Impact of Test Method on Sulfide Stress Cracking Fracture Toughness

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When dealing with the risk of Sulfide Stress Cracking (SSC) of Oil Country Tubular Goods (OCTG) as casings and tubings, two main approaches are: either the determination of the threshold stress on a smooth tensile specimen (NACE TM0177 Method A) or the fracture toughness KISSC on a Double Cantilever Beam (DCB) specimen (NACE TM0177 Method D).
Relevance of KISSC for predicting the behavior of real pipes in service has been assessed in the frame of API PRAC project Phase II.1 Hollow tensile specimens sampled in the wall of sour service seamless pipes were exposed to sour environments either on Inner Diameter (ID), Outer Diameter (OD) or both and subjected to a tensile loading (see also references

Product Number: 51323-19118-SG

Author: Florian Thebault, Christelle Gomes, Daniela Garcia

Publication Date: 2023

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Double Cantilever Beam (DCB) test as per Method D of NACE TM0177 is the most used method for the evaluation of the Sulfide Stress Cracking fracture toughness KISSC. Klimit has been introduced in the latest revision of the standard for suppressing the influence of the wedge thickness on KISSC. Recently, Single Edge Notch Tension (SENT) has been proposed as an alternative geometry for obtaining threshold KISSC values. The present work aims to further compare these two testing methodologies on a C110 in two environments presenting different abilities to hydrogenate the material. Results show that KISSC-SENT tends to be more conservative than Klimit, especially in mild sour environment. This observation is explained by crack arrest versus crack initiation configurations and time-dependent hydrogen absorption inside specimens. This interpretation is strengthened by crack growth resistance curve performed on SENT specimens presenting different surface coating configurations able to delay hydrogen uptake rate.