Study On Performance And Safety Design Of Fatigue And Fracture In Large-scale Railway Steel Welded Bridge

First part of the present dissertation is devoted to experimental study of fatigue and fracture behaviour of steel structure which was needed for the design of Wuhu Changjiang Bridge and the composite bridges in Tsinghai-Tibet railway line. The first group of experiments is fatigue tests carried out by using large size samples to imitate 12 kinds of welded fatigue details with 44mm plate butt weld, integral joint and precision cutting technology in order to find out the sensitive places where the fatigue crack tends to create and their S-N curve equations. Those fatigue resistance obtained from the tests renewed the design code, and the conclusion had played a guide role for the safety design in Wuhu Changjiang Bridge. The results of study were accepted in the design Code for steel structure of railway bridge TB 10002.2-99. The second group of experiments is the fatigue crack growth rate test at -50℃low temperature, which was carried for base metal and weld of 14MnNbq with plate thickness 24 mm. The conclusion showed that under -50℃low temperature the material constants in power-law equation of midrange fatigue crack growth proposed by Paris have smaller 'C'and larger 'm', and fatigue crack threshold ? Kth was higher than that obtained under room temperature. Furthermore, the results demonstrated that the S-N curve at -50℃is above the curve that is obtained in room temperature, and its slope is flatter slightly than that of room temperature. On bases of this study decision to use welded steel structure for composite bridge in Tsinghai-Tibet Railway Line was approved by authorities concerned. The third group of experiments is Crack Tip Open Displacement (CTOD) Test under series of temperature. The three-point bend samples were designed for different type of steels, i.e. 14MnNbq and 16Mnq, different plate thickness and welded joints that are applied in practice. The CTOD critical value δc-versus-temperature equations were obtained based on test data. In order to get regular pattern from the complicated and scattered data, a method of emilinating individuation was proposed by the author. For implementation of above mentioned tests special facilities and testing method were developed by the author, which are useful for conducting similar experiments in the future. The fourth group of experiments is Charpy V Notch Impact Test under series of temperatures. All of the samples used are made to correspond to the samples used in the CTOD tests. Formulas representing the relationship between impact energy and temperature are elicited from the tests. These results provided rich and reliable data for determination of the coefficients in fatigue design and anti-fracture design. Second part of the dissertation focused on study of criterion for preventing fracture in bridge code. The conclutions are as follows. (1) Design stress and material impact toughness are two dominant factors that should be taken into consideration in drafting the design code for preventing fracture. (2) In aspect of stress control the design stress and some factors which exerts additional stress in structure are taken as active force. The resistant force is obtained from the CTOD test equation. On bases of these couple of forces the COD design curve formulas checking the limit stresses for preventing brittle fracture are derived and both allowable stress and allowable temperature for different plate thickness are determined. (3) It was found from experiments that certain relationship exits between absorb energy in CTOD and impact test value. On bases of this phenomema the transition state from ductility to brittleness in CTOD test is used as reference to study the criterion of lowest impact energy and its testing temperature. The test temperature is determined by the critical transition temperature of the CTOD test. And the temperature shift is proposed by the author. (4) A draft of fatique design method for Tsinghai-Tibet line is proposed. The study in this dissertation pays much attention to solve practical engineering problem. Conclusions are made on bases of large quantity of experimental data one hand and theoretical analyses using elastic-placstic fracture mechanics on the other hand, they are rigorous and reliable. The recommendations presented in this dissertation may be accepted for drafting bridge design code to adopt anti-fracture criterion.