Crack Arrest Testing of High Strength Steels

Fracture propagation is a major concern for the safe operation of gas transmission pipelines. Ductile fracture resistance, which is required according to line pipe standards, is commonly assessed by Charpy impact testing. If fracture occurs during pipe operation, fracture propagation is required to appear in ductile manner. The prerequisite for this is the demonstration of sufficient shear fracture in the BDWT test and minimum required Charpy impact energy. A combination of both requirements ensures avoidance of brittle fracture as well as control of ductile fracture propagation. The experimental chain of evidence and the Battelle-Two-Curve (BTC) model which is the most widely applied model to predict resistance against fracture propagation have been developed on basis of welded pipes of grade ≤ X70. The model has been calibrated against test data obtained from pipes with Charpy impact energy values below 100 J. In recent years, new material concepts were developed to increase material strength and material toughness. On the one hand, increase in material toughness, which is evaluated by Charpy impact testing, is often achieved by an increase in crack initiation resistance. On the other hand, crack propagation resistance, which is determined by BDWT testing with an instrumented striker, can remain on the same level. Increased material toughness and crack initiation resistance can be manifested by incomplete fracture of Charpy impact specimens in the upper shelf (ductile fracture). Actual Charpy impact test standards for metallic materials do not coincide with each other regarding the validity of Charpy energy of unbroken specimens. Increased crack initiation resistance also affects fracture initiation mechanism in BDWT tests, leading to invalid test results according test standards. Invalidity can be expressed by inverse fracture appearance. To avoid inverse fracture, crack initiation energy can be reduced by changing notch type and therefore changing the constraint in the root of the notch. BDWT test standards also do not agree with each other concerning allowable notch types. While the pressed notch type is the preferred one for low toughness steels and the Chevron notch type for higher toughness steels according some test standards, other test standards allow only for a pressed notch type. Being semi-empirical by nature, the BTC concept strongly depends on the input parameters derived from different material tests. Changing test conditions can have a direct impact on the assessment results.