The Study On CTOD Fracture Toughness Of Steel Bridge

With the enhancement of our country and the development of domestic technique of bridge fabrication, the number of huge steel bridges that will appear is rising. So the control of their fracture toughness appears more important. Now, in our country, the toughness indicator is still Charpy impact toughness and it is not advisable to use impact toughness for huge steel bridge. There are examples of application of CTOD during fabrication of steel bridges, but the number found is only two. Based on previous research, this dissertation is mainly about study on CTOD fracture toughness of steel bridge.The first half of this dissertation introduces test methods of toughness of structural steel for bridges. Because Charpy impact toughness has the advantages of sample operation and low fee, it is applied speedily in engineering. Though series of impact tests under different temperatures, we could establish a curve of impact absorbing energy versus temperature. Then the transformation temperature of impact toughness could be obtained. Deep notched wide plate test is based on linear plastic fracture mechanics and its purpose is to get the critical plane stress intensity factor KC of materials tested, namely resisting force. The relationship of KC and impact transformation temperature, established through statistical means, is amended and then the limit thickness of structure under different temperatures could be determined. However, it is a little dangerous to use the toughness indicator of Charpy impact test for huge steel bridges. Neither, wide plate test is not effective in the situation of large-scale yielding. In order to test the most approximate toughness of real steel bridges and to take full advantage of materials, CTOD test is advisable.The second half of this dissertation talks about the scatter of CTOD values and the effect of temperature and thickness to CTOD values for bridge steel. Just as impact test, CTOD test results appear temperature-transformation phenomenon. In transformation region of CTOD-temperature curve, CTOD values have conspicuous scatter which cannot be neglected. Processing CTOD data of 14MnNbq steel by means of statistical fit, the result is that CTOD values obey the laws of normal distribution, logarithmic normal distribution and three-parameter Weibull distribution. The most effective distribution is the last one. Temperature and thickness are main factors that affect CTOD value and also are the most interested ones. The effect that temperature brings to CTOD value could be shown clearly through a S-shape curve which could be got by series of CTOD tests under different temperature. The effect of thickness to CTOD value is presented by a math formula which is deduced theoretically. At last, according to the characteristics of CTOD-temperature curve, a kind of CTOD criterion is proposed. Define the half of sum of upper platform value and lower platform value as acceptable value of bridge steel and the corresponding temperature as CTOD ductile-brittle transformation temperature. This criterion could be applied to control of fracture toughness of steel bridge.