Fracture Evaluation of Welded Joints with Embedded Flaw from Cold Cracking

Abstract

Prevention of cleavage fracture is one of the most significant problems in securing the structural integrity of welded structures. Although embedded flaws from cold cracking are realistic flaws that should be considered for the structural integrity assessment of a welded structure, experimental fracture evaluations through wide-plate tensile tests using a welded joint with an embedded flaw from cold cracking have never been conducted. In this study, therefore, a manufacturing method of welded joints with intentionally introduced embedded flaws from cold cracking is first developed based on the y-groove weld cracking test. A series of weld cracking tests is next performed by the above method. It is determined that a more deeply embedded flaw is more stably cracked as higher heat-input and more diffusible hydrogen is applied. Subsequently, a wide-plate tensile test for the welded joint with the embedded flaw using the established technique is performed to evaluate the fracture load and fracture behavior. It is determined that the introduced embedded flaw is nearly rectangular and positioned near the fusion line in the heat-affected zone. A fracture-origin survey of the wide-plate tensile specimen reveals that the cleavage fracture occurred from a local brittle zone at the tip of the introduced embedded flaw. Consequently, it is confirmed that this test meets the requirements for application to an evaluation on the significance of defects. Finally, fracture evaluation is performed by comparing the fracture stress with allowable stress to verify the applicability of the evaluation method using a failure assessment diagram (FAD) of the cold-cracking embedded flaw. It is determined that the fracture load is sufficiently higher than the allowable stress; therefore, fracture will not occur even if similar embedded flaws exist in the welded structures. Furthermore, it is confirmed that a BS7910-based FAD evaluation can be applied to cold-cracking embedded flaws with a specific degree of safety margin.