Determination of Fracture Initiation in H2 Environments From Rising Displacement Tests

Abstract Fracture toughness behavior in high pressure H2 is often determined using standard methods in ASTM E1820 except at slower rates to ensure sufficient hydrogen uptake during the test. The baseline method tracks crack extension during the test from periodic unloads and specimen compliance. This method, however, is not effective for capturing crack initiation and behavior at small crack extensions. Currently the preferred method to determine crack initiation in these tests is Electrical Potential Difference (EPD). However, determination of the early stages of crack extension, including initiation, requires accurate identification of the point of crack initiation. Typically, the deviation in the EPD vs crack mouth opening (CMOD) signal is used to identify the point of crack initiation. Another approach to identify the point of crack initiation is based on a resolvable crack growth rate approach. Standards, such as ASTM E1820, provide guidance on determining crack initiation, but the practice contains subjectivity. Work performed across two laboratories on the same pipeline material under similar conditions of ∼ 200 bara H2 is used to compare different approaches to identify the point of crack initiation, its impact on the J-R curve and subsequent identification of single-point J-integral values along the J-R curve, such as J0-H, J0.05-H, and JIC-H (where the number represents the amount of crack extension in millimeters, as an offset from the blunting line). A good alignment of the assessed toughness values JIC-H is observed across the two labs for the same material and similar test conditions. The assessments highlight the need to develop additional guidance and improved methodology to identify the point of crack initiation and the associated J0-H value.