In view of the shortcomings of the nominal stress method,the damage mechanics method and the fracture mechanics method,the fatigue performance of the riveted structure is evaluated by using the main S-N curve based on the structural stress method.The mesh insensitivity of the structural stress method in the riveted joint structure is verified by dividing meshes of different sizes for the riveted joint plate with riveting holes in finite element.By scanning the three-dimensional topography of the corroded riveted butt joint specimens and inverse modeling,calculating the structural stress of the fatigue cracking section,combined with the fatigue test data verifying the suitability of the main S-N curve for corrosion.（3）The influence of different corrosion morphology characteristics on the deterioration of fatigue effect is quantitatively analyzed,and the fatigue effect deterioration equation is established and verified with the two parameters of section unevenness coefficientη_p and section thickness weakening rateη_ave as independent variables,and the fatigue effect deterioration coefficientγas the dependent variable.Compared the traditional multivariate nonlinear fitting regression method based on the Levenberg-Marquardt algorithm and the nonlinear fitting regression method based on support vector regression（SVR）.（4）According to the identification results of the most dangerous joint of the whole bridge,the upper chord joint of the outer main girders in the transition area from the support stiffening position to the mid-span position are more prone to fatigue failure.The full bridge model and the local solid model of the riveted joint are established,and the corrosion morphology features are introduced into the inclined web member of the joint model to verify the validity and applicability of the fatigue effect degradation equation under the actual complex load.The characteristics of corrosion morphology were introduced,and the migration law of fatigue failure mode under actual complex loads was studied.