Session: MF-01-01 The Noel O'Dowd Memorial Symposium on Fracture: Application of Fracture Mechanics in Failure Assessment

Paper Number: 155865

155865 - 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 linepipe steels for the DOT/PHMSA MegaRule toughness requirements, it was necessary to make 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 linepipe manufacturers. There were also about 1,000 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 the ductile initiation would occur, while other more rigorous fracture mechanics based method 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 will show the developed relationships that allowed the user to take Charpy data at one temperature (typically average of three specimens at 50F), and predict the lowest temperature for ductile fracture initiation of a surface crack in that material. Then that 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 was 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. From full-scale testing, the SEN(T) specimen give 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.

During the evaluations, it was found that much 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 larger J_i values compared to the preferred plane strain specimen geometry. An experimental correlation for both SEN(B) and C(T) specimens was then used to determine the preferred specimen geometry, which was used as a reference value when going to the low constraint specimen typical of surface cracks. It was also found that when looking at Charpy upper-shelf energy versus J_i values, that the J_i 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 J_i 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 CTOD/ligament length normalized toughness was constant in these cases. The SEN(T) toughness at an a/W=0.7 is close to the J_i value from a preferred C(T) specimen geometry (W/B=2 and a/W=0.5).

With the above efforts, it was then possible to determine the brittle to-ductile transition temperature from limited Charpy data, relate the Charpy plateau energy to the C(T) and SENT) J_i toughness as a function of the surface crack a/t.

Presenting Author: Gery Wilkowski Engineering Mechanics Corporation of Columbus

Presenting Author Biography: To be added later.

Authors:

Gery M. Wilkowski Engineering Mechanics Corporation of Columbus