Study On Fatigue Crack Growth Of 14MnNbq Steel And Welds

Fatigue is the main reason to failures of engineering structures and components. It is unavoidable that fatigue cracks initialize when steel bridges are constructed and fatigue cracks grow when the bridge is working. According to the statistic data, 90 percent of failures occurred in steel welded bridges result from fatigue. Therefore, it is very important for welded structures to study the mechanical properties concerned with fatigue crack growth in the steels and welds.The purpose of this thesis is to study the fatigue crack growth of 14MnNbq steel and welds, which are used in Tian Xing Zhou Bridge. Firstly, we introduced the experiments conditions and methods in our research: the experiments proceeded according to related national standards; specimen use center crack tension specimen, The welds welding procedure refer to Tian Xing Zhou bridge design technology.Afterwards, this thesis displayed 14MnNbq steel and welds fatigue crack growth data, which were obtained by experiments that under 3 different stress ratio conditions, the results of Paris formula fitting were also given. The results indicate that fatigue crack growth rate data and stress intensity factor range present high linear relationship under log-log plot. It is found that14MnNbq welds HAZ(heat affected zone) is the weak area. Comparing with base metal, welded seam and HAZ, the fatigue crack growth rate in HAZ is higher during crack initiation and stabilizes growth stage. Meanwhile, 14MnNbq fracture toughness result was also given by experiments.Finally, we introduced the research on full-range fatigue crack performance of 14MnNbq steel and welds. A four-parameter Forman formula fitted full-rang experiment data is used, and full-range fatigue crack growth expression of 14MnNbq steel and welds is presented. It is found that fatigue test data and theoretical predictions agree well with each other under different stress ratio and in three stages of fatigue crack growth. Therefore, the four-parameter Forman formula can be used to represent full-range fatigue crack performance of 14MnNbq steel and welds effectively. All the experiment results and theory forecast meet fatigue crack growth general rule.