Hydrogen-assisted Fatigue Crack Research Articles

Hydrogen-assisted localized plasticity driven by dislocation pinning-depinning: Finite element simulations

For the development of a hydrogen energy society, understanding hydrogen-assisted fatigue crack propagation is crucial. Hydrogen-assisted fatigue crack propagation exhibits an exceptionally rapid growth rate, with only small plastic deformation observed at the macroscopic level. Nevertheless, the presence of dislocation structures, typically associated with conventional fatigue failure, has been reported near the crack tip. Explaining this phenomenon solely based on hydrogen-induced softening or hardening is challenging. Therefore, this study focuses on the newly proposed concept of temporary hardening resulting from hydrogen-induced pinning and depinning. In this research, we conducted a numerical analysis using an elastoplastic analysis coupled with hydrogen diffusion analysis to simulate the pinning and depinning effects caused by hydrogen. Our simulation results demonstrated that hydrogen-induced pinning and depinning led to higher strains at the crack tip compared to cases without these effects, resulting in small-scale macroscopic plastic deformation.

Hydrogen-assisted fatigue crack propagation behavior of selective laser–melted Inconel 718 alloy

Hydrogen-assisted fatigue crack propagation behavior of selective laser melted Inconel 718 alloy was investigated under in situ electrochemical hydrogen charging, and by multi-scale microstructural analysis. Results suggest that hydrogen significantly increases the fatigue crack growth rate (FCGR), and such an effect is intensified for the post-heat-treatment sample by an acceleration of 2.8 times. Rapid hydrogen diffusion along grain boundaries, and planar-dislocation-slip-induced local hydrogen concentration induced abundant of intergranular cracking, and a decrease of deformation expansion. The suppressed dislocation emission and mobility by hydrogen significantly confine the plastic deformation ahead of the crack tip, thus accelerated the FCGR in the presence of hydrogen.

Effects of hydrogen pressure on hydrogen-assisted fatigue crack growth of Cr-Mo steel

The fatigue crack growth rates (FCGRs) of 4130X steel in 45, 70, 87.5 and 100 MPa hydrogen were measured, and the hydrogen concentrations at the crack tip of test specimens were analyzed by establishing a hydrogen transport model. This model can describe the hydrogen behaviors in the process of environmental embrittlement. Results show that the FCGR increases with the increase in hydrogen pressure. However, the acceleration reaches the threshold under 87.5 MPa. Hydrogen pressure has the same influence on hydrogen concentration, and the threshold of the effect is probably related to the fact that the hydrogen absorption on the material surface reaches the limit.

Hydrogen-assisted fatigue crack propagation behavior of a C–N co-doped interstitial high-entropy alloy

The hydrogen-assisted fatigue crack propagation behavior of a C–N co-doped interstitial high-entropy alloy (iHEA) was investigated under in-situ electrochemical hydrogen charging. The cracking path, deformation microstructure, and fractography were characterized via multiple methods. Results suggest that hydrogen uptake shows a certain acceleration on the fatigue crack propagation rate (FCGR) of the iHEA, and such acceleration increases are intensified when increasing the hydrogen-charging current density. Crack propagation along grain boundaries and a remarkable decrease in the extent of plastic deformation immediately near to the crack surface have also been observed. This decrement of critical plastic strain for fatigue crack propagation primarily accelerates the FCGR.

Dual grain size-effects on hydrogen-assisted fatigue crack growth in 1 GPa-class medium-carbon martensitic steel

The influence of prior austenite grain (PAG) size ranging from 6 to 90 μm on the fatigue crack growth (FCG) of a tempered 0.41%C lath martensitic steel was investigated in a 90 MPa H2 gas environment at ambient temperature. Under the presence of H, severe FCG accelerations accompanying intergranular (IG) cracking along PAG boundaries as well as quasi-cleavage (QC) fracture along martensite block boundaries and {011} crystallographic planes were systematically found. Smaller PAG mitigated the acceleration at a relatively high-stress intensity via suppressing IG cracking. However, PAG refinement instead escalated the acceleration at a low-stress intensity factor where QC facture prevailed. This inverse grain size effect was discussed in terms of the plasticity criterion in triggering QC and its possible dependence on the microstructural length scale.

Fatigue Crack Propagation in Pearlitic Steel under Pressurized Gaseous Hydrogen: Influences of Microstructure Size and Strength Level

For the wall-thickness reduction of the components destined for pressurized gaseous hydrogen, widespread use of high-strength martensitic steels has long been desired. However, their strong susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG) is still limiting their proactive applications. Here, we instead focused on pearlite as another potential reinforcing agent for the development of new hydrogen-compatible steels with acceptable cost performance. Fatigue crack growth (FCG) behavior of three eutectoid steels with different microstructure sizes (i.e., ferrite/cementite interlamellar spacing, colony and block sizes) and strength levels was investigated in a 90 MPa hydrogen gas, an essential evaluation when attempting to perform a defect tolerant design of the components used for high-pressure gases.

A cyclic cohesive zone model for predicting hydrogen assisted fatigue crack growth (FCG) of subsea pipeline steels

A cyclic cohesive zone model for predicting hydrogen assisted fatigue crack growth is proposed, in which the degradation of cohesive strength and accumulated cohesive length is taken into account and an inverse Logistic formula is proposed for describing the degradation model of the accumulated cohesive length. A comparison between prediction results of the proposed model and experimental data of fatigue crack growth rates (FCGRs) of API 5L Grade B, X42 and X52 pipeline steels under hydrogen environment is then performed and the results show that the FCGRs predicted by the proposed model are in good agreement with the experimental data.

Hydrogen-assisted fatigue crack growth: Pre-charging vs in-situ testing in gaseous environments

We investigate the implications of conducting hydrogen-assisted fatigue crack growth experiments in a hydrogen gas environment (in-situ hydrogen charging) or in air (following exposure to hydrogen gas). The study is conducted on welded 42CrMo4 steel, a primary candidate for the future hydrogen transport infrastructure, allowing us to additionally gain insight into the differences in behavior between the base steel and the coarse grain heat affected zone. The results reveal significant differences between the two testing approaches and the two weld regions. The differences are particularly remarkable for the comparison of testing methodologies, with fatigue crack growth rates being more than one order of magnitude higher over relevant loading regimes when the samples are tested in a hydrogen-containing environment, relative to the pre-charged samples. Aided by finite element modelling and microscopy analysis, these differences are discussed and rationalized. Independent of the testing approach, the heat affected zone showed a higher susceptibility to hydrogen embrittlement. Similar microstructural behavior is observed for both testing approaches, with the base metal exhibiting martensite lath decohesion while the heat affected zone experienced both martensite lath decohesion and intergranular fracture.

Resistance of pearlite against hydrogen-assisted fatigue crack growth

Recent progress in realizing hydrogen society requires higher-strength steels having decent hydrogen embrittlement resistance via optimized microstructural controls. The present communication first reports the fatigue crack growth (FCG) property of pearlitic steel in a high-pressure, i.e., 90 MPa, hydrogen gas environment to probe this standard microstructure's potency for becoming a constructive reinforcing agent in future hydrogen-compatible structural materials. As-transformed eutectoid steel with 1080 MPa-tensile strength exhibited hydrogen-induced FCG acceleration to a lesser extent relative to martensite with an equivalent strength level and occasionally to pure ferritic iron. Furthermore, the superiority over martensite was augmented under slow loading-rate conditions, suppressing time-dependent cracking, a critical issue in implementing defect tolerant design on high-strength steels for hydrogen gas applications.

Cohesive zone modelling of hydrogen assisted fatigue crack growth: The role of trapping

We investigate the influence of microstructural traps in hydrogen-assisted fatigue crack growth. To this end, a new formulation combining multi-trap stress-assisted diffusion, mechanism-based strain gradient plasticity and a hydrogen- and fatigue-dependent cohesive zone model is presented and numerically implemented. The results show that the ratio of loading frequency to effective diffusivity governs fatigue crack growth behaviour. Increasing the density of beneficial traps, not involved in the fracture process, results in lower fatigue crack growth rates. The combinations of loading frequency and carbide trap densities that minimise embrittlement susceptibility are identified, providing the foundation for a rational design of hydrogen-resistant alloys.

Effects of hydrogen and load frequency on the fatigue crack propagation behavior of selective laser melted Inconel 718 alloy

Hydrogen-assisted fatigue crack propagation behavior of a selective laser melted Inconel 718 alloy was investigated under in situ electrochemical hydrogen charging, and by multi-scale microstructural analysis. Results show that hydrogen significantly accelerates fatigue crack growth rate (FCGR), and such effect is intensified when decreasing load frequency. Crack propagation along cellular structure boundaries and decrease of plasticity along the crack have been evidenced. Hydrogen decreases the critical strain for crack propagation and results in the acceleration of FCGR. Lower load frequency induces higher amplitude of concentration fluctuation and larger penetration depth of hydrogen ahead of the crack tip, thus further promotes the acceleration of FCGR.

Hydrogen-assisted fatigue propagation paths in corner cracks emanating from holes in tension-loaded plates

Hydrogen-assisted fatigue crack growth (corrosion-fatigue) is studied in corner cracks emanating from holes placed in plates under tensile loading, comparing the results with those obtained in the case of fatigue in air. To this end, numerical modeling has been carried out to evaluate the crack front advance based on the Paris equation and the stress intensity factor (SIF) obtained by Raju and Newman. There is a preferential crack path, the approach to it being quicker for the case of fatigue in air than for the case of corrosion-fatigue.

Dual roles of pearlite microstructure to interfere/facilitate gaseous hydrogen-assisted fatigue crack growth in plain carbon steels

Fatigue crack growth of two carbon steels with different pearlite volume fractions were studied in pressurized gaseous hydrogen environment. Notably, pearlite was found to mitigate hydrogen-assisted fatigue crack acceleration. This positive impact of pearlite was ascribed to ferrite/cementite lamellar aligned perpendicularly to the cracking direction, which functioned as barriers to intermittently arrest the crack propagation. Meanwhile, brittle delamination fracture ensued in the pearlite lamellar lying parallel to the crack-plane increased the crack growth rate and compromised the above positive effect to some extent. The material behavior is rationalized in light of fractographical observations and microstructural analyses of the crack-wake.

Controlling factors and mechanisms of fatigue crack growth influenced by high pressure of gaseous hydrogen in a commercially pure iron

This paper proposes an extended discussion of experimental results previously published by the authors and from literature about fatigue crack growth influenced by high pressure of gaseous hydrogen in a commercially pure iron. Firstly a summary of these results obtained for different exposure conditions in terms of gas pressure and loading frequency is presented. Two characteristic regimes, the first at low ΔK values characterized by a moderated fatigue crack growth enhancement and a nearly fully intergranular mode, and a highly-accelerated regime with a quasi-cleavage failure mode with the formation of coarse striations. The mechanisms controlling the different regimes of Hydrogen-Assisted Fatigue Crack Growth are discussed on the basis of observations of fracture surfaces and of deformation substructures within the plastic zone while assessing the capability of different models from literature to account for these observations. In a last section, the dependency of the transition between the different regimes as well as the crack growth rate enhancement in the highly-accelerated regime with respect to the hydrogen gas pressure and loading frequency is analyzed.

Atomistic simulation of hydrogen-induced plastic zone compression during cyclic loading

To investigate the effect of hydrogen atoms on the size of the plastic deformation zone, molecular dynamics (MD) simulations were performed on a single crack model. The model uses pre-charged hydrogen to quantify the compression effect of hydrogen atoms on the plastic zone during cyclic loading. The results show that stress release at the crack tip occurs mainly in the form of plastic deformation, and the degree of compression in the plastic zone increases with increasing hydrogen concentration. A compression factor, which considers hydrogen concentration, is found with the help of the simulation results. A modified fatigue crack growth rate (FCGR) model, combined with the compression factor, was then used to predict the hydrogen-assisted fatigue crack growth rate. The proposed model shows excellent agreement with the experimental data for X100 steel, and it provides a new framework to describe hydrogen-assisted cracking.

A mechanism behind hydrogen-assisted fatigue crack growth in ferrite-pearlite steel focusing on its behavior in gaseous environment at elevated temperature

Hydrogen-assisted fatigue crack growth in gaseous environment was comparatively examined at room temperature (RT) and 423 K, based on analysis of the deformation structure evolution around crack-wakes using scanning electron microscopy techniques. In hydrogen-gas at RT, the propagating crack displayed weakly-evolved dislocation arrangement, accompanied by a significant acceleration of fatigue crack growth. However, in hydrogen-gas at 423 K, the crack-wake plasticity was well-evolved and analogous to that observed in an inert environment. This apparent recovery of deformation micro structure coincided with suppressed crack growth acceleration, the rationale for which can be interpreted by the trapping/de-trapping equilibrium between hydrogen and dislocations.

Fatigue crack growth rates in high pressure hydrogen gas for multiple X100 pipeline welds accounting for crack location and residual stress

Fatigue crack growth rates (FCGR) of multiple X100 pipeline steel welds and heat affected zones were measured in high-pressure hydrogen gas to investigate their behavior compared to lower strength pipeline welds. A total of five high strength welds and two heat affected zones (HAZ) were examined all of which were fabricated using the same X100 base material. Different welding wires and techniques were used to fabricate the welds to provide a variety of end products to evaluate susceptibility to fatigue in high pressure hydrogen gas. Residual stresses were measured for each weld and HAZ using the slitting method and the effect of residual stress on the stress intensity factor, Kres, was determined. Using Kres, the fatigue crack growth rate curves were corrected to remove the effects of residual stress by examining the influence of Kres on stress ratio, R. Comparisons were then made between the high strength welds, which were corrected for residual stress, and lower strength welds from the literature. It was found that the higher strength welds and heat affected zones exhibited comparable fatigue crack growth rates to lower strength welds, as the FCGR data of the high strength welds overlaid the lower strength welds. This suggests that despite distinct differences in strength and microstructure between the different welds, hydrogen-assisted fatigue crack growth susceptibility is similar. A comparison was made between the Kres measured in extracted coupons and residual stress estimates provided in relevant welded pipe assessment standards such as API 579-1/ASME FFS-1. It was found the residual stress values in the test coupons extracted from welded pipe were significantly lower than those expected in the intact welded pipes and highlights the importance in quantifying and removing coupon residual stresses when fatigue crack growth rates are measured and including expected weld joint residual stress when making structural assessments.

Role of retained austenite in hydrogen trapping and hydrogen-assisted fatigue fracture of high-strength steels

The Interaction of hydrogen with retained austenite under fatigue loading of dual-phase and complex-phase high-strength steels with strength of about 1200 MPa was studied. A load-controlled fatigue test was performed in the air with the maximum applied tensile strength of 900 MPa, which is just below the offset yield point of the studied steels. The trapping of hydrogen accumulated into the studied steels under the fatigue loading was studied by thermal desorption spectroscopy. Measurements of hydrogen trapping evolution and microstructure changes during fatigue testing reveal a complicated hydrogen trapping behavior driven by hydrogen interaction with deformation defects and retained austenite. Hydrogen concentration increases in the studied steels during the fatigue testing in the air without preceding hydrogen charging. The fracture surfaces were studied by scanning electron microscopy evidencing the relationship between the hydrogen concentration increase related to retained austenite and initiation of the intergranular fatigue fracture. The role of retained austenite in hydrogen-assisted fatigue cracking is discussed and a possible mechanism of the hydrogen-assisted fatigue crack initiation in the high-strength steels is proposed.

Hydrogen isotope effect on the embrittlement and fatigue crack growth of steel

The deleterious effect of hydrogen on steel is a long-standing problem. Embrittlement in the presence of hydrogen is, in part, a consequence of the fast diffusion of hydrogen in ferritic steels. Because of the identical chemical properties but large differences in mass between hydrogen isotopes, interaction of hydrogen isotopes with steel structures are of interest as a tool to study the role of diffusion in hydrogen embrittlement under all-else-equal conditions. In this paper, we present tensile and fatigue data for steel samples measured in air, in hydrogen, and in deuterium. Tensile measurements show deuterium causes a loss of ductility comparable to the loss due to hydrogen. In fatigue, a large pressure effect on deuterium-assisted fatigue crack growth was observed, an effect which is not exhibited in uniaxial tensile measurements. With sufficient pressure to show an enhanced fatigue crack growth effect, the fatigue crack growth rate is well-predicted assuming a scaling of the diffusion coefficient by 1/2. These results suggest surface adsorption kinetics play a large role in hydrogen-assisted fatigue crack growth at low pressure. At higher pressures, hydrogen-assisted fatigue crack growth is primarily affected by the kinetics of diffusion of hydrogen within the steel lattice.

The role of intergranular fracture on hydrogen-assisted fatigue crack propagation in pure iron at a low stress intensity range

Hydrogen-assisted fatigue crack growth (HAFCG) in pure iron at a relatively low stress intensity range exhibits brittle-like intergranular (IG) fracture, while the macroscopic crack acceleration is not significant. The present study focuses on the mechanism of IG fracture in terms of the microscopic deformation structures near the crack propagation paths. We found that the IG fracture is attributed to hydrogen-enhanced dislocation structure evolution and subsequent microvoid formation along the grain boundaries. The impact of such IG cracking on the macroscopic fatigue crack growth (FCG) acceleration is evaluated according to the dependency of IG fracture tendency on the hydrogen gas pressure during testing. It is demonstrated for the first time that increased hydrogen pressure results in a larger fraction of IG fracture and correspondingly faster FCG. On the other hand, the gaseous hydrogen environment also has a positive role in decelerating the FCG rate relative to air due to the absence of oxygen and water vapor. The macroscopic crack propagation rate in hydrogen gas is eventually determined by the competition between the said positive and negative influences.