Brittle Crack Arrestability and Inverse Fracture in DWTT

Abstract

The drop-weight tear test (DWTT) has been widely used to evaluate the resistance of linepipe steels against brittle crack propagation, and the shear area fraction SA% in the DWTT has been adopted in the requirement for the linepipe steels. However, recent studies have pointed out the issue of ambiguity in evaluation of the DWTT when a ductile crack initiates from the notch and then transits to a brittle crack during ductile crack propagation. This fracture behavior is termed “inverse fracture.” According to the API Recommended Practice 5L3 (API RP 5L3), a test is considered invalid when a DWTT specimen shows inverse fracture. In this case, it is difficult to examine the acceptance criterion (85% shear area transition temperature) for linepipe steels. Because the purpose of the DWTT is to evaluate the brittle crack arrestability of the steels in a pressurized linepipe, the DWTT results should be examined with a propagating brittle crack arrest test. A large-scale brittle crack arrest test called the West Jefferson test is generally conducted to reproduce the crack propagation and arrest behavior in actual linepipes. However, it is somewhat difficult to control the lower test temperature and to initiate brittle crack in recent high-toughness steels in this burst test. Although the test stress conditions of the uniaxial tension in the plate tension brittle crack arrest test and the biaxial tension in a pressurized pipe are different, the plate tension brittle crack arrest test has the advantages of accurate control of the test temperature, test stress, and brittle crack initiation in comparison with the actual pipe burst test. Therefore, in this study, the brittle crack arrestability of linepipe steel which showed inverse fracture in the DWTT was investigated by conducting plate tension brittle crack arrest tests under an isothermal condition (crack arrest temperature test (CAT test)), which simulates the condition of the actual pipelines in service. This study also investigated the local shear lip thickness fraction in the CAT tests together with the shear area fraction SA% measured in DWTTs. Based on the results, the effect of brittle crack arrestability on inverse fracture appearance in the DWTTs was discussed in comparison with the brittle crack arrest behavior in the CAT tests.