Research In Numerical Simulation Of Fatigue Crack Propagation In Long-Span Steel Bridges

Fatigue cracking is one of the typical failure models of bridge compents and connection details. The initiation and propagation of fatigue crackle or crack not only affects the safety of the structure, but also triggers a lot of maintenance costs. Therefore, the rational and accurate prediction of crack growth is of great significance for the maintenance and strengthening of the fatigue sensitive details. In this study, the numeircal simulation on the crack propagation under random loads is investigated regarding some fatigue vulnerable details of the long-span cable-stayed bridge. The work conducted in this paper is summarized as follows:(1) The fracture mechanics theories and methods regarding fatigue crack and crack propagation are investigated. An introduction to the three cracking patterns and their corresponding causes, the asymptotic field of a cracktip, means of calculation of fracture parameters, models of fatigue crack propagation and simulation methods of crack propagation is made, and the applicability of various methods is discussed.(2) A numerical simulation method about fatigue crack propagation in large span steel bridges subjected to random vehicle load and ambient temperature is presented. A comprehensive probabilistic vehicle load model for fatigue analysis and ambient temperature model are established using data from the toll station and health monitoring system, respectively. Based on multi-scale modeling and the submodel technique, simulation on crack propagation is realized through local remeshing. The minimum strain energy density method is adopted to distinguish propagation direction of the composite crack.(3) Statistical analysis of cracks on longitudinal diaphragms in steel box girder of a cable-stayed bridge is conducted based on field inspection. Distribution of cracks, characteristics and propagation law are discussed. The role of the longitudinal diaphragms and the influence on bridge performance when damaged are investigated using finite element analysis, as well as the reason for cracking of longitudinal diaphragms. It is found that the bridge withstands large stress amplitude and alternating loads. Thereafter, the fatigue life of longitudinal diaphragms in the Runyang Cable-stayed Bridge (RCB) is predicted though probabilistic finite element analyses, and the results are basically identical with the field inspection.(4) Based on the simulation method of crack propagation proposed in this paper, analysis on crack propagation in different details of orthotropic steel decks is conducted. The contribution of each crack pattern to crack propagation and the time-history curve of the stress intensity factor are analyzed. According to the linear elastic fracture mechanics, crack propagation life of each detail is predicted.The above work provide a means to understand the fatigue damage mechanism of large span steel bridges and to simulate the process of the fatigue crack initiation and propagation, which give references to the design optimization, maintenance and retrofits.