SUBCRITICAL CRACK GROWTH IN HIGH-PRESSURE HYDROGEN AND HYDROGEN WITH OXYGEN IMPURITY

In this study, the effects of oxygen content on hydrogen environment-assisted cracking are studied for several pipeline and commercial steels. Characterizing the effects of low oxygen impurities in hydrogen gas on subcritical crack growth in high pressure hydrogen environments can help inform fracture mechanics-based design and evaluate if oxygen can be utilized to mitigate hydrogen effects over long timescales.
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CRACK GROWTH-BASED FATIGUE LIFE PREDICTION FOR AGING PIPELINE STEEL IN HYDROGEN WITH PRE-EXISTING CORROSION

Using existing pipeline infrastructure for hydrogen transport is under prime focus nationwide and globally. As a result, several studies were conducted under gaseous and electrochemically charged hydrogen environments. However, most existing studies focused on virgin material degradation under a hydrogen environment but did not include the effect of pre-existing damage due to aging, such as corrosion. This study focuses on a hydrogen-assisted fatigue crack growth model that can capture the growth rate behavior for various line pipe steels at various operating conditions. Pre-existing corrosion (both general material loss and pitting) effects are naturally included as surface irregularities in the form of roughness. Modified stress intensity factor solutions for surface roughness and crack growth kinetics function are integrated for fatigue life prediction. Model predictions are validated with collected experimental data from the open literature. Several future research directions are recommended based on the current findings.
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NUMERICAL ANALYSIS OF HYDROGEN DIFFUSION AROUND THE NOTCH UNDER CYCLIC LOADING WITH AN OVERLOAD

Hydrogen embrittlement is known to be induced by a local increase in hydrogen concentration in materials. Therefore, it is important to elucidate the mechanism of hydrogen concentration behavior, which is the cause of hydrogen embrittlement, to prevent hydrogen embrittlement. One of authors proposed a numerical analysis method that couples stress analysis using the finite element method and hydrogen diffusion analysis using the finite difference method. This analysis has clarified that there is the effect of loading waveforms on hydrogen concentration behavior. In this study, hydrogen diffusion concentration behavior analysis under fatigue conditions with an overload was performed, and it was shown that hydrogen concentration may be enhanced by an overload.
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MATERIAL DISSOLUTION AT THE CRACK TIP

Despite a long-documented history of environmental effects, an understanding of the controlling mechanisms remains clouded. At fault are several challenges. First, multiple mechanisms can act simultaneously, e.g., dissolution, oxide fracture, oxide formation, material redeposition, and hydrogen embrittlement. Second, the scale of the material separation process on which the environment acts is atomistic, inhibiting direct observation. Considering these challenges, atomistic modeling can serve as a powerful probe to illuminate the mechanisms governing environmental effects and providing a means to study the material separation processes under the action of isolated mechanisms. Here, we report on the results of atomistic simulations specifically constructed to illuminate the role of material dissolution at the tip of a long crack. In cases of both brittle and ductile materials, we find that material dissolution can free arrested cracks. Beyond this, we find material dissolution to play a dole role, accelerating crack growth in the cases of brittle materials under sub-critical loading and accelerating crack tip blunting in the case of ductile materials. We find the result to be largely independent of loading magnitude and type, i.e. static vs fatigue. In total these results provide guidance for the development of continuum scale crack growth rules.
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THREE-DIMENSIONAL ANALYSIS ON HYDROGEN-RELATED INTERGRANULAR CRACK PROPAGATION IN MARTENSITIC STEEL

This study investigated three-dimensional propagation behavior of hydrogen-related intergranular crack in martensitic steel by X-ray computed tomography and FIB-SEM serial sectioning. Macroscopic analysis using X-ray computed tomography revealed that the crack morphology exhibited more continuous in the hydrogen-charged specimen. Through FIB-SEM serial sectioning, we found that the crack-tip blunting and ductile rupture of un-cracked ligaments were associated with a certain grain boundary segment in the uncharged specimen. In the case of hydrogen-related intergranular crack propagation, even very fine low-angle grain boundary segments (sub micro-meter size) could act as obstacles to crack propagation. Based on the results, we can propose that misorientation of each grain boundary segment has a large influence on local crack arrestability of intergranular crack propagation.
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EFFECTS OF CRACK TIP STRESS RELAXATION ON SUBCRITICAL CRACK GROWTH IN SILICATE GLASSES: THRESHOLD AND STOCHASTICITY

Silicate glass is a non-equilibrium material and as such evolves over time to reduce internal energy through thermally activated structural rearrangement. This statement is perhaps especially true in the highly stressed region around a crack tip. At the atomistic scale, structural changes to accommodate crack growth or to mediate stress relaxation become indistinguishable. Here, we present a simple expression for static fatigue threshold using slow crack growth power law parameters and a structural relaxation time scale. Using subcritical crack growth data from the literature and measured threshold data, this model is demonstrated for soda lime silicate glass. In addition, we discuss the impact of crack tip relaxation on statistical lifetime prediction and evolution of flaw populations.
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MODELING OF HYDROGEN EMBRITLLEMENT USING MIXED NONLOCAL FINITE ELEMENTS

Industrial power generation and transmission structures are designed to have a service life of 40 years. Knowledge of the evolution of material behavior over long periods of time is therefore crucial to ensure the safety and reliability of these facilities. Due to the continuously increasing power demand, new energy sources are needed. As part of the decarbonization of these sources, hydrogen will play an important role as an energy vector. However, hydrogen can easily diffuse in materials, inducing premature failure with reduced ductility and toughness. This phenomenon, called hydrogen embrittlement (HE), is a complex mechanism which combines mechanical and chemical loadings. Therefore, this work presents a strategy to simulate HE by the finite element method integrating plasticity and damage coupled to hydrogen diffusion. Since damage is highly dependent on local stresses and hydrostatic pressure mixed formulations in displacement, pressure and volume variation have been proposed to control volumetric locking. To represent ductile rupture, the Gurson-Tvergaard-Needleman (GTN) model based on an implicit gradient nonlocal formulation with two internal lengths is considered, which allows regularizing void growth and strain-controlled nucleation. All the implementations and simulations have been carried out using the Z-set software.
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FATIGUE DESIGN SENSITIVITIES OF STATIONARY TYPE 2 HIGH-PRESSURE HYDROGEN VESSELS

Common manufacturing processes for type 2 high-pressure hydrogen storage vessels use a surrogate measure of the desired residual stress, e.g., target strain on the external surface or target internal pressure. The critical value of these measures is chosen to impart residual stress sufficient to achieve a certain fatigue performance for the targeted operational pressure cycling and with given assumptions about the liner and overwrap geometry and materials. This paper uses computational simulation to study the sensitivities of fatigue performance to associated design specifications and assumptions.
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MECHANICAL CHARACTERIZATION AND DEFECT ANALYSIS OF NATURAL GAS PIPELINE STEEL TOWARDS HYDROGEN INJECTION

Repurposing of natural gas infrastructure towards hydrogen injection implies its mechanical viability assurance. This study focuses on the structural integrity assessment of vintage steel API 5L Grade B (used in natural gas infrastructure), especially in what concerns the ductility loss due to hydrogen embrittlement and its effect on common damage occurrence, such as plain denting.
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ENVIRONMENTAL STRESS CRACKING RESISTANCE OF HIPS UNDER CYCLIC LOADING USING CRACKED ROUND BAR SPECIMENS

The service life of polymers depends strongly on their loading conditions and the environment surrounding them. Prolonged contact of a polymer with an oily or fatty environment increases the tendency of crazing and thus shorten the service life. The objective of this paper was to investigate two different high-impact polystyrene polymers (HIPS) in terms of their environmental stress cracking resistance (ESCR) in air and sunflower oil environments by cyclic testing. It was shown that the HIPS grade with bigger rubber particles, even though it has lower short-term mechanical performance in tensile modulus, yield strength, and notched impact strength, is preferrable in terms of ESCR and should be used in fatty environment applications.
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