Inverse Fracture: What is it All About?

The transition temperature behavior of pipes subjected to propagating fracture is assessed by means of Battelle Drop Weight Tear (BDWT) tests. These consist of notched specimens which are subjected to impact loading under a three point bend test configuration. The fracture surface is assessed to derive shear or brittle portions that are macroscopically visible. These have been shown to correspond well to the fracture surface of a pipe subjected to a propagating fracture. Historically, it is required for the test to either exhibit entirely ductile fracture or a combination of brittle and ductile provided that the test initiates in a brittle fashion with propagation in a ductile manner. Conversely, tests exhibiting ductile initiation with brittle or mixed brittle and ductile propagation are not acceptable to some standards/specifications. In recent times, this requirement has been softened in certain standards/specifications but it remains to be explained what these two diverging opinions are based upon and which one of the two is the correct one. This nominally unacceptable fracture mode has been termed inverse or abnormal fracture and is frequently observed with modern pipeline steels. Other than inverse fracture character, these specimens show every sign of highly ductile material being tested. The unanswered question is: does the brittle area reflect material properties or is it caused by the specific test conditions meaning that it is simply a testing issue? Furthermore, the reason to retain the requirement is not obvious and a procedure is missing on how to assess this type of specimen. The only possibility outlined in testing standards to avoid inverse fracture is the use of an alternative notch, the so-called Chevron notch that is supposed to facilitate brittle crack initiation. Mostly, the problem remains unsolved and further guidance is not given. Current research aims at avoiding invalid results by choosing different notches that could potentially be successful but also on checking the correspondence of results of testing inverse material to pipe behavior in West Jefferson tests. Alternatives discussed to avoid inverse fracture are, apart from the above named one, static pre-cracked notches and fatigue notches. Within this paper, test results of specimens notched with all of these notches are presented. None of these were successful in eliminating inverse fracture. Literature studies were conducted to understand the background of the requirement. Investigations of material behavior after having been subject to heavy pre-strain were carried out. The question is, on basis of the available evidence, which conclusions can be drawn and if these are sound enough to derive general guidance.