Research into Brittle Fracture of Connections with Weld Defects--Targeting at Establishment of Practical Quality Control Criteria for Steel Constructions

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The final goal of this investigation is to establish practical quality control criteria for welded joints in steel building structures for avoidance of brittle fracture. In order to attain this goal, it is required to establish a methodology for predicting quantitatively an initiation of brittle fracture after full-scale yielding of the structure, which frequently occurred during the 1995 Kobe Earthquake.The research is divisible into two main subjects. The first subject concerns prediction of brittle fracture occurring in full-scale beam-to-column connections utilizing a newly developed CTOD design curve approach. Cyclic loading tests were conducted on beam-to-column assemblies with artificial weld defects. Specimens were analyzed by a nonlinear finite element method. The vulnerability to brittle fracture of numerically modeled specimens were assessed by means of the CTOD design curve approach. The results or assessment reproduced well the test results.The second subject is to assess … More the possibility of simpler welded connections by the failure assessment diagram (FAD). Simple welded T-joins with fatigue cracks along the weld toes were tested to fracture. Fatigue cracks, installed intentionally, were to represent weld defects. The specimens were modeled by a nonlinear finite element method. The numerical models suggested that these specimens were susceptible to brittle fracture because the resistance of the models fell on one of the FAD curves. Test results, however, showed that specimens had much greater resistance than the predicted FAD curves, although specimens eventually failed by brittle fracture after sustaining large plastic deformation as well as ductile crack extension. These experimental and analytical results demonstrated that the two issues, namely, the relationships between the level of constraint at notch roots and the critical CTOD and ductile crack extension preceding brittle fracture, remained to be studied further.Both the CTOD design curve approach and FAD were originally developed for the practical use for quality assurance of various structures. However, these two methods were found applicable to predict an initiation of brittle fracture, if they were combined with a nonlinear numerical analysis. These two methods may be used as a basis for quality control criteria in the future. Less