Pragmatic Trends for Estimating Constraint Effects on Upper-Shelf Fracture Toughness for Pipe Flaw Evaluation

Abstract During efforts for a PRCI project to assess the toughness for critical flaw size evaluations of vintage axially surface-cracked line-pipe steels for the DOT/PHMSA MegaRule toughness requirements, it was necessary to develop a pragmatically simple procedure to assess the toughness inputs for various types of burst pressure analyses. Various databases were compiled together that had over 25,000 Charpy tests on different pipe base metal and axial seam welds from a variety of vintage line-pipe manufacturers. There were also about 500 traditional high-constraint fracture mechanics test specimen data. Some of the burst pressure analyses empirically used the Charpy upper-shelf energy if it can be shown that ductile initiation would occur, while other more rigorous fracture-mechanics-based methods should use the low constraint fracture toughness representative of a surface-cracked pipe. Some failure avoidance procedures may use higher constraint fracture specimens with a semi-theoretical basis. This paper shows the developed relationships that allow the user to take Charpy data at one temperature (typically the average of three specimens at 50°F) and predict the lowest temperature for ductile fracture initiation of a surface crack in that material. Ductile fracture of a surface-cracked pipe is typically about 200°F lower than the Charpy transition temperature. From full-scale testing, the SEN(T) specimen gives close to the same brittle-to-ductile transition temperature as the surface-cracked pipe, while higher constraint specimens [i.e., SEN(B) or C(T)] have warmer brittle fracture initiation transition temperatures than a surface-cracked pipe. Statistically 99.76 percent of the pipe base metals examined (5,750 pipes) had ductile initiation at typical US minimum ground temperatures. Some vintage weld, however, were predicted to have cleavage fracture initiation, which is a topic of a companion paper. Once it is known that ductile fracture initiation will occur for surface crack in the pipe of interest, then the Charpy data (typical energy and shear area percent), can be used to determine the Charpy upper-shelf energy. The effects of different Charpy specimen sizes were included for the transition temperature and upper-shelf determinations. Alternatively, the high-constraint fracture toughness data (typically C(T) or SEN(B) specimens) might be desired to be used. During the evaluations, it was found that many of the specimen tests in the database used non-standard ASTM C(T) or SEN(B) specimen dimensions, i.e., W/B=2 and a/W=0.5 is the preferred specimen, but W/B>7 was used in many cases. The high W/B specimens give higher Ji values than the preferred plane strain specimen geometry. An experimental correlation for both SEN(B) and C(T) specimens was then used to estimate the toughness that would be obtained from the preferred specimen geometry. The preferred plane-strain specimen geometry is used as a reference toughness value for correlation to the low-constraint specimen typical of surface cracks. It was also found that when looking at Charpy upper-shelf energy versus Ji values, the Ji values were also a function of the C(T) specimen size – an additional constraint aspect. Next was the toughness correlations for the low-constraint toughness, where past work showed that the SEN(T) specimen gives a better estimate of the surface-crack toughness. From dozens of different material tests, it was found that the Ji values (and J-R curves) decrease as the a/t of the crack increases in a linear manner. It was also seen that the SEN(T) toughness increased linearly as the specimen width increased (even if a/W was constant). Interestingly, the CTODi/ligament length normalized toughness was constant in these cases. The SEN(T) toughness at an a/W=0.7 is close to the Ji value from a preferred C(T) specimen geometry (W/B=2 and a/W=0.5), with added conditions as given in this paper. With the above efforts, it was then possible to determine the brittle-to-ductile transition temperature from limited Charpy data and relate the Charpy plateau energy to the C(T) and SENT) Ji toughness as a function of the surface crack a/t.