Abstract

Single-Edge-Notch Tension, SEN(T) specimens have been used for fracture toughness determinations of base, weld, and heat-affected-zone (HAZ) materials. While the experimental results and computational finite element analysis (FEA) of SENT specimens with cracks in base and weld metals are relatively straight-forward, the analysis of SENT specimens with cracks in the HAZ poses several complexities that are highlighted through computational FEA in this work. SENT 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, SEN(B) and compact tension, C(T), specimens. The lower constraint condition gives higher upper-shelf toughness values, and also a lower brittle-to-ductile transition temperature. Also, 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 traditional methods of evaluation allow for a direct interpretation and application of elastic-plastic fracture mechanics approaches such as those that have evolved over the last five decades, for SEN(T) specimens with a flaw/crack in base materials and to some extent in weld materials, the presence of cracking in the HAZ complicates such routine approaches.

This work initially highlights findings from an earlier published SEN(T) experiment that was investigated and reported in support of the development of recommended practices and worldwide standards. From that experiment the presence of mode-mixity in HAZ fracture experiments was identified, and their characterization and determination of how to account for them in the recommended practices are under development. This work supplements the earlier experimental findings through computational FEA that shows the effects of mode-mixity and also highlights the differences when the initial crack is located in different regions of the HAZ and is of different lengths so as to keep the initial crack and the crack growth within the HAZ region. Of particular interest is the extent of crack growth that can be used for J-R and CTOD-R curves depicting fracture resistance curves for HAZ material cracks. Towards such an end, this work starts with an experimental discussion, followed by computational FEA to provide initial guidelines and a framework for future discussions. The goal being to create a better procedural analysis and treatment of the elastic-plastic crack growth in HAZ materials which will benefit the industry. 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.

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