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

本研究的最终目的是建立实用的质量控制标准，焊接接头在钢建筑结构，以避免脆性断裂。为了实现这一目标，需要建立一种方法来定量预测结构在满量程屈服后的脆性断裂开始，这在1995年科比地震中经常发生。第一个主题是利用新开发的CTOD设计曲线方法预测全尺寸梁柱连接中发生的脆性断裂。对带有人工焊接缺陷的梁柱组合件进行了循环荷载试验。通过非线性有限元方法对试样进行了分析。通过CTOD设计曲线方法评估了数值模拟试件的脆性断裂脆弱性。结果或评估很好地再现了测试结果。第二个主题是评估 ...更多信息 通过失效评定图（FAD），简化焊接连接的可能性。对沿焊趾沿着有疲劳裂纹的简单焊接T型接头进行断裂试验。有意安装的疲劳裂纹代表焊接缺陷。通过非线性有限元方法对试件进行建模。数值模型表明，这些标本是易受脆性断裂，因为模型的阻力落在FAD曲线之一。然而，试验结果表明，试样有更大的阻力比预测的FAD曲线，虽然试样最终失败的脆性断裂后，持续大的塑性变形以及韧性裂纹扩展。这些实验和分析结果表明，两个问题，即在缺口根部的约束水平和临界CTOD和延性裂纹扩展之前的脆性断裂之间的关系，仍有待进一步研究。CTOD设计曲线法和FAD最初开发的实际使用的各种结构的质量保证。然而，如果将这两种方法与非线性数值分析相结合，则发现它们适用于预测脆性断裂的开始。这两种方法可作为今后质量控制标准的基础。少

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