Procedure Proposition using a Compact SENT Geometry for Fracture Toughness Assessment in Sour Environment

The DCB method is a crack arrest test, i.e., the specimen is self-loaded with a wedge and the constant displacement configuration makes the SIF decrease as the crack grows. Thus, K_ISSC provided from the DCB test could be different from the toughness of a cracked structure under a constant load scenario. The objective of this paper is to present a new experimental method using a compact Single Edge Notched Tension (SENT) specimen for assessing the SIF threshold for crack growth initiation. The test method provides the same framework as Method A of the NACE TM0177 standard. A numerical solution of SIF as a function of load and geometric parameters is derived from Finite Element Analysis. Tests are performed using the DCB and SENT geometries on C110 grade. The obtained results from the two methods are compared and a test protocol for conducting the crack initiation test is proposed.

For many decades several standardized and not standardized methods based on different pre-cracked specimen geometries have been used for quantifying the stress intensity threshold K_Ith under environmentally assisted cracking conditions ^{(1; 2)}. Since its implementation in 1990 ⁽²⁾ as method D of NACE TM0177 standard ⁽³⁾ the Double Cantilever Beam (DCB) test method remains in continuous improvements and updates beyond the fact that it is widely used as a tool for quality control, new alloy evaluation, or even material performance mapping regarding environment parameters as temperature, pH, H₂S content and chemical species concentration. Thus, DCB geometry is considered as the most known and used test method to evaluate material fracture toughness under Stress Corrosion Cracking and Sulfide Stress Cracking conditions.

Over the years, numerous studies were done to highlight some difficulties faced when carrying out tests with NACE TM0177 method D and to understand the effect of some test parameters on the stress intensity factor threshold as arm displacement and pre-cracking parameters ^{(4; 5),} notch type and initial crack length ^{(4; 6; 7; 8)} and side groove ^{(9; 10; 11).} Nevertheless, nowadays the DCB test is still a debated issue regarding both the intrinsic variables that are inherent to the specimen geometry and the data validation requirements as specified in the NACE TM0177 standard. Besides, the DCB test is a crack arrest test, i.e. the specimen is self-loaded with a wedge that is considered as constant displacement configuration and where the stress intensity factor (SIF) decreases as the crack grows. Thus, obtain K^ISCC from the DCB test could be different from a critical scenario where a cracked structure is under constant load and the applied stress intensity factor is an initiation crack propagation threshold K^threshold.