L51635 Optimizing Techniques for CTOD Testing of Girth Welds in Linepipe

Appendix A of the sixteenth edition of API Standard 1104 allows relaxation of the flaw acceptance criteria, as defined in the standard, provided that specified minimum fracture toughness values are achieved (Ref.1). The fracture toughness is measured using the crack tip opening displacement (CTOD) test procedure as described in BS 5762 published by the British Standards Institute. In allowing relaxed defect acceptance criteria, the achievement of a specified CTOD value ensures that the flaws in girth welds will remain stable during installation and service. Thus, despite the presence of flaws, the pipeline can be operated safely without risk of unstable fracture, e.g. brittle fracture. However, to ensure that this is the case, it is vital that the testing procedure be designed to identify and quantify the lowest toughness region. If this is not done, there is a risk that the flaw will sample low toughness material and initiate an unstable fracture leading to failure of the girth weld. This report describes a research program to investigate various notching techniques with the objective of determining the most appropriate method of selecting the notch location for CTOD specimens notched into the HAZ region of pipeline girth welds. It was found that for the weld samples studied in this investigation, the API 1104 Appendix A method of conducting HAZ CTOD tests can produce scattered results and does not necessarily guarantee lower bound estimates of toughness. Of the various notching techniques studied, specimens notched at the intersection of the fusion boundary with the specimen mid-thickness produced the most consistent results.