Issue 35

G. Gobbi et alii, Frattura ed Integrità Strutturale, 35 (2016) 260-270; DOI: 10.3221/IGF-ESIS.35.30 261 of components. Hydrogen embrittlement phenomenon interests different fields such as mechanical, structural and energetic. For some industrial environment, this problem is widely recognized and studied in literature. For instance, oil&gas industry [1] in which atomic hydrogen is released as product of chemical reactions in environments where the infrastructures operate, pressure vessels for hydrogen storage and transportation [2] and lately even energy devices that use hydrogen as alternative energy carrier. However, other applications in which the presence of hydrogen is less evident have been pointed out recently thanks to the ongoing research on this topic. An example is reported in [3], dealing with wind turbine gearbox bearings, where the hydrogen presence has a deleterious effect in combination with rolling contact fatigue. In this case, it is suggested that hydrogen comes from decomposition of lubricating oil [4] or from water contamination. Scientific literature also reports some failures occurred in threaded fasteners, as in [5] where the possible sources of hydrogen are related to thermal treatments. However, independently on the source that generates atomic hydrogen, the most crucial phase is the diffusion process of hydrogen through the material lattice. According to [6], usually the concentration of hydrogen is split into two parts: the content of hydrogen in the interstitial lattice sites (NILS) driven by hydrostatic pressure, and the amount accumulated in correspondence of the so-called trap sites. In turn, these can be divided in reversible and irreversible based on the hydrogen binding energy (potential energy at microscopic scale). Reversible traps, or low binding energy traps, are mainly related to dislocations and plastic flow. In fact, in [6] the authors showed that plastic strain and hydrogen concentration in reversible traps have similar trends in front of a crack tip. The presence of a crack in a component induces hydrogen atoms to move from the free surface towards the tip. Indeed, crack initiation and propagation are deeply influenced by hydrogen presence and diffusion. In terms of diffusion coefficient the motion of hydrogen through NILS is represented by an ideal lattice diffusivity, D L . The diffusion can be limited or increased by traps and in these circumstances, a trap-affected or apparent diffusivity, D H , is considered. Hydrogen embrittlement is mostly governed by this second diffusion coefficient. Traps effect is not univocal [7, 8]. Indeed, literature reports that hydrogen in reversible traps is in equilibrium with the one in NILS, and it is an “obstacle” to its transport, thus D H <D L . Anyway, a large population of reversible traps will have a high probability of hydrogen release at room temperature and it can provide a reservoir of mobile (diffusible) hydrogen to supply the crack-tip fracture-process zone and thus increase hydrogen embrittlement. Quenched and tempered high strength steels have microstructural features that increase the strength, but also provide for many hydrogen-trapping sites [9]. Contrary, irreversible traps, often saturated even at low hydrogen concentrations, no longer interact with the dissolved hydrogen in the lattice, and D H tends to D L [10]. However, a homogeneous distribution of high-energy traps can prevent that hydrogen localize at low energy sites and shield its segregation at the crack tip. Regarding embrittlement mechanisms, several authors proposed possible theories but the most acceptable in literature are three: HEDE (Hydrogen Enhanced DEcohesion, [11]), HELP (Hydrogen Enhanced Local Plasticity, [12]) and hydrides formation and cleavage. These physical models try to explain the embrittlement phenomenon starting from nano and microscopic considerations, and propose that the damage located at crack tip is proportional to hydrogen concentration into the steel [13]. Therefore, it is clear that hydrogen embrittlement is a complex mechanism from both mechanical and physical-chemical standpoints. Based, on these considerations, it is hence really important to develop a tool able to assess the sensitivity of steels to hydrogen embrittlement. Literature reports several numerical models that try to simulate properly the embrittlement process considering interacting factors and the involved parameters mainly microstructure dependent. Among these numerical models, cohesive zone models (CZM) seem the most promising to reproduce at macro-scale the effect of hydrogen on mechanical properties of steels/alloys. These finite elements models assemble hydrogen diffusion process together with fracture mechanical behavior through the implementation of cohesive elements. A traction separation law (TSL), whose formulation includes parameters able to describe crack initiation and propagation, governs these particular elements. Therefore, the effect of embrittlement is simulated simply by reduction of the cohesive energy based on the hydrogen content, omitting the micro-mechanisms. The major contributions to this approach come from [14, 15, 16]. Even the model presented in the current manuscript belongs to this category. A complex finite element model including three steps of simulations is developed to reproduce the mechanical behavior of a steel, named AISI 4130, during a toughness tests in hydrogen-contaminated environment. This type of test was carried out during an extensive campaign of experimental tests, being suitable to describe the embrittling effect in steels due to hydrogen. Experimental fracture data are used to calibrate the mechanical behavior of cohesive elements describing crack propagation. Considerations about the