Hydrogen embrittlement in steels: predictive models and integrity assessment
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Hydrogen embrittlement (HE) is a well-recognized challenge as it can degrade the mechanical properties of most structural steels, leading to premature failure of hydrogen infrastructures. Continuum scale simulation is an effective tool to predict and assess HE in practical applications. In this thesis, we have developed three tools to simulate HE: a microstructure-informed and mixed-mode cohesive zone modelling approach, a combined cohesive zone model (CZM) and complete Gurson model (CGM) approach, and a framework unifying hydrogen enhanced plasticity and decohesion. To verify the effectiveness of the models, the hydrogen-induced failure behavior of X65 pipeline steel under in-situ hydrogen charging conditions is simulated, through which a hydrogeninduced failure criterion is obtained. Microstructure has been found to influence almost all aspects of HE, such as hydrogen absorption, diffusion, trapping, and crack nucleation. Therefore, a practical HE simulation tool should take the microstructural features into consideration. The developed microstructure-informed cohesive zone approach involves three steps: image processing and finite element representation of the experimentally observed microstructure, stress-driven hydrogen diffusion and diffusion coupled cohesive zone modelling of fracture considering mixed failure mode. Simulation in a duplex stainless steel (austenite and ferrite phases) shows that the effective hydrogen diffusion coefficient is larger and the hydrogen fracture resistance is lower for the microstructure with polygonal and randomly distributed austenite phase, compared to the microstructure with slender and laminated austenite phase. This finding is consistent with experimental observations. Moreover, the strength of the interface in the shear direction dominates the fracture mode and initiation site, highlighting the importance of considering mixed failure mode. This technique can serve as a predictive material design tool towards improved hydrogen resistance. HE manifests as a fracture mode transition from ductile in hydrogen-free condition to brittle in hydrogen environment. Thus, capturing the ductile-to-brittle transition (DBT) is essential for HE simulation. In the combined CGM and CZM approach (CGM+CZM), DBT is rationalized as the competition between fracture due to micro-void growth and coalescence and fracture in the cohesive zone. It is found that the fracture mode is dependent on the ratio between the cohesive strength and the yield strength of the material, and brittle fracture only occurs when the strength ratio is below a critical value yc. The transition ratio yc is material dependent. Corresponding to the yc, a critical hydrogen concentration CHc is revealed, which is material dependent as well. This approach is applicable to most metals in HE scenarios and low temperature embrittlement scenarios. One step further, to simulate not only the hydrogen-induced DBT behavior but also the interaction between hydrogen enhanced plasticity and decohesion, a unified framework (H-CGM+) has been developed for the first time. The CGM, designed to predict ductile failure by voiding, is extended to the CGM+ by incorporating a decohesion failure criterion. It is demonstrated that the result produced with CGM+ is comparable to that with the CGM+CZM approach. In H-CGM+, hydrogen enhanced plasticity is accounted for through acceleration of the voiding process, while hydrogen induced decohesion is realized by a degradation of the decohesion threshold. The interplay between plasticitydominated and decohesion-dominated failure modes driven by varied conditions, i.e. hydrogen concentration, trapping, and material’s microstructure, can be well captured by H-CGM+. This framework can predict a realistic level of embrittlement, as well as the suppression of dimples in a hydrogen-induced fracture surface. H-CGM+, being generic, versatile, and easy to implement, is thus applied to model the hydrogen-induced failure behavior in X65 pipeline steel. The loading curves of slow strain-rate tensile tests under in-situ hydrogen charging (in-situ SSRT) conditions of different notched specimens are simulated. Both global stress-strain curve and local failure initiation site can be well captured, providing a critical combination of localized strain and hydrogen concentration (0.31 and 2.8 wppm), which triggers the hydrogeninduced failure initiation. This failure criterion is independent of stress triaxiality and can be a good reference for the safety assessment of hydrogen pipelines.