Mode Mixity in the Fracture Toughness Characterization of HAZ Material Using SEN(T) Testing

Single-Edge-Notch-Tension, SEN(T), specimens have been found to provide good similitude for surface cracks in pipes, where a surface-cracked structure has lower constraint condition than bend-bars and C(T). The lower constraint condition gives higher upper-shelf toughness values, and also a lower brittle-to-ductile transition temperature. Also, the SENT specimen eliminates concern of material anisotropy since the crack growth direction in the SENT is the same as in a surface-cracked pipe. While the existing recommended and industrial practices for SEN(T) have been developed based on assumption of monomaterial across the crack, their applicability for the evaluation of fracture toughness of heat-affected-zone (HAZ) is evaluated in this paper. When conducting tests on SEN(T) specimens with prescribed notch/crack in the heat-affected-zone (HAZ), the asymmetric deformation around the crack causes the occurrence of a combination of Mode-I (crack opening) and Mode-II (crack in-plane shearing) behavior. The extent of this mode mixity is dependent on the relative difference between the material properties of the adjacent girth weld and pipe base metals, as well as the amount of crack growth in the test. This mode mixity affects the measurement of the crack-tip-opening-displacement (CTOD) and evaluation of fracture mechanics parameter, J. The CTOD-R curve depicts the change in toughness with crack growth, in a manner similar to the J-R curve methodology. Observations also show a mismatch in the length of the crack growth that is measured on the fracture surface, attributable to the material deformation differences across the two adjacent materials (weld and base metals). This paper discusses the experimental observations of Mode-I and Mode-II behavior seen in tests of SEN(T) specimens with notch/crack in the HAZ and as the crack propagates through the weld/HAZ thickness. The paper addresses the issues related to and the changes needed to account for such behavior in the development of recommended practices or standards for SEN(T) testing of weld/HAZ. The effects of mode mixity in HAZ testing is critical to the development of crack growth resistance, CTOD-R and J-R curves employed in Engineering Critical Assessment (ECA) of pipelines.