The meaning of the thresholds of hydrogen-assisted cracking

We analyze the concept of threshold stress intensity factorK _HAC and its importance for the processes of hydrogen-assisted cracking (HAC). We discuss the self-similarity of the zone near the crack tip well-described by the stress intensity factorsK, indicate the ambiguities encountered in the determination ofK _HAC and the experimental parameters affecting the thresholds of hydrogen-assisted cracking parallel withK. It is shown thatK _HAC has the physical meaning of the lower bound of the stress-intensity factors corresponding to the initiation of hydrogen-assisted cracking (if all other conditions are arbitrary). To obtain reliable estimates ofK _HAC, one must impose additional restrictions on the experimental procedure of its evaluation.     

A new theory describing the hydrogen-assisted intergranular cracking of high-strength steels

A thermodynamic analysis of hydrogen-assisted intergranular brittle fracture of high-strength steels has been made. In this analysis the functional relationship between cohesive energy and hydrogen coverage is derived in the case of solute equilibrium constraint during the decohering process. This relationship is evaluated and discussed in the presence of a triaxial stress field. The variation of threshold-stress intensity, K _th, with hydrogen fugacity is calculated by a criterion for hydrogen-assisted intergranular fracture, and is also considered as it relates to the effects of several material parameters, such as trap-binding energy at a grain boundary, yield strength and work-hardening exponent. In particular the fracture mode transition by hydrogen-assisted cracking is discussed as it relates to the effects of hydrogen on the K _th necessary for the occurrence of the respective fracture modes.     

Hydrogen-assisted stress cracking of high-strength wheel bolts

Early failures occurred with two sizes of wheel bolts used for attaching front wheels and dual rear wheels to heavy truck hubs. Failure resulted in fracture of the bolts, and was a response to the material and process specification which produced a steel microstructure highly susceptible to hydrogen-assisted stress cracking. The microstructure resulted from an alloy steel that was carburized, heat treated, and then zinc plated. This combination of material and processing produced a high-strength SAE Grade 8 bolt (equivalent to an ISO 10.9 grade) with a hard, brittle case and an anodic zinc coating. A slight misalignment of the wheel bolt coupled with a ball seat mounting design for the wheel nuts created a combined axial and bending stress that exceeded the threshold for hydrogen-assisted stress cracking.     

Influence of alloying on hydrogen-assisted cracking and diffusible hydrogen content in Cr-Mo steel welds

Study of hydrogen-assisted cracking and measurement of diffusible hydrogen content in different Cr-Mo steel welds shows that under identical conditions, susceptibility to cracking increased and diffusible hydrogen content decrease with increase in alloy content. Hydrogen permeation studies show that hydrogen diffusivity decreases and solubility increases with increase in alloy content. Thus decrease in diffusible hydrogen content with increase in alloying is attributed to increase in apparent solubility and decrease in apparent diffusivity of hydrogen. Analysis of the results indicates that variation of diffusible hydrogen content and apparent diffusivity of hydrogen with alloy content can be represented as a function of carbon equivalent CE₁ originally proposed to predict the hardness in the heat-affected zone of alloy steel welds.     

A fractographic study of hydrogen-assisted cracking and liquid-metal embrittlement in nickel

Metallographic and fractographic studies of crack growth in nickel polycrystals and single crystals in a number of environments are described. Brittle intercrystalline and transcrystalline cleavage-like fractures were observed for specimens tested in liquid mercury, liquid lithium, liquid sodium, gaseous hydrogen, and for hydrogen-charged specimens tested in air. Brittle fractures were associated with considerable slip, and dimples/tear ridges were observed on fracture surfaces, suggesting that crack growth occurred by localized plastic flow. There were remarkable similarities between adsorption-induced liquid-metal embrittlement and hydrogen-assisted cracking which, along with other observations, suggested that adsorbed hydrogen at crack tips was responsible for hydrogen-assisted cracking. It is concluded that adsorbed atoms weaken interatomic bonds at crack tips thereby facilitating the nucleation of dislocations and promoting crack growth by localized plastic flow.     

Fracture mechanics approach to hydrogen assisted cracking: Analysis of theK-dominance condition

We summarize the achievements of a major research line at the Department of Materials Science of the University of La Coruña (Spain) in the field of environmentally-assisted cracking in general and hydrogen degradation in particular. We analyze the meaning and significance of the fracture-mechanics approach to hydrogen-assisted cracking and study the problem ofK-dominance not only over the mechanical aspects of the phenomenon but also over the environmental (physicochemical) factors affecting the entire coupled process of hydrogenation and failure. Two key factors capable of violating the unambiguous behavior of the kinetic diagram of crack growthv=v (K) are discussed: the role of the far stress-strain field (i.e., the stress-strain field which is notK-dominated) and the effect of the history of hydrogenation and crack growth. For this purpose, we consider the stress-strain-assisted diffusion of hydrogen regarded as a rate-controlling factor of hydrogen-assisted cracking under sustained or quasistatic loads. It is shown that the far field produces a minor effect on the near-tip diffusion of hydrogen. This can only increase the spread in crack growth rates in the near-threshold part of thev (K)-curve. As far as the influence of the history is concerned, we discovered that the processes of hydrogenation and crack growth are coupled, eachof them affects the other, and, hence, the kinetic diagram of crack growthv=v (K) is not one-to-one as should be for an intrinsic property of the material. However, there exists a special mode of steady-state crack growth when hydrogen-assisted cracking becomes aK-dominated process and the corresponding plot of the steady-state ratev as a function ofK becomes unambiguous (as a characteristic curve of the material) and, thus, can be used in the engineering practice for a more conservative evaluation of the crack-growth resistance of materials and structural integrity.     

Hydrogen-enhanced stress-corrosion cracking of aluminum alloys

We present the results of the investigation of Al-Zn-Mg and Al-Zn-Cu-Mg alloys in NaCl solutions at low strain rates. The contribution of hydrogen to the process of stress-corrosion cracking is analyzed by taking into account the influence of the admixtures of arsenic trioxide and residual hydrogen (remaining after the processes of release and cathodic polarization) on the susceptibility of metals to this kind of cracking. A mechanism of hydrogen-assisted stress-corrosion cracking taking into account the time dependence of the microstructure of grain boundaries, concentration of hydrogen, and its distribution is suggested on the basis of the concept of critical concentration of hydrogen. Technical University of Gdansk, Poland. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 4, pp. 20–26, July–August, 1998.     

Fractographic and numerical study of hydrogen–plasticity interactions near a crack tip

This paper offers a fractographic and numerical study of hydrogen–plasticity interactions in the vicinity of a crack tip in a high-strength pearlitic steel subjected to previous cyclic (fatigue) precracking and posterior hydrogen-assisted cracking (HAC) under rising (monotonic) loading conditions. Experiments demonstrate that heavier cyclic preloading improves the HAC behaviour of the steel. Fractographic analysis shows that the microdamage produced by hydrogen is detectable through a specific microscopic topography: tearing topography surface or TTS. A high resolution numerical modelling is performed to reveal the elastoplastic stress–strain field in the vicinity of the crack tip subjected to cyclic preloading and subsequent monotonic loading up to the fracture instant in the HAC tests, and the calculated plastic zone extent is compared with the hydrogen-assisted microdamage region (TTS). Results demonstrate that the TTS depth has no relation with the active plastic zone dimension, i.e., with the size of the only region in which there is dislocation movement, so hydrogen transport cannot be attributed to dislocation dragging, but rather to random-walk lattice diffusion. It is, however, stress-assisted diffusion in which the hydrostatic stress field plays a relevant role. The beneficial effect of crack-tip plastic straining on HAC behaviour might be produced by the delay of hydrogen entry caused by residual compressive stresses and by the enhanced trapping of hydrogen as a consequence of the increase of dislocation density after cyclic plastic straining.     

Hydrogen-assisted cracking studies of 4340 steel by using the optical method of caustics

The optical method of caustics is used to measure the effect of hydrogen on the localized deformation within the fracture process zone (PZ). In particular, the role of the PZ in hydrogen-assisted cracking (HAC) of 4340 steel in hydrogen gas at 1 atm is examined. No change in the caustic diameter was detected prior to crack initiation but an “anomalous” enlargement of the caustic diameter as a function of crack growth was observed as a result of crack tunneling. The “center” of the caustic follows the position of the internal crack front, thus providing a simple means to monitor the growth of internal cracks. It is demonstrated that the technique provides a sensitive means for detecting HAC initiation (the smallest crack depth change detected is 35 μm) and insight into the three-dimensional character of the PZ.     

Hydrogen embrittlement of a bimaterial

Commonly, within the energy industry, the corrosion resistance of pressure vessel steels is increased by the addition of an overlay coating comprising a nickel-based alloy or a stainless steel. However, the interface between the two alloys is prone to hydrogen-assisted cracking, due to for example carbide precipitation near the interface. In the present study, the sensitivity of the tensile strength of the interface to hydrogen concentration is measured for both notched and un-notched specimens made from the overlay welding of 690 nickel alloy on a low alloy steel A533B. An elastic–plastic finite element analysis is used to determine the stress and strain state near the notch root, and thereby to calculate the local distribution of hydrogen within the lattice and at traps. The observed strength of the notched specimens is best rationalised by assuming that the local cohesive strength of the interface is a function of the lattice hydrogen concentration, with a negligible influence of the trapped hydrogen. The scatter in specimen strength, and the relative strength of the notched and un-notched specimens, are adequately described by Weibull statistics, with a low value of Weibull modulus equal to 3.4.     

Burnthrough prediction in pipeline welding

Welding of branch connections on gas pipelines at full line pressure is frequently an operational necessity. Weld parameters must be selected so that heat inputs are low enough to avoid burnthrough yet not so low that hydrogen-assisted cold cracking occurs. Current techniques rely on the observation that burnthrough does not occur if peak temperatures on the inner surface are kept below 980^°C. At these temperatures, rate-dependent flow is the dominant mechanism. The problem is one of creep rupture occurring at temperatures in excess of 980^°C with times of the order of seconds. Material constitutive models for the analysis of welding must include both rate-dependent and rate-independent plastic flow as well as the effects of phase transformations. Material properties at elevated temperatures are usually not available for pipeline steels and must be extrapolated from values at lower temperatures. An exploratory study using 3D thermal-mechanical finite element analysis of welding on pressurized vessels is presented and includes comparisons with experiment. The agreement is encouraging. The material failure does occur in appropriate locations but the predictions are generally overconservative. Estimated material properties, especially damage and rupture properties at high temperatures could be improved. This revised version was published online in July 2006 with corrections to the Cover Date.     

Role of Microstructural Anisotropy in the Hydrogen-Assisted Fracture of Pearlitic Steel Notched Bars

This paper analyzes the role of microstructural anisotropy generated by cold drawing in the hydrogen-assisted fracture of progressively drawn pearlitic steel notched bars. Very different notched geometries were used to generate quite distinct stress triaxiality distributions (and thus very different constraint levels) in the vicinity of the notch tip, and fracture surfaces were classified in accordance with three micromechanical models of hydrogen-assisted micro-damage on the basis of the micro-fracture maps assembled by scanning electron microscopy over the fractured area of the notched specimens.     

The effect of hydrogen induced cracking on the integrity of steel components

The occurrence of hydrogen embrittlement is a much researched phenomenon, known to cause mechanical property degradation and catastrophic failures. The ductility loss brought about by hydrogen ingress is encountered even in unstressed bodies where such cracking is termed hydrogen induced cracking (HIC) and is in phenomenological contrast to catastrophic failures encountered by stressed bodies subjected to hydrogen producing environments. This article will discuss HIC in some detail. This form of cracking is especially detrimental and often observed in oil country tubular goods (OCTG) which are subjected to sour gas. Consequently, the significance of HIC is most appreciated by oil companies at various stages of oil extraction, transportation and storage. In this article the chemical and metallurgical genesis of HIC, its harmful impact on material and component integrity are discussed. It has been noted that MnS inclusions are extremely harmful to this form of cracking. Similarly, centreline segregation in the ingot stage and deoxidation practices during steelmaking were found to affect HIC. Some case studies of HIC obtained from literature are presented. The variables affecting the propensity to HIC are provided in brief. Suitable measures to reduce or eliminate HIC in steels are also discussed.     

Ductility of thin copper films on rough polymer substrates

Electronic components in modern flexible electronics are connected by interconnects, which typically have the form of metal films resting on polymer substrates. This paper firstly studies experimentally the ductility of Cu films deposited on polyimide substrate with roughened surface (due to sandblasting) and finds that, upon tensile loading along the direction of film surface, the density of surface cracks in the film decreases with increasing surface roughness. The method of finite elements is subsequently employed to study the distribution of tensile stresses in the film and their influence on film cracking (initiation and propagation). It is demonstrated that a rough (curved) interface can reduce the tensile stresses along the film surface so as to restrain channel cracking of the film. Finally, the cohesive zone model is used to study the initiation and spreading of damage in the film and interfacial debonding of the curved interface. Both the interfacial damage and interface crack length are reduced as a result of interface roughening.     

Creep and shrinkage analysis of composite systems under axial load and biaxial bending

The instantaneous and creep and shrinkage behaviour of composite steel-concrete sections subjected to axial load and biaxial bending is examined analytically using the age-adjusted effective modulus method and a stress relaxation procedure. Any cross-section that can be discretized in rectangular elements can be analysed using the approach described in this study. Material non-linearities (concrete cracking and crushing as well as steel yielding) are included in the formulation by empolying an interative numerical procedure. The analytical method was implemented in a computer program and case studies demonstrate that composite columns under complex loads experience a considerable redistribution of stresses and strains, causing the degree of cracking and the secondary load effects (P-delta) to increase.     

Corrosion-assisted cracking in progressively drawn pearlitic steels: A materials science approach

This is a summary of the achievements of a major research line at the Materials Science Department of the University of La Coruña (Spain) in the field of environmentally assisted cracking in general and hydrogen degradation in particular. It deals with a materials science approach to the study of corrosion-assisted cracking of progressively drawn pearlitic steels for use in civil engineering. The approach is based on the fundamental idea of materials science of linking the microstructure of different steels (progressively oriented as a consequence of the process of manufacturing by cold drawing) with their macroscopic stress-corrosion behavior (increasingly anisotropic as the degree of cold drawing increases). In the first part of the research, we performed metallographic analysis and gave special attention to the evolution with cold drawing of the two basic microstructural levels: pearlite colonies (first level) and pearlitic lamellae (second level). For the first microstructural level, we observed progressive eleongation and orientation of the pearlite colonies in the direction of cold drawing (axis of the wire). For the second microstructural level, the analysis revealed an increase in the closeness of packing (with a decrease in the interlamellar spacing) and progressive orientation of the pearlitic lamellar microstructure in the direction of cold drawing. Therefore, in the process of cold drawing, both the pearlite colonies and pearlitic lamellar microstructure tend to align in a direction quasiparallel to the axis of the wire. In the second part of the research, we carried out a program of stress-corrosion-cracking tests under both cathodic and anodic conditions to promote two very different mechanisms of cracking: hydrogen-assisted cracking (HAC) and localized anodic dissolution (LAD). Both types of stress corrosion tests confirm that steels subjected to cold drawing exhibit anisotropic behavior connected with clear changes in the direction of crack propagation, which approaches the axis of the wire or the direction of cold drawing. For both mechanisms (HAC and LAD), there is a strong correlation between the angles of microstructural orientation (on the levels of pearlitic colonies and lamellae) and the angles of propagation of macroscopic cracks, which clearly demonstrates the influence of the oriented microstructure (and, thus, of the process of manufacturing by increasing cold drawing) on the macroscopic corrosion-assisted behavior of steel wires.     

Evaluation of hydrogen effect on the fatigue crack growth behavior of medium-Mn steels via in-situ hydrogen plasma charging in an environmental scanning electron microscope

Fatigue crack growth(FCG)tests were conducted on a medium-Mn steel annealed at two intercritical annealing temperatures,resulting in different austenite(γ)to ferrite(α)phase fractions and different γ(meta-)stabilities.Novel in-situ hydrogen plasma charging was combined with in-situ cyclic loading in an environmental scanning electron microscope(ESEM).The in-situ hydrogen plasma charging increased the fatigue crack growth rate(FCGR)by up to two times in comparison with the reference tests in vacuum.Fractographic investigations showed a brittle-like crack growth or boundary cracking manner in the hydrogen environment while a ductile transgranular manner in vacuum.For both materials,the plastic deformation zone showed a reduced size along the hydrogen-influenced fracture path in comparison with that in vacuum.The difference in the hydrogen-assisted FCG of the medium-Mn steel with different microstructures was explained in terms of phase fraction,phase stability,yielding strength and hydrogen distribution.This refined study can help to understand the FCG mechanism without or with hydrogen under in-situ hydrogen charging conditions and can provide some insights from the applications point of view.     

Frequent Failures of Motor Shaft in Seawater Desalination Plant: Some Case Studies

The failure of a shaft from a motor in a pump or a compressor has been a phenomenon of common occurrence in seawater desalination plants. The origin of the problem in majority of cases is either the inability of the material to withstand the level of dynamic stresses to which shaft is subjected during operation and/or inadequacy of the design. The shortcoming in the design may be responsible for initiating localized corrosion which ultimately leads to failure of the component. The mode of failure of the shaft could be stress-related failure such as stress corrosion cracking, mechanical fatigue or corrosion fatigue, and/or localized corrosion such as crevice corrosion. This paper describes some recent case studies related to shaft failures in seawater desalination plants. The case studies include shearing of a shaft in brine recycle pump in which a combination of environment, design, and stresses played important role in failure. In another case, ingress of chloride inside the key slot was the main cause of the problem. The failure in a high pressure seawater pump in a SWRO plant occurred due to cracking in the middle of the shaft.     

Random hydrogen-assisted fatigue crack growth in steel plates

A stochastic analysis of hydrogen-assisted fatigue crack growth in steel plates is presented. First, a simplified deterministic model of the process is proposed. It captures the basic empirical property that the influence of hydrogen diminishes, as the crack growth rate increases. However, it only applies to cases, when diffusion is rate limiting. Next, the model parameters are randomized to reflect the uncertainty inherent in the physical situation. On the basis of the obtained stochastic equation, probabilistic moments of the time, in which the crack reaches a critical length, are computed. Theoretical results are illustrated by a numerical example.     

Micromechanisms of damage in unidirectional fiber reinforced composites: 3D computational analysis

Numerical micromechanical investigations of the mechanical behavior and damage evolution of glass fiber reinforced composites are presented. A program code for the automatic generation of 3D micromechanical unit cell models of composites with damageable elements is developed, and used in the numerical experiments. The effect of the statistical variability of fiber strengths, viscosity of the polymer matrix as well as the interaction between the damage processes in matrix, fibers and interface are investigated numerically. It is demonstrated that fibers with constant strength ensure higher strength of a composite at the pre-critical load, while the fibers with randomly distributed strengths lead to the higher strength of the composite at post-critical loads. In the case of randomly distributed fiber strengths, the damage growth in fibers seems to be almost independent from the crack length in matrix, while the influence of matrix cracks on the beginning of fiber cracking is clearly seen for the case of the constant fiber strength. Competition between the matrix cracking and interface debonding was observed in the simulations: in the areas with intensive interface cracking, both fiber fracture and the matrix cracking are delayed. Reversely, in the area, where a long matrix crack is formed, the fiber cracking does not lead to the interface damage.