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

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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