Effect of transient trapping on hydrogen transport near a blunting crack tip

The paper revisits the way transient trapping is introduced in the literature based on the Sofronis and McMeeking model (Sofronis and McMeeking, 1989) [1] of hydrogen transport. It is shown that the direct use of the improved formulation made by Krom et al. (1999) [2] for transient trapping may lead to non-physical results of hydrogen concentration in case of an insulated system. The use of McNabb and Foster trapping kinetic equation is more relevant, and its ability to model both trap creation and kinetic trapping is investigated on a Small Scale Yielding configuration for the sake of comparison with a reference case from the literature. A parametric study is conducted, exhibiting differences with literature, and emphasizes on the significant effect of trapping kinetics on the hydrogen distribution.     

Influence of grain boundary misorientation on hydrogen embrittlement in bi-crystal nickel

Computational techniques and tools have been developed to understand hydrogen embrittlement and hydrogen induced intergranular cracking based on grain boundary (GB) engineering with the help of computational materials engineering. This study can help to optimize GB misorientation configurations by identifying the cases that would improve the material properties increasing resistance to hydrogen embrittlement. In order to understand and optimize, it is important to understand the influence of misorientation angle on the atomic clustered hydrogen distribution under the impact of dilatational stress distributions. In this study, a number of bi-crystal models with tilt grain boundary (TGB) misorientation angles (θ) ranging between 0°≤ θ ≤ 90° were developed, with rotation performed about the [001] axis, using numerical microstructural finite element analysis. Subsequently, local stress and strain concentrations generated along the TGB (due to the difference in individual neighbouring crystals elastic anisotropy response as functions of misorientation angles) were evaluated when bi-crystals were subjected to overall uniform applied traction. Finally, the hydrogen distribution and segregations as a function of misorientation angles were studied. In real nickel, as opposed to the numerical model, geometrically necessary dislocations are generated due to GB misorientation. The generated dislocation motion along TGBs in response to dilatational mismatch varies depending on the misorientation angles. These generated dislocation motions affect the stress, strain and hydrogen distribution. Hydrogen segregates along these dislocations acting as traps and since the dislocation distribution varies depending on misorientation angles the hydrogen traps are also influenced by misorientation angles. From the results of numerical modelling it has been observed that the local stress, strain and hydrogen distributions are inhomogeneous, affected by the misorientation angles, orientations of neighbouring crystal and boundary conditions. In real material, as opposed to the numerical model, the clustered atomic hydrogens are segregated in traps near to the TGB due to the influence of dislocations developed under the effects of applied mechanical stress. The numerical model predicts maximum hydrogen concentrations are accumulated on the TGB with misorientation angles ranging between 15°< θ < 45°. This investigation reinforces the importance of GB engineering for designing and optimizing these materials to decrease hydrogen segregation arising from TGB misorientation angles.     

Experimental study and numerical simulation on the SSCC in FV520B stainless steel exposed to H2S+Cl− Environment

Constant displacement loading tests using wedge opening loading specimens were carried out in aqueous hydrogen sulfide solution containing sodium chloride to investigate the susceptibility of stress corrosion cracking (SCC) of FV520B precipitation hardening martensitic stainless steel. Results of the SCC tests indicated that the stress corrosion critical stress intensity factor (K_ISCC) dramatically decreased in the corrosion medium investigated and decreased with the increasing of H₂S concentration. Microstructures of fracture surfaces were analyzed using a scanning electron microscope (SEM) with an energy dispersive X-ray spectroscopy (EDS). The fracture surface was typical of sulfide stress corrosion fracture. In addition, large amount of intermittent arc-crack on the side surfaces around the tip of main crack formed even no main crack propagated.A sequentially coupling finite element analysis (FEA) program was utilized to simulate the stress field and calculate the diffused hydrogen concentration distribution of specimen exposed to the corrosion medium investigated. The FEA results indicated that corrosion pit affected the stress and diffusion hydrogen distribution around the corrosion pit where large stress gradients formed. Side surface cracks initiated from those corrosion pits and propagated under the synergy of stress and hydrogen. The effect of the corrosion pit on hydrostatic stress distribution was limited in superficial zone near the side surface, thus side surface cracks propagated along the hoop direction rather than along the direction of specimen thickness. Based on the morphology observation and FEA results, it can be concluded that the SCC mechanism of FV520B steel was hydrogen embrittlement mainly and combination of anodic dissolution. Simultaneously, corrosion pitting was the precondition of side surface crack formation while the stress induced hydrogen diffusion was the dominant factor.     

Effects of vanadium precipitates on hydrogen trapping efficiency and hydrogen induced cracking resistance in X80 pipeline steel

In this study, the number and size distribution of vanadium precipitates and their effects on hydrogen trapping efficiency and hydrogen-induced cracking (HIC) susceptibility were investigated in X80 pipeline steel. The results showed that as the vanadium content increased, the number of nanoscale vanadium precipitates clearly increased. Furthermore, the amount of hydrogen atoms trapped by vanadium precipitates gradually increased and the hydrogen diffusion coefficient decreased from 4.74 × 10^?6 cm² s^?1 in the vanadium-free V0 steel to 8.48 × 10^?7 cm² s^?1 in the V4 steel with 0.16% V, according to hydrogen permeation results. It also reduced the possibility of hydrogen atoms diffusing into the sites of harmful defects such as large-size oxides and elongated MnS inclusions, where cracks were caused more easily. In addition, the V3 steel with 0.12% V, containing the largest number of vanadium carbide particles of less than 60 nm, had the lowest HIC susceptibility.     

Investigation on hydrogen induced cracking behaviors of Ni-base alloy

Hydrogen embrittlement of a Ni-base alloy at room temperature was investigated by slow strain rate tensile test (SSRT) under precharging or dynamic charging conditions. It was found that hydrogen embrittlement susceptibility of this alloy increased with increasing charging current density in both charging conditions. In-situ observation of hydrogen induced cracking revealed that surface crack initiation at both grain boundaries and slip bands, which should be attributed to decomposition of hydride phase during aging at room temperature. SSRT result exhibited that hydrogen diffusion in the alloy could be facilitated by deformation and as a result induced transgranular fracture of the sample. Both hydrogen induced cracking and the interaction between hydrogen and deformation played combined roles on hydrogen embrittlement of this alloy.     

Comparison of mode-Ⅰ crack propagation of tube subjected to internal hydrogen static and detonation loading

An elaborate numerical model with progressive damage and failure and fluid-structure coupling was developed to study the crack propagations of tubes under hydrogen static and detonation loads. The numerical model was verified with experiment in terms of crack behaviors and fracture patterns. The tube responses, crack propagations, pressure histories, crack lengths and speeds as well as the energy storages were obtained and analyzed in detailed. It was found the static load case has higher stored energy inside the tube, which causes the larger crack length and speed, as well as the severer bending deformation of tube. The forward crack first run faster under detonation load, but it will be caught up by the backward crack in the late period. The crack growths are incremental under both types of loads. However, the crack growth under detonation load has certain regular patterns, where the oscillating crack speed has a dominant frequency that can be calculated by the proposed formula. Moreover, the quantitative relationship between detonation load speed and the incremental crack growth length was revealed, which is fundamentally and practically useful.     

Dependence of hydrogen embrittlement on hydrogen in the surface layer in type 304 stainless steel

Hydrogen embrittlement (HE) together with the hydrogen transport behavior in hydrogen-charged type 304 stainless steel was investigated by combined tension and outgassing experiments. The hydrogen release rate and HE of hydrogen-charged 304 specimens increase with the hydrogen pressure for hydrogen-charging (or hydrogen content) and almost no HE is observed below the hydrogen content of 8.5 mass ppm. Baking at 433 K for 48 h can eliminate HE of the hydrogen-charged 304 specimen, while removing the surface layer will restore HE, which indicates that hydrogen in the surface layer plays the primary role in HE. Scanning electron microscopy (SEM) and scanning tunnel microscopy (STM) observations show that particles attributed to the strain-induced α′ martensite formation break away from the matrix and the small holes form during deformation on the specimen surface. With increasing strain, the connection among small holes along {111} slip planes of austenite will cause crack initiation on the surface, and then the hydrogen induced crack propagates from the surface to interior.     

Numerical simulation of the transient hydrogen trapping process using an analytical approximation of the McNabb and Foster equation

In order to simulate hydrogen charging and discharging cycles of mechanically loaded structures, an analytical solution for the differential equation of trapping kinetics is proposed, as a generalization of the Oriani's equilibrium relationship.This solution has been implemented in the Abaqus finite element software, and validated by comparison with the one-dimensional kinetic MRE Hydrogen Isotope Inventory Processes Code (HIIPC). Last, the results of an application on a 3D structure are presented.     

Finite element analysis of hydrogen diffusion/plasticity coupled behaviors of low-alloy ferritic steel at large strain

Hydrogen embrittlement is commonly considered as a typical failure mechanism for low-alloy ferritic steel under high pressure hydrogen environment. Currently, the hydrogen enhanced localized plasticity theory has been largely recognized for studying the hydrogen embrittlement mechanism by introducing the localized plastic flow and the hydrogen induced strain concept. However, the hydrogen induced strain and the plastic strain are often solved respectively in this theory, which may weaken the effect of hydrogen on the plastic deformation. The purpose of this paper is to propose a modified theoretical model from the microstructural level by emphasizing the coupling mechanism between the hydrogen diffusion and the plastic deformation at large strain, where the hydrogen induced strain is superimposed on the equivalent plastic strain instead of on the strain components. Fully implicit backward Euler algorithm by finite element analysis (FEA) under the corotational configuration is used to implement the proposed model, where the hydrogen induced strain is involved in the stress return process within each iteration, indicating a more direct interaction between them than existing works. FEA by using finite element software ABAQUS-UMAT subroutine is performed for the smooth tensile specimen and the notch specimen respectively under slow tensile strain rate loading and different hydrogen pressure. Developed direct coupling model is expected to further gain insight into the hydrogen embrittlement effect on the plastic deformation, especially at the trapping sites.     

Hydrogen trapping and hydrogen induced cracking of welded X100 pipeline steel in H2S environments

Hydrogen induced cracking (HIC) susceptibility of the welded X100 pipeline steel was evaluated in NACE “A” solution at room temperature according to the NACE TM0284-2011 standard. Both the kinetic parameters of the permeability $\left(J_{\infty }L\right)$, the apparent diffusivity (D_app) and the concentration of reversible and irreversible hydrogen in the base metal and welded joint of X100 pipeline steel were quantitatively investigated by hydrogen permeation test. The results showed that the welded joint with an inhomogeneous microstructure had a higher trap density and more susceptible to HIC due to two orders of magnitude larger in the concentration of irreversible hydrogen than that of base metal, though all presenting poor HIC resistance for both base metal and the welded joint. The HIC cracks initiated from the inclusions enriching in Al, Ca, Si, Mn. The cracks are primarily transgranular, accompanying with limited intergranular ones.     

Structural Control of Piezoelectric Laminated Beams under Thermal Load

The dynamic analysis and the active vibration control of piezoelectric laminated beams under thermal load are presented. The beam is modeled using two noded finite elements with four mechanical and a variable number of electric potential degrees of freedom at each node. In the thickness direction, the thermal and the electric fields are approximated as piecewise linear across an arbitrary number of sublayers in a layer. Cubic Hermite interpolation is used for the deflection and electric potentials at the sublayers and linear interpolation is used for the axial displacement and the shear rotation. The thermal field is computed using a consistent six-noded thermal finite element with a quadratic interpolation along longitudinal direction and a linear interpolation along thickness direction. The temperature distribution and undamped natural frequencies are obtained for composite and sandwich beam under cantilever and clamped-clamped boundary conditions and compared with 2D-FE Abaqus results. The finite element equations derived are converted into modal model to represent them in the state space form. This model is then reduced using Hankel norm for designing the LQG controller. Optimal control technique is used to control the vibration of the beam. The designed LQG controller controls the tip deflection of composite and sandwich cantilever beams and midpoint deflection of clamped-clamped beams.