Failure behavior of multiple-axial part-through-wall flaws in alloy 690TT steam generator tubes

This study investigates the interaction effect of multiple-axial part-through-wall (PTW) flaws on the failure behavior of Alloy 690TT steam generator (SG) tubes. Burst tests of tubes with single and multiple flaws were conducted at room temperature (RT). The flaws were made by the electrodischarge machining (EDM) method on the outer surface of the specimens. Six different configurations of multiple flaws were considered to see the interaction effect; two and three collinear, two and three parallel, and two and three nonaligned flaws. In all cases, an axial flaw with a constant depth of 50% wall-thickness was considered, and the following variables were systematically varied; the axial and/or circumferential separating ligament lengths between flaws, the flaw length, and the number of flaws. Effects of these variables on the failure pressure and failure mode were investigated based on experimental data. The effects of separating ligament lengths and flaw lengths on the failure pressure were dependent on the type of flaw configuration. For collinear and nonaligned flaws, the decrease in failure pressure by the interaction of multiple flaws became significant as the number of flaws increased. The failure mode of multiple flaws was strongly dependent on the length of the flaws.