Hydrogen-assisted Fatigue Crack Propagation 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.

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.

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 crack-propagation in a Ni-based superalloy 718, revealed via crack-path crystallography and deformation microstructures

Fatigue crack-growth (FCG) of Ni-based superalloy 718 was investigated under gaseous hydrogen environment (external hydrogen) and uniformly pre-charged state (internal hydrogen). Under external hydrogen, intergranular fracture predominated, whereas dislocation slip-band or twin boundary fracture were prevalent under internal hydrogen. This failure mode divergence encompassed unique characteristics of macroscale FCG response, leading to both cycle- and time-dependent cracking. The intergranular cracking was ascribed to short-circuit diffusion of hydrogen along grain boundaries. Meanwhile, the material’s inherently inhomogeneous deformation mode exerts harmfulness when hydrogen was uniformly distributed inside the specimen, causing slip-bands or twin boundaries to become the weakest links for fracture.

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.

Interpretation of hydrogen-assisted fatigue crack propagation in BCC iron based on dislocation structure evolution around the crack wake

A new model for hydrogen-assisted fatigue crack growth (HAFCG) in BCC iron under a gaseous hydrogen environment has been established based on various methods of observation, i.e., electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM), to elucidate the precise mechanism of HAFCG. The FCG in gaseous hydrogen showed two distinguishing regimes corresponding to the unaccelerated regime at a relatively low stress intensity factor range, ΔK, and the accelerated regime at a relatively high ΔK. The fracture surface in the unaccelerated regime was covered by ductile transgranular and intergranular features, while mainly quasi-cleavage features were observed in the accelerated regime. The EBSD and ECCI results demonstrated considerably lower amounts of plastic deformation, i.e., less plasticity, around the crack path in the accelerated regime. The TEM results confirmed that the dislocation structure immediately beneath the crack in the accelerated regime showed significantly lower development and that the fracture surface in the quasi-cleavage regions was parallel to the {100} plane. These observations suggest that the HAFCG in pure iron may be attributed to “less plasticity” rather than “localized plasticity” around the crack tip.

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part II: Accelerated regime manifested by quasi-cleavage fracture at relatively high stress intensity range values

Hydrogen effect on fatigue performance at relatively high values of stress intensity factor range, ΔK, of pure BCC iron has been studied with a combination of various electron microscopy techniques. Hydrogen-assisted fatigue crack growth rate is manifested by a change of fracture features at the fracture surface from ductile transgranular in air to quasi-cleavage in hydrogen gas. Grain reference orientation deviation (GROD) analysis has shown a dramatic suppression of plastic deformation around the crack wake in samples fatigued in hydrogen. These results were verified by preparing site-specific specimens from different fracture features by using Focused Ion Beam (FIB) technique and observing them with Transmission Electron Microscope (TEM). The FIB lamella taken from the sample fatigued in air was decorated with dislocation cell structure indicating high amount of plasticity, while the lamella taken from the quasi-cleavage surface of the sample fatigued in hydrogen revealed a distribution of dislocation tangles which corresponds to smaller plastic strain amplitude involved at the point of fracture. These results show that a combination of critical hydrogen concentration and critical stress during fatigue crack growth at high ΔK values triggers cleavage-like fracture due to reduction of cohesive force between matrix atoms.

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part I: Intergranular crack propagation at relatively low stress intensities

The role of hydrogen on intergranular (IG) fracture in hydrogen-assisted fatigue crack growth (HAFCG) of a pure iron at low stress intensity was discussed in terms of the microscopic deformation structures near crack propagation paths. The main cause of IG fracture was assumed to be the hydrogen-enhanced dislocation structure evolution and subsequent microvoids formation along the grain boundaries. Additionally, the impact of such IG cracking on the macroscopic FCG rate was evaluated according to the dependency of IG fracture propensity on the hydrogen gas pressure. It was first demonstrated that the increased hydrogen pressure results in the larger area fraction of IG and corresponding faster FCG rate. Moreover, gaseous hydrogen environment also had a positive influence on the FCG rate due to the absence of oxygen and water vapor. The macroscopic crack propagation rate was controlled by the competition process of said positive and negative effects.

Temperature effects on the mechanism of time independent hydrogen assisted fatigue crack propagation in steels: Marrow, T.J., King, J.E. and Cotterill, P.J. Acta Metall. Mater. Aug. 1992 40, (8), 2059–2068

An inverse heat conduction problem in a system is solved using a non-integer identified model as the direct model for the estimation procedure. This method is efficient when some governing parameters of the heat transfer equations, such as thermal conductivity or thermal resistance, are not known precisely. Reliability of the inversion depends on the precision of the identified model. From considerations on the analytical solutions in simple cases and on the definition of non-integer (or fractional) derivative, the non-integer model appears to be the most adapted. However, some experiments do need to be carried out on the physical thermal system before it can be identified. An application that consists in estimating the heat flux in a turning tool insert during machining is presented. First, identification is performed using a specific apparatus that permits a simultaneous measurement of temperature and heat flux in the insert. Then, during machining, heat flux can be estimated from temperature using this identified model.

Temperature effects on the mechanism of time independent hydrogen assisted fatigue crack propagation in steels

The effects of temperature on hydrogen assisted fatigue crack propagation are investigated in three steels in the low-to-medium strength range; a low alloy structural steel, a super duplex stainless steel, and a super ferritic stainless steel. Significant enhancement of crack growth rates is observed in hydrogen gas at atmospheric pressure in all three materials. Failure occurs via a mechanism of time independent, transgranular, cyclic cleavage over a frequency range of 0.1–5 Hz. Increasing the temperature in hydrogen up to 80°C markedly reduces the degree of embrittlement in the structural and super ferritic steels. No such effect is observed in the duplex stainless steel until the temperature exceeds 120°C. The temperature response may be understood by considering the interaction between absorbed hydrogen and micro-structural traps, which are generated in the zone of intense plastic deformation ahead of the fatigue crack tip.

Effect of mean stress on hydrogen-assisted fatigue crack propagation in duplex stainless steel: Marrow, T.J., Hippsley, C.A. and King, J.E. Acta Metall. Mater. June 1991 39, (6), 1367–1376

The present paper attempts to generate fatigue crack growth data for RPV steels in a pressurised water reactor (PWR) environment at lower temperatures than those normally encountered at the PWR water outlet temperatures of around 290°C.Significant effects of frequency on the fatigue crack growth behaviour were observed in ambient PWR water at low and high R-ratio. The enhanced fatigue crack growth rates at low frequency were coincident with the appearance of significant amounts of isolated intergranular failure facets.At a PWR water temperature of 120°C, marked effects of frequency and R-ratio were recorded. Indeed lowering the frequency and increasing the R-ratio significantly increased fatigue crack growth rates as a result of environmental assisted crack (EAC) growth which was typified by the appearance of significant amounts of flat, fan-shaped EAC growth on the fatigue fracture surfaces.A powerful effect of material on the fatigue crack growth rates were evident in a PWR environment at 120°C inasmuch that the RPV A508 steel exhibited growth rates which, at a ΔK level of 30 MPa√m, were over an order of magnitude faster than those recorded for an A533B RPV steel. Such differences were explained in terms of non-metallic sulphide composition because while the A533B steel contained many more sulphide inclusions, over 80% of them were pure manganese sulphide inclusions while the A508 steel contained nearly 90% active sulphide inclusions which were manganese sulphides containing significant amounts of iron, chromium, aluminium and phosphorous and traces of sodium, potassium, zinc and nickel. Súch active sulphide inclusions, which were either completely or partially dissolved in PWR water at 120°C, produced surface active chemical species that promote EAC growth, while the predominantly pure manganese sulphides prevalent in the A533B steel were not attacked and remained undissolved.

The correlation between grain size and plastic zone size for environmental hydrogen assisted fatigue crack propagation

The correlation between grain size and plastic zone size for environmental hydrogen assisted fatigue crack propagation