Evaluation of Fracture Toughness of Field-Welded Joints Containing Defects--Targeting at Establishment of Optimum Quality Control Criteria for Steel Construction

钢结构建筑框架现场焊接接头的缺陷很难完全消除。当工程师发现焊接缺陷时，他们需要有可靠的标准来决定是否进行修复。本研究的目的是建立一个定量评估焊接缺陷脆性断裂风险的程序。在神户地震中观察到的材料大量屈服后发生的脆性断裂，不能用经典断裂力学方法预先预测。由于裂纹尖端塑性约束的丧失使这些区域的应力松弛，因此不能仅通过CTOD或J积分等材料断裂参数来预测脆性断裂的发生。为了考虑塑性约束的影响，提出了表观J积分，将其定义为高应力区与紧致拉伸试样裂纹尖端体积相等时裂纹尖端的J积分。用表观J积分代替CT试样测得的J积分，改进了失效评估图方法。为了重现神户地震中观察到的脆性断裂，进行了一系列试验。结果表明，失效评估图方法能较好地预测试验结果。在这些试验中，研究了由焊缝缺陷引起的全贯通槽焊接接头脆性断裂。在这些点上引入疲劳裂纹，形成人工焊接缺陷。在此基础上，对采用部分节点焊透（PJP）坡口焊接的梁柱连接进行了全尺寸试验。PJP坡口焊接接头存在内部不连续，而上述焊缝缺陷属于表面裂纹。连接不能承受脆性断裂。CTOD设计曲线方法预测这些连接没有脆性断裂，并支持了测试结果。上述研究的一个发现是，通过设计带有细节的连接，将裂纹驱动力限制在一定范围内，可以避免脆性断裂的发生。作为这种连接细节的一个例子，在循环荷载下测试了一个由盖板加固的全尺寸梁柱连接。试验结果表明，该接头具有足够的塑性旋转能力。少

It is difficult to eliminate defects completely from field-welded joints of steel building frames. When engineers find weld defects, they need to have reliable criteria to decide whether to repair or not. The goal of this research is to establish a procedure for the quantitative assessment of risks of brittle fractures from weld defects.Brittle fractures occurring after extensive yielding of materials, as observed during the Kobe Earthquake, cannot be pre-dieted by means of classical fracture mechanics procedures. Since loss of plastic constraint at crack tips relaxes the stress at these regions, an initiation of brittle fracture cannot be predicted only by a fracture parameter of material like CTOD or J integral. In order to take into account the effect of plastic constraint, an apparent J-integral, which is defined as the J integral at the crack tip when volume of highly stressed region becomes equal to that at the crack tip of a compact tension (CT) specimen, is proposed. The failur … More e assessment diagram approach was modified by using the apparent J integral in place of the J-integral measured by CT specimens. A series of tests were conducted to reproduce brittle fractures observed during the Kobe Earthquake. The failure assessment diagram approach was found to predict test results well. In these tests brittle fractures starting from weld defects existing at the ends of complete joint penetration groove welded joints were investigated. Artificial weld defects were prepared at these points by introducing fatigue cracks.Further, full scale testing of beam-to-column connections with partial joint penetration (PJP) groove welded joints was conducted. The PJP groove welded joints have internal discontinuities, while weld defects mentioned above belong to surface cracks. The connections did not sustain brittle fracture. A CTOD design curve approach predicted that these connections are free from brittle fracture and supported the test results.One of the findings from the above investigations is that occurrences of brittle fracture can be avoided by designing connections with details such that the crack driving force is limited within a certain value. As an example of such connection details, a full-scale beam-to-column connection reinforced by cover plates was tested under cyclic loads. The test results showed that this connection had sufficient plastic rotation capacity. Less