Analysis Of The Probability Fatigue Fracture Behavior Of Cable Steel Wires In A Suspension Bridge

Bridges in the service would be damaged under the long-term repeated loads.Suspension bridges are most applied in the large-span bridges. The cables in thesuspension bridges are most important for the loading capacity of bridge structures.The cables must have a high anti-fatigue property.The structural components with a smooth surface in fact have many distributedmicro-defects that are invisible by the naked eyes. It is not easy for the homogeneousmaterials to occur the fatigue failure. Alternatively, the discontinuity caused by themicro-defects in the material would induce the stress concentration. This is the mainreason of the fracture of structural components. By the coupling effects of the cyclicloads and the environmental corrosions, the micro-defects would grow from themicro- to macro-scale to result in the final fracture of the structural components. Thematerial failure process is indeed a continuous physical process. In this whole failureprocess, the trans-scale problem must be considered, i.e., from the micro-scale to themacro-scale. The fatigue failure is also a trans-scale problem that passes two differentscales, say micro- to macro-scale. For the existing suspension bridges, the seriousdestruction for the whole structure little occurs. However, we sometime have tochange the cables in the suspension bridges due to fatigue failure or environmentalcorrosions. This is also a great cost.The traditional fatigue analysis approaches are firstly concluded in this work. Inthis basis, a new trans-scale analytical model for the fatigue problems is thenintroduced. Many effects can affect the fatigue property of a material. It can beconcluded that the material micro-structure, the maximum cyclic stress, the amplitudeof the cyclic stress and the material damage evolution mode are the main factors. Theinfluences of these factors on the fatigue life of the structural components areinvestigated in this work.The trans-scale fatigue model is applied in this work. Then, the parameters in themodel are determined accordingly. Because the distributed micro-defects can not bemeasured in the practices, it is assumed that they obey the normal distribution. Themean value and the variance of the normal distribution can be prescribed. Therefore,we can calculate the fatigue life. The theoretical results are compared to the testresults. The result shows that the proposed model is valid for the description of the fatigue process from micro to macro. Next, the proposed trans-scale model is appliedto the fatigue problem of the cables in a real suspension bridge. The a-N curves (i.e.,crack length a versus cycle number N curves) for the steel wires in the cables of thesuspension bridge are then calculated under different sizes of initial defects anddifferent stress levels. Lishui Large Bridge in Zhangjiajie of Hunan province is takenas the engineering example in this work. The FEM model for the whole bridge isestablished by the software ANSYS. The cable forces in the suspension bridge arecalculate and the stress levels of the steel wires in the cables are evaluated accordinglywith the consideration of the additional effects of the cable tightening or loosening byother factors such as structural creep, wind loads and so on. Finally, the fatiguefracture behaviors of steel wires in the cables of suspension bridge are calculated anddiscussed in details. The research conclusion can be useful for the engineeringapplication.