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  Title A detailed comparison of separation characteristics between laboratory-scale fracture tests and a full-scale burst test
  Author(s) Dr Bradley J. Davis, Dr Guillaume Michal, Prof. Cheng Lu, and Prof. Valerie Linton  
  Abstract SEPARATIONS ARE SMALL FISSURES that form along the rolling plane of line pipe steels as the steel deforms and the stress state reaches the critical value to initiate a separation. Separations are often observed on the fracture surfaces of tensile, Charpy, and drop-weight tear test (DWTT) specimens---the key tests for determining the fracture arrest capabilities of line pipe steel. However, when comparing laboratory-scale material tests to a full-scale burst test, the separation appearance on the fracture surface is dissimilar. This indicates that laboratory-scale tests may not capture separations’ influence on fracture behaviour compared to full-scale burst tests (FSBT).

For

this study, the fracture surfaces of Charpy impact and DWTTs specimens were evaluated for their separation characteristics based on the separation index (SI). This was then compared with the separations found of the fracture surface of a full-scale burst test containing CO2. Slices at the front, middle, and tail of each fractured full- scale test pipe section were extracted and their surfaces analysed for their separation characteristics. This was then compared with the corresponding Charpy and DWTT specimen fracture surfaces.

With the separations measured across all fracture faces, the SI of the FSBT surfaces consistently fell below those measured on Charpy surfaces but above those measured on DWTT specimens. The closest similarity between FSBT and DWTT surfaces was seen right before fracture arrest, where the fracture velocity was lowest. When comparing the SI between specimen types, only the Charpy and DWTT surfaces showed a strong correlation to each other.?

 
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