Abstract
The Original Net-Section-Collapse (NSC) analysis was developed in the 1970s for prediction of the maximum (failure) moment for a circumferential flaw in a pipe, and is used widely in pipe flaw assessments. A large number of past pipe tests show that deep surface cracks can break through the thickness and result in leaks; hence, the maximum moment of that surface-cracked pipe was below the maximum moment for the circumferential through-wall crack with the same length. In these cases, the applied moment has to be increased for the resulting leak to grow as a through-wall crack. Hence, load-controlled leak-before-break (LBB) fracture behavior has been experimentally observed although it is not predictable by the Original NSC analysis. Recently, Original NSC analysis for circumferential surface-cracked pipes under combined bending and axial tension were enhanced through the development of the “Apparent Net-Section Collapse” methodology to explain inconsistencies with the Original NSC. “Apparent NSC” methodology was developed considering surface-cracked pipe test data developed from external (OD) surface-cracked pipe tests conducted at room temperature (RT) with a vast majority conducted under pure bending and unpressurized conditions.
Since it is undesirable to have leakage in many applications, the deficiency in the Original NSC analysis was shown experimentally, and the recently developed “Apparent NSC” methodology applied to a carefully planned matrix of pipe and elbow tests conducted on TP304 stainless steel and Alloy600 materials with different flaw dimensions (composed of short and shallow to long and deep surface cracks), in the range of normalized crack depth, a/t = 0.4 to 0.8 and crack length, 2θψ = 90° to 180°. The tests were conducted under conditions similar to a pressurized water reactor (PWR), and consistent with the International Piping Integrity Research Group (IPIRG-2) [1] test conditions, namely a temperature of 550°F (288°C) and an internal pressure of 2,250 psi. The loads corresponding to the surface-crack initiation, maximum load, and leakage events were recorded from each of the surface-cracked pipe and elbow tests. The data were used to understand the predictable nature of the “Apparent NSC” methodology and to develop an understanding of the fracture behavior of surface-cracked pipes leading to correlation of these results to LBB behavior. Further, the results were correlated between the material composition and the variation of the experimental and predicted bending stress from NSC loads to observations from the previous IPIRG-2 program, where the experimental burst loads were characterized with respect to the flow stress assumptions. The material composition such as variation in sulfur content, and the crack-initiation and crack growth based on elastic-plastic fracture mechanics were used to explain the variability of the flow stress assumption when used in a NSC/limit-load type of analysis. The investigation also showed comparison of predictions based on various flow stress (σf) definitions assumed using yield and ultimate stresses obtained from the tensile tests conducted on the pipe and elbow materials at 550°F (288°C) and applied to the Original NSC and “Apparent NSC” methodologies. The moment predictions using ASME elbow stress indices (B2, C2 used in design) or the IPIRG-2 parameter (Ψec) for the circumferentially surface-cracked elbows were also compared to the experimental maximum moments for the tested elbows.
