Mode Mixity in the Fracture Toughness Evaluation of Heat-Affected-Zone Material Using SEN(T) Experiment

Abstract

Single-edge-notch-tension, SEN(T), specimens have been found to provide a good similitude for surface cracks in pipes, where a surface-cracked structure has lower constraint condition than single-edge notch bend, SEN(B) (bend-bars), and compact-tension, C(T) specimens. This lower constraint condition gives higher upper-shelf toughness values, and also a lower brittle-to-ductile transition temperature. Additionally, the SEN(T) specimen eliminates concern of material anisotropy since the crack growth direction in the SEN(T) specimen 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 homogenous or monomaterial across the crack, their applicability for the evaluation of fracture toughness of the heat-affected zone (HAZ) were evaluated in this investigation. When conducting experiments on SEN(T) specimens with a prescribed notch/crack in the 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 experiment. 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 experiments conducted on SEN(T) specimens with a 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 toward the development of recommended practices or standards for SEN(T) experiments of weld/HAZ. The effects of mode mixity in HAZ SEN(T) experiments is critical to the development of crack growth resistance, CTOD-R and J–R curves employed in engineering critical assessment (ECA) of pipelines.