Weld crack assessments in API X65 pipeline: failure assessment diagrams with variations in representative mechanical properties

Applying accurate material properties to failure assessment diagrams (FAD) for flaw assessment has been problematic, particularly for welded joints in structures such as natural gas pipeline. In a study of API X65-graded natural gas pipeline, we evaluated material properties such as tensile properties and fracture toughness for the base metal, weld metal and heat-affected zone (HAZ), and investigated the influence on flaw assessment of variations in material properties of three regions. In particular, microtensile tests and crack-tip-opening-displacement (CTOD) tests made it possible to construct an HAZ-focused FAD reflecting HAZ properties. It was found that, when crack-like flaws exist in the HAZ, the HAZ-focused FAD yields a more accurate assessment than FADs constructed according to current codes.