The tensile properties of line pipe are usually determined using a flattened strap tensile sample which is obtained by cutting a long transverse sample from the pipe and then flattening it prior to machining the final tensile coupon. Although, several documents have been published to standardize this test, variability in the reported yield stress for the same material tested by different labs continues to be an issue particularly in high strength line pipe (X70 and X80).
Pipe properties are influenced by the pipe forming operations which introduce plastic strain into the steel. As well, the flattening of the tensile blank reverses the deformation and leads to Bauchinger effects which further alter the tensile properties of the material. There is no standard available for the flattening process and pipe manufacturers and operators continue to seek a best practice for the process. In addition, other factors such as placement of extensometer on a flattenend tensile specimen during the tensile testing have been considered a source of variation in the results.
Several projects were conducted to identify the source of variability and to standardize the flattening and testing process among the Evraz QA Labs. These initiatives included: a round robin tensile testing program in which tensile tests were performed on flat plates and subsequently on flattened strap specimens produced from a sister plate; examination of a 2-step flattening procedure against a 1-step method, and investigation of the extensometer placement (ID, OD or side mounted) on the recorded stress-strain behaviour.
The flattening process was found to be the main source of variability of yield stress. No real trend was observed resulting from extensometer position. Other testing practices such as specimen gripping, zeroing the load and positioning at the start of the test, and the dimensional variability within the reduced section of a specimen were also found to contribute to yield stress variability. Best practices for determination of yield stress using flattened strap tensile samples are discussed.