PSI - Issue 28

Alla V. Balueva et al. / Procedia Structural Integrity 28 (2020) 873–885 Author name / Structural Integrity Procedia 00 (2019) 000–000

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creating micro voids that become macroscopic fractures under internal pressure. It is most closely associated with brittle, intergranular fracture types (Laureys et al., 2016). This is an old theory that continues to be cited in current research, and is favored by engineers as a framework for failure analysis because it is easily adapted to existing models from fracture mechanics (Jemblie et al., 2017). That said, a majority of researches do not advocate HEDE as a generalized explanation for HE. Recent qualitative experiments show a clear change in the granular structure of the metals in a hydrogen environment, which is not expressed under the HEDE theory. Several researchers have identified slip bands that form as a result of changing plasticity in the solid, which plays a large role in the propagation of fractures (Zhang et al., 2017). 1b. Hydrogen-Enhanced Localized Plasticity (HELP) HELP is a newer theory that describes HE as the result of combined reduced ductility around the crack nucleation and internal stresses brought about by hydrogen diffusion (Zhang et al., 2017; Laureys et al., 2016). HELP is understood to cause HE in a metal solid by effectively increasing the velocity of dislocation motion and slip-band formation under stress. This results in a loss of ductility and lowers the ability of the solid to resist cracking. The HELP model is considered one of the most promising for a general understanding of HE. HELP is heavily associated with slip-band fracture types. Recently, researchers have amended the HELP framework and made several notable improvements (Laureys et al., 2016). 1c. Absorption-Induced Decohision Emission (AIDE) AIDE is a more recently theory advocated for HE by some authors. The idea is that absorbed hydrogen lowers the atomic bonding strength of the metal, resulting in separation and decohesions to form in the micro-structure. AIDE has been connected with cleavage-like and dimpled intergranular fracture types (Laureys et al., 2016; Sun et al., 2020). The theory is not as well developed as HELP or HEDE, but is promising none-the-less. 1d. Multi-Mechanistic Models At least one author has promoted a model of HE that incorporates multiple theories. Specifically, a recent article described a theoretical model in which HELP and HEDE worked synergistically to achieve HE failure (Laureys et al., 2016). Other such models likely exist. 1.2. Recent Developments in HELP theory Several new concepts have been added to the overall HELP framework in recent years. A new relationship was described between the two driving forces in the HELP process: Localized Plastic Flow and Hydrogen Induced Stress (Cui et al., 2017). They outlined a numerical method for solving for both sets of values in tandem, as opposed to solving them independently. Using this model, they found hydrogen concentration to be most heavily dependent on both the diffusion rate and the rate of plastic deformation. They also noted that hydrogen trapped in micro-structural voids was more influential on the plastic deformation than hydrogen diffused into the molecular lattice. In another article, researchers tested three of the most commonly cited mechanisms that govern help. Using numerical methods, they found dislocation motion was actually limited by the present hydrogen, in accordance with solute-drag theory (Song and Curtin, 2014). This contradicts the concept that hydrogen promoted decohesion by increasing dislocation mobility. They also found no evidence to suggest that hydrogen changed the way dislocations interface with each other under stress. They advocated a theory in which hydrogen interacts with defects in the solid, which would otherwise help to inhibit dislocation mobility. By doing so, hydrogen is supposed to indirectly increase mobility. One experimental observation supported the HELP framework by identifying Dislocation Slip Bands as the nucleation site for fractures (Zhang et al., 2017). Specifically, the authors identified slip bands with the highest Schmidt factor to be the most likely sites for fractures to form. Another research collaboration effort described the effect of HELP in the different phases of TRIP-assisted high strength steel (Claeys et al., 2020). Different phases of the steel are characterized by different crystallographic structures (namely FCC and BCC). The different phases were