PSI - Issue 68

A.H. Jabbari et al. / Procedia Structural Integrity 68 (2025) 874–879 Jabbari et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Harsh environments may accelerate the initiation and propagation of cracks in susceptible materials exposed to mechanical loading. For instance, exposure of materials to atomic hydrogen during manufacturing or service can cause hydrogen embrittlement, which reduces the cracking resistance and thus the strength under separating loading (Gangloff and Somerday, 2012). Therefore, susceptible materials should be tested under well-defined conditions either in gaseous hydrogen atmosphere or using electrolytic hydrogen charging for determining their resistance to hydrogen- assisted cracking (HAC). For this purpose, different standardized testing methods can be employed. For investigating HAC in steels, the ISO 11114-4 standard (ISO 11114-4, 2017) proposes three different testing methods including (i) disc testing, (ii) fracture mechanics testing, and (iii) constant-displacement testing. In method (i), disc-shaped specimens are subjected to increasing gas pressure until cracking or bursting. In method (ii), fatigue pre-cracked specimens placed inside a chamber of pressurized hydrogen gas are subjected to incrementally increasing tensile loading. In method (iii), compact tension (CT) specimens placed in pressurized hydrogen gas are loaded by constant displacement, which requires simpler equipment but longer testing time than methods (i) and (ii). For examining stress-corrosion cracking based on the constant-displacement method, the ISO 7539-6 standard (ISO 7539-6, 2018) proposes using modified wedge-opening-loaded (MWOL) specimens or double cantilever beam (DCB) specimens that are both loaded using bolts. The ASTM E1681 standard (ASTM E1681, 2003) also proposes using bolt-loaded compact specimens, also known as MWOL specimens, for testing environment-assisted cracking based on the constant displacement method. For examining stress-corrosion cracking in H 2 S environments, the NACE TM0177 standard (ANSI/NACE TM0177, 2016) proposes using DCB specimens loaded by double-tapered wedges. Using wedge-loaded DCB specimens for investigating environment-assisted cracking is quite simple and does not require any special equipment, even not a clip-on gauge for measuring the notch opening width. Nevertheless, as the specimens must be tested in pressurized hydrogen gas atmosphere, pressure autoclaves may have limitations with respect to the specimen dimensions. Hence, in small autoclaves testing standardized DCB specimens that are 4 in (101.6 mm) long is impossible. In comparison, wedge-loaded CT specimens have more compact dimensions and they can be easier manufactured than bolt-loaded MWOL specimens, but determining the actual load applied during testing is more challenging. In order to address this issue, the present study numerically investigates the feasibility of employing wedge-loaded CT specimens for evaluating environment-assisted cracking by comparing them directly with wedge-loaded DCB specimens. 2. Methodology Fig. 1 shows the geometries of CT and DCB specimens and of the wedges used for opening the notches and consequently the cracks. To simulate inserting the wedges, the ABAQUS FEA finite element (FE) software was employed using an implicit solver. For each type of specimens, the simulations were carried out considering either (i) purely elastic behavior of steel with the Young’s modulus of 210 GPa, or (ii) perfectly elastoplastic behavior of steel with the Young’s modulus of 210 GPa and the yield strength of 880 MPa. According to the ISO 9809-1 standard (ISO 9809-1, 2019), the susceptibility of steels with ultimate tensile strength (UTS) > 880 MPa should be tested with respect to hydrogen embrittlement. Thus, to see the maximum effect of plastic deformation, a steel with the lowest strength was selected. Considering the defined purely elastic and perfectly elastoplastic behaviors, the behavior of each steel which should be tested with respect to its hydrogen embrittlement susceptibility will therefore fall inside this range.

Fig. 1. Geometries of (a) CT specimen (ISO 11114-4, 2017) and (b) DCB specimen (ANSI/NACE TM0177, 2016), and geometries of wedges used for loading (c) the CT specimen and (d) the DCB specimen.