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.
EXTENDED ABSTRACT
Themes: Hydrogen Embrittlement and Environmentally Assisted Cracking
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.
EXTENDED ABSTRACT
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.
EXTENDED ABSTRACT
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.
EXTENDED ABSTRACT
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.
EXTENDED ABSTRACT
IN-SITU NEUTRON IMAGING AND MODELING OF HYDROGEN EMBRITTLEMENT IN HIGH STRENGTH STEELS
A new experimental method to investigate hydrogen assisted cracking is presented in this paper. By combining electrochemical pre-charging, fracture mechanics and neutron imaging it is possible to get large experimental data which can give insight into the local fracture process zone. Furthermore, it can be used to calibrate FEM-models which considers crack propagation, embrittlement, and H diffusion from a moving stress field.
EXTENDED ABSTRACT
HYDROGEN EMBRITTLEMENT BEHAVIOR OF A 1.5 GPA CLASS DUAL-PHASE STEEL
In the current work, using tensile tests, we evaluate the hydrogen embrittlement behavior of a 1.5 GPa dual-phase (DP) steel consisting of ~75% martensite. Contrary to previous studies of DP steel with ultimate tensile strength (UTS) ≤1.2 GPa, a predominant brittle fracture is observed in the DP steel in the absence of hydrogen. Conventionally, in the absence of hydrogen, ferrite is reported to arrest cracks, resulting in a ductile fracture. However, ferrite undergoes {100} brittle cleavage cracking. Furthermore, the morphology of the martensite crack is found to have an influence on ferrite {100} cleavage cracking. The micro-mechanisms are presented in detail. Subsequently, we investigated the effect of hydrogen on the degradation of tensile properties. Hydrogen caused a significant deterioration of UTS, from 1.5 GPa to 0.9 GPa. The damage mechanisms of hydrogen-induced fracture are discussed in detail.
EXTENDED ABSTRACT
MODDELING OF INTERGRANULAR STRESS CORROSION CRACKING MECHANISM THROUGH COUPLING OF SLIP-OXIDATION AND COHESIVE ZONE MODEL
A finite element model was proposed for intergranular stress corrosion cracking modelling (IGSCC). The model is based around a moving integration point formulation which enables the model to track the oxide, dissolution, and crack tip. The formulation is introduced in the cohesive element. The model also relies on an electrochemical model, based on the slip-oxidation model and a diffusion model. The model is dependent on the plastic strain rate and creep strains for oxide rupture to evaluate the effect of creep and plastic strain on crack growth and oxide thickness in IGSCC.
EXTENDED ABSTRACT
FRACTURE TOUGHNESS OF ZIRCALOY-4 CLADDING IN CASE OF DELAYED HYDRIDE CRACKING
Spent nuclear fuels are stored after their use in reactors. Dry storage can favor the appearance in the fuel rod cladding of a mechanical-chemical phenomenon referred to as Delayed Hydride Cracking (DHC). DHC is divided into three iterative steps: (i) diffusion of hydrogen in solid solution; (ii) precipitation of this hydrogen into hydrides; (iii) brittle fracture of hydrides. To assess the risk of occurrence of this phenomenon, the fracture toughness is determined by calculating the stress intensity factor below which DHC is not observed (KIDHC) based on an experimental procedure and a numerical model.
EXTENDED ABSTRACT
INTEGRATED MODELING OF STRESS CORROSION CRACKING IN SUPERALLOYS
The reliability of turbine blades is strongly dependent on the humidity, contamination, stress, and temperature to which they are exposed during operation. In many cases, cracks initiate simultaneously at multiple locations, which can result in crack arrest (shielding) or (coalescence). This presentation will explore an integrated computational and experimental approach that evaluates crack interaction in CMSX-4 superalloy using C-Ring tests with a layer of contaminant salt exposed to 550C. A phase-field model calculates the diffusion of species and reduces the material critical energy release rate accordingly. The model, which is parameterised to enable cracking above a threshold stress, predicts the critical crack spacing that results in shielding or coalescence. In addition, the integration of X-Ray microscopy (XRM) characterisation with FEM modelling demonstrates univocally the role of crack interaction in stress corrosion cracking. We conclude discussing the value of integrating models and experiments to understand complex failure mechanisms.
EXTENDED ABSTRACT