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The model describing the corrosion-mechanical fracture of the underground gas pipeline with semi-elliptical external surface crack is developed taking into account the intensification of crack growth in the pipe steel 17H1S by diffusible hydrogen. The model is grounded on the energy approach to fracture combined with the hydrogen accelerated soil corrosion cracking mechanism. The formula for the soil corrosion rate is derived as the sum of two components: the rate of regular soil corrosion due to contact of steel with the soil, and the term characterizing its acceleration by hydrogen. Corresponding mathematical model (differential equation with initial and final conditions) is built up to determine the residual lifetime of a pipe of gas pipeline subjected to hydrogenation from the transported gas, soil corrosion, long term sustained and transient loadings caused, respectively, by gas pressure in the pipe and by start/stop valve operations. As a result of model implementation, the pipe residual lifetimes are evaluated considering pipe hydrogenation from the inner surface, soil corrosion at the outer surface as well as transient loading. From the analysis of results, it is concluded that pipe wall hydrogenation, as well as transient loading, lead to significant acceleration of corrosion-mechanical crack growth in the pipe, and thus, reduce its residual lifetime.

The calculation model is developed to evaluate the residual lifetime of a pipe with a crack considering in-service degradation of the pipeline steel and the effect of soil corrosion. The model is based on the first law of thermodynamics. The problem of determination of the period of subcritical crack growth in a pipe is reduced to the differential equation subjected to definite initial and final conditions. Obtained equation together with the boundary conditions are used for prediction of the residual lifetime of the pipe made of the X52 pipe steel. The pipe contains external surface semi-elliptical crack. The task consists in the determination of the time of crack propagation through the pipe wall till its decompression. According to the experimental data, cracks in the studied steel under long-term loading and soil corrosion propagate mainly at constant rate, which is different for the as-received pipeline steel and the steel after 30 years in service. Based on these data, the equation for approximate determination of the crack growth rate for arbitrary service time of the pipe made of the X52 steel is proposed. To evaluate the crack propagation time, the energy approach is employed. For a crack that remains semi-elliptical during propagation, the problem is reduced to the differential equation system that determines the variations of the crack semi-axes. The system solution under definite initial and final conditions renders the expression for the pipe residual lifetime. The dependences of pipe residual lifetime on initial crack depth and pipe operation time are derived. The strong influence of the time of steel previous service on the pipe residual lifetime is manifested.

Generalised continuum model of hydrogen transport to fracture loci is developed for the purposes of analysis of the hydrogenous environment assisted fracture (HEAF). The model combines the notions of the theories of gas flow, surface science, and diffusion and trapping in stressed solids. Derived flux and balance equations describe the species migration across different states (gas, adsorbed specie at the gas-metal interface, interstitial solute in metal bulk) and a variety of corresponding sites of energy minimums along the potential relief for hydrogen in a system. The model accounts for the local kinetics of hydrogen interchange between the closest dissimilar neighbour sites and for the nonlocal interaction of hydrogen trapping in definite positions with the species wandering in their farer surroundings. In particular situations, certain balance equations of the model may degenerate into equilibrium constraints, as well as some terms in the generalised equations may be insignificant. A series of known theories of hydrogen transport in material-environment system can be recovered then as particular limit cases of the generalised model. Presented theory can help clarifying the advantages and limitations of particularised models so that appropriate one may be chosen for the analysis of a particular HEAF case.

Continuum modelling of hydrogen diffusion in metals, which accounts for both trapping and an imposed force field, is revisited. A generalised model of hydrogen diffusion and trapping is developed as a continuous interpretation of the discrete random-walk theory. A system of nonlinear equations describing the phenomenon of diffusion with multiple types of traps is derived without the assumption of a local equilibrium among hydrogen populations in dissimilar positions. Lattice-trap interchange kinetics can degenerate into local equilibrium as a limit case. Moreover, certain terms in general equations may be negligible in specific situations. By removing these terms, known particularised models of hydrogen diffusion and trapping are recovered. Determining the terms, which are disregarded in reduced models, enables a straightforward assessment of the applicability of these models. The advantages and limitations of particularised models applied to hydrogen embrittlement analyses are discussed.

The present paper offers a continuum modelling of trap-affected hydrogen diffusion in metals and alloys, accounting for different physical variables of both macroscopic nature (i.e., related to continuum mechanics, e.g., stress and strain) and microscopic characteristics (material microstructure, traps, etc.). To this end, the model of hydrogen diffusion assisted by the gradients of both hydrostatic stress and cumulative plastic strain,stress-and-strain assisted hydrogen diffusion, proposed and frequently used by the authors of the present paper (Toribio & Kharin) is analysed in addition to other well-known models such as those proposed by (i) McNabb & Foster, (ii) Oriani, (iii) Leblond & Dubois, (iv) Sofronis & McMeeking, (v) Krom and Bakker, showing their physical and mathematical differences and similarities to account for different physical variables.

Hydrogen diffusion within a metal or alloy is conditioned by the stress-strain state therein. For that reason it is feasible to consider that hydrogen diffuses in the material obeying a Fick type diffusion law including an additional term to account for the effect of the stress state represented by the hydrostatic stress. In this paper the hydrogen transport by diffusion in metals is modelled in notched specimens where loading generates a triaxiality stress state. To this end, two different approaches of stress-assisted hydrogen diffusion, one-dimensional (1D) and two-dimensional (2D), were compared in the vicinity of the notch tip in four notched specimens with diverse triaxiality level at two different loading rates. The obtained results show that the 2D approach predicts lower values of hydrogen concentration than the 1D approach, so that a loss of directionality of hydrogen diffusion, depending on both notch geometry parameters (radius and depth) and loading rate, appears when a 2D approach is considered.

This work analyses the effects of hydrogen-assisted rolling-contact fatigue on bearings of wind turbines. To this end, the well-known ball-on-rod test to evaluate the resistance to rolling-contact fatigue was simulated by the finite element method for obtaining the stress-strain state undergone by the bearings considering their in-service conditions. Results of this paper allow a simple estimation of the hydrogen amount in potential damage zones of the specimen.

Residual stress and strain states produced by wire drawing play an essential role in the main cause of failure of cold drawn wires: hydrogen embrittlement (HE), because of the influence of such fields on hydrogen diffusion within the material lattice. Therefore, variations on stress and strain fields, due to changes in the wire drawing process conditions, could modify the service life of these structural components. In this work the influence on HE of two parameters of the wire drawing process (the inlet die angle and the die bearing length) are analyzed by means of diverse numerical simulations by the finite element method (FEM). According to the obtained results, the effects of residual stress and strain fields produced by wire drawing on HE are less dangerous when the inlet die angle decreases or when the bearing length exceeds a characteristic value (wire radius), with a remarkable reduction of the driving forces for hydrogen diffusion. Consequently, wires drawn under such conditions (lower inlet die angle and longer bearing length) will exhibit a lower susceptibility to HE, thereby increasing their resistance to engineering failure.

Prestressing steels, obtained by cold drawing, are highly susceptible to hydrogen embrittlement (HE) phenomena. Stress and strain fields produced by cold drawing play an essential role in this process since they affect hydrogen diffusion. Therefore, variations of such fields due to changes in drawing conditions could modify life in-service of these structural components. In this work the effect on HE of a parameter of the wire drawing process, thebearing length, is analyzed by means of diverse numerical simulations by the finite element method (FEM). The results of this work allow the definition of acharacteristic valueof the die bearing length equal to the wire radius, and demonstrate that the effects of stress-strain fields produced by wire drawing on HE are reduced when the bearing length exceeds such a characteristic value, so that the optimum cold drawing process is that with a bearing length higher than the wire radius.

Finite-deformation elastoplastic analysis of plane-strain crack subjected to mode I cyclic loading with constant amplitude at various load ranges and ratios, as well as with solitary over- and under-load cycles, is presented. The Laird-Smith mechanism of crack advance by cyclic blunting and re-sharpening, which transfers material from the crack tip towards its flanks, is visualised. In the present modelling, crack closure has never been detected. As well, the supposed origin of plasticity induced crack closure (PICC), which is habitually attributed to filling-in a crack with material stretched out of the crack plane behind the tip, is ruled out. Nevertheless, the rate of simulated fatigue crack growth (FCG) by blunting–re-sharpening reproduces the key experimental trends concerning the effects of ΔK and single over-/under-load peaks. Calculated compliance curves are bent in spite of the absence of PICC, which raises the doubt about their trustworthiness as the means of the crack closure detection and assessment. This way, performed modelling manifests ambiguities concerning PICC as the universal intrinsic factor able to control FCG ubiquitously. On the other hand, having PICC absent, calculated near-tip stress–strain responses, being driving forces of fatigue damage and crack advance by bond-breaking, manifest affinities with the experimental FCG trends without intervention of PICC, too. This implicates both independent parameters of crack-tip cyclic loading under small scale yielding, such as the couple Kmax and Kmin, or Kmax and ΔK, or other equivalent one, as the indispensable variables that drive FCG directly without mediation of PICC.

Hydrogen embrittlement (HE) of prestressing steel is a major technological problem in damage tolerance and structural integrity of structures in harsh environments, it being strongly influenced by residual stresses and plastic strains after cold-drawing. This paper calculates the distributions of such variables ( mechanical approach ) and later the stress-and-strain assisted diffusion of hydrogen ( chemical approach ) in a real manufacturing process by multi-step cold drawing. Results confirm the relevant role of plastic strain in HE of prestressing steel and thus the necessity of considering it in hydrogen diffusion calculations, because stress-only assisted diffusion leads to dramatic (and non conservative) underestimation of the hydrogen content.

Numerical analyses of plasticity-induced fatigue crack growth (FCG) were performed for high-strength steels. FCG proceeds by large crack tip deformations promoting transfer of material from the tip apex to the crack flanks, thus rendering plastic crack growth based on plastic deformations without bond breaking or material splitting at the crack tip (Laird–Smith mechanism of FCG by blunting and re-sharpening). Plastic FCG rates (d a/ d N ) p were obtained and numerical results were fitted with a Paris-like equation (d a/ d N ) p = C Δ K m . An experimental validation of the simulations was performed by comparing the numerical Paris-like equation with real (experimental) Paris equations in the steels.

The premature contact of crack surfaces attributable to the near-tip plastic strains under cyclic loading, which is commonly referred to as plasticity-induced crack closure (PICC), has long been focused as supposedly controlling factor of fatigue crack growth (FCG). Nevertheless, when the plane-strain near-tip constraint is approached, PICC lacks of straightforward evidence, so that its significance in FCG, and even the very existence, remain debatable. To add insight into this matter, large-deformation elastoplastic simulations of plane-strain crack under constant amplitude load cycling at different load ranges and ratios, as well as with an overload, have been carried out. Performed modelling visualises the Laird-Smith conceptual mechanism of FCG by crack-tip blunting and re-sharpening. The simulations reproduce experimental trends of FCG concerning the roles of ΔK and overload, whilst PICC has not occurred yet. Near-tip deformation patterns discard the filling-in a crack with material stretched out of its plane in the wake behind the tip as the origin of PICC. Despite the absence of closure, load-deformation curves appear bent, which raises doubts about the trustworthiness of closure assessment from the compliance variation. This points out the ambiguities of PICC as a supposedly ubiquitous factor that controls FCG.

Cold drawn wires of eutectoid pearlitic steel are widely used for prestressing concrete structures that usually work in harsh environments, so that stress corrosion cracking of prestressing steel is a problem of major technological concern. In this paper a previous model developed by the authors is applied to analyse the influence of the residual stress-and-strain profiles after cold drawing on the hydrogen embrittlement susceptibility of prestressing steel wires. Numerical results show the relevant role of both residual stress and strain field in hydrogen diffusion in this material

This paper analyzes the evolution of Vickers micro-hardness in progressively drawn eutectoid steels focussing the study on the characteristic sections of the wires. The average value of the Vickers micro-hardness was calculated for each section according to its symmetry, thus studying the variation of these values with cold drawing. The indentation marks after the tests demonstrate the anisotropy caused by the cold drawing process itself on the eutectoid steel, and a finite element modelling allows one to finish material characterization.

The present work is based on previous research on the one-dimensional (1D) analysis of the hydrogen diffusion process, and proposes a numerical approach of the same phenomenon in two-dimensional (2D) situations, e.g. notches. The weighted residual method was used to solve numerically the differential equations set out when the geometry was discretized through the application of the finite element method. Three-node triangular elements were used in the discretization, due to its simplicity, and a numerical algorithm was numerically implemented to obtain the hydrogen concentration distribution in the material at different time increments. The model is a powerful tool to analyze hydrogen embrittlement phenomena in structural materials.

The paper presents an analysis of the failure in service of a lifting platform used for tree pruning. Different fracture mechanics techniques were used to reveal the causes of failure such as analysis of the fracture surface, mechanical and microstructural characterization of the material and fracture mechanics tests to determine the critical value of the stress intensity factor and to characterize the subcritical crack growth under fatigue. Results yield do conclude that the platform failure cause was the subcritical crack growth in the welded joint where the welding was discontinuous, thereby producing a stress concentration effect similar to that of a crack.

High-resolution elastoplastic simulations of a plane-strain tensile crack under cyclic loading visualised the Laird-Smith mechanism of fatigue crack growth, which manifested the effects of K and overload, but no signs of crack closure. Simulations discarded the supposed mechanism of plasticity-induced crack closure (PICC) via out-of-plane stretching of material elements filling-in the crack and coming in contact at unloading. Specimen compliance curves turned out to be nonlinear merely by virtue of plasticity with no contribution of crack closure. Thus the results manifested the ambiguity of main justifications about the phenomenon of PICC.

The plane-strain crack subjected to mode I cyclic loading under small scale yielding was analysed. The influence of the load range, load ratio and overload on the near-tip deformation-, stress- and strain-fields was studied. Although the near-tip zones of appreciable cyclic plastic flow for all loading regimes matched closely one another, when scaled with (ΔK/σY)2, the activities of plastic flow within them manifested dependence on Kmax and Kmin, as well as on overload. Cyclic trajectories of the crack-tip opening displacement (CTOD) converged to stable self-similar loops of the sizes proportional to ΔK2 and positions in CTOD-K plane dependent on the maximum K along the whole loading route, including an overload. Computed near-tip deformation evidenced plastic crack advance, this way visualising of the Laird–Smith concept of fatigue cracking. This crack growth by blunting-resharpening accelerated with rising ΔK and was halted by an overload. Crack closure upon unloading had no place. The affinities were revealed between computed near-tip stress–strain variables and the experimental trends of the fatigue crack growth rate, such as its dependence on Kmax and Kmin (or ΔK and Kmax), and retardation by overload. Thus, the effects of loading parameters on fatigue cracking, hitherto associated with crack closure, are attributable to the stress–strain fields in front of it as the direct drives of the key fatigue constituents – damage accumulation and bond breaking.

Hydrogen embrittlement plays an important role in environmental cracking of prestressing steel wires. The ammonium thiocyanate test (ATT) was implemented as a standard for determination of their susceptibility to environmentally assisted fracture. However, ATT reveals neither how hydrogen induced fracture (HIF) goes on in steel nor the roles of important manufacturing and service factors. To this end, the knowledge of the residual stresses and plastic strains in wires due to cold drawing, as well as the hydrogenation from harsh environments, are the keys to successful predictions of wire lives. This paper advances previous analyses of HIF in cold drawn prestressing wires via numerical modelling, firstly, of the cold drawing process to obtain the distributions of residual stresses and plastic strains, and secondly, of the stress–strain assisted hydrogen diffusion in wires towards creation the conditions for HIF nucleation. Generated results prove the relevant role of residual stress-and-strain field in hydrogen diffusion in the wires, as well as their possible consequences for HIF.

Finite-deformation elastoplastic analysis of a crack subjected to mode I cyclic loading under small scale yielding was performed. The influence of the load range, load ratio and overload on the crack tip deformations is presented. Cyclic crack tip opening displacements agreed with predictions of simpler models, where available. Crack closure was not detected. Plastic crack advance was evidenced. Its rate per cycle reproduced common trends of the fatigue cracking dependence on loading range and overload.

We analyze the influence of the distributions of residual stresses and strains formed after the surface treatment of cold-drawn steel wires used as the reinforcement of prestressed reinforced concrete on the susceptibility of these wires to hydrogen embrittlement characterized by the time to fracture in the tests carried out according to the regulations of the FIP (Federation Internationale de la Precontrainte). A computational model is proposed for the prediction of the durability of wires in corrosive hydrogenated media. In the model, the accumulation of hydrogen in potential sites of fracture as a result of diffusion under the influence of stresses and plastic strains is analyzed and a criterion of critical concentration of hydrogen specifying the time of local fracture depending on the stress-strain state is formulated. The enables us to predict the influence of specific features of the distributions of residual stresses and plastic strains after various types of thermal treatment on the durability of wires under the conditions of hydrogen embrittlement.

We present the results of numerical analysis of the influence of the mode of preliminary cyclic loading on hydrogen-induced crack growth in high-strength steel with regard for the stress-strain state and diffusion of hydrogen. The elastoplastic stress-strain state at the crack tip under cyclic loading followed by monotonic loading is simulated for the case of high strains. The diffusion of hydrogen at the crack tip is modeled by taking into account the evolution of the stress-strain state from postcyclic compression to tension in the course of hydrogenation. It is shown that the accumulation of hydrogen in the process zone depends on the distribution of residual stresses induced by cyclic preloading.

Stress corrosion cracking (SCC) of metals is an issue of major concern in engineering since this phenomenon causes many catastrophic failures of structural components in aggressive environments. SCC is even more harmful under cathodic conditions promoting the phenomenon known as hydrogen assisted cracking (HAC), hydrogen assisted fracture (HAF) or hydrogen embrittlement (HE). A common way to assess the susceptibility of a given material to HAC, HAF or HE is to subject a cracked rod to a constant extension rate tension (CERT) test until it fractures in this harsh environment. This paper analyzes the influence of a residual stress field generated by fatigue precracking on the sample’s posterior susceptibility to HAC. To achieve this goal, numerical simulations were carried out of hydrogen diffusion assisted by the stress field. Firstly, a mechanical simulation of the fatigue precracking was developed for revealing the residual stress field after diverse cyclic loading scenarios and posterior stress field evolution during CERT loading. Afterwards, a simulation of hydrogen diffusion assisted by stress was carried out considering the residual stresses after fatigue and the superposed rising stresses caused by CERT loading. Results reveal the key role of the residual stress field after fatigue precracking in the HAC phenomena in cracked steel rods as well as the beneficial effect of compressive residual stress.

Offshore wind parks at locations further from the shore often involve serious difficulties, e.g. the maintenance. The bearings of offshore wind turbines are prone to suffer hydrogen-assisted rolling-contact fatigue (HA-RCF). Three important aspects linked with bearing failures are being extensively researched: (i) rolling contact fatigue (RCF), (ii) influence of carbide particles on fatigue life, and (iii) local microplastic strain accumulation via ratcheting. However, there is no reference related to bearing failure in harsh environment. This way, this paper helps to gain a better understanding of the influence of hydrogen on the service life of offshore wind turbine bearings through a numerical study. So, the widely used RCF ball-on-rod test was simulated by finite element method in order to obtain the stress-strain state inside the bearings during life in service and, from this, to elucidate the potential places where the hydrogen could be more harmful and, therefore, where the bearing material should be improved.

Analysis of stress corrosion cracking (SCC) is performed with emphasis on the validity of the fracture mechanics approach, whose keystone is the crack growth kinetics curve, i.e., the plot "crack growth rate v vs. stress intensity factor K", as an intrinsic characteristic of a system {material—environment}. The uniqueness of the v(K)-curve as an attribute of a given material-environment couple forms the backbone of the approach and guarantees the soundness of the applications. However, ample experimental evidence does exist of the nonuniqueness of the v(K)-curves. Apart from K, crack growth rate depends on a family of variables related to the pre-SCC loading history, geometry and testing/service routine. The origins of the uncertainty of the fracture mechanics characterisation of SCC are analysed in general terms. In parallel, hydrogen assisted cracking is addressed in explicit terms as an important particular case. Two kinds of factors responsible for the essential weakness of the fracture mechanics approach to SCC are revealed: (1) crack tip plasticity which suppress the K-dominance of the near tip stress-strain field depending on the history of crack formation/extension even under small scale yielding within an elastic K-controlled crack tip zone; (2) time-dependent reactions between the material and the environment which affect the tip shape and nearby plasticity. Suggestions to consolidate the customary fracture mechanics approach are outlined. A rigourous treatment of SCC in terms of local values of governing parameters just at the crack tip is emphasised. In addition, a safe approach is recommended for design against SCC based on the idea of "the worst crack tip state" intrinsic for each material-environment system.

Prestressing steel wires are manufactured from a hot-rolled bar, which is heavily cold drawn to produce a highly resistant material. This manufacturing process generates very intense plastic deformations in the material and causes severe changes in its pearlitic microstructure, thus leading to anisotropic stress corrosion behavior in the form of environmentally assisted longitudinal splitting and, thus, mixed-mode stress corrosion cracking. This work describes experimental evidence of mixed-mode hydrogen-assisted cracking of high-strength steel and discusses the role of cyclic load history, because fatigue precracking is a fundamental technique of crack generation for posterior stress corrosion testing, and it has been reported that fatigue preloading may substantially alter the results from stress corrosion cracking tests, especially in the case of hydrogen-assisted cracking.

Analysis of hydrogen assisted cracking is performed with emphasis on the validity of the linear elastic fracture mechanics approach. Its soundness is ensured by the uniqueness of the crack growth kinetics curve `crack growth rate v versus stress intensity factor K' for a material–environment system. The ability of this relationship to match the similitude of crack tip events is revised for the whole chain of process components: `hydrogen supply to metal—entry into and accumulation in the process zone—degradation and crack advance'. Some of the factors able to violate the uniqueness of the `material's' v(K)-curve are discussed: (i) the role of far field in hydrogen transportation to the fracture zone; (ii) near tip environmental interactions and process zone hydrogenation dependent on the cracking process history; (iii) mechanical nonlinear effects caused by crack tip plasticity which may be significant even under small-scale yielding. Suggestions for a refinement of the customary fracture mechanics approach are given.

— This paper analyses the validity of the fracture mechanics approach to hydrogen assisted cracking in metals on the basis of K-dominance over both the stress-strain state and the accumulation of hydrogen in the fracture process zone. Stress-strain assisted diffusion of hydrogen is considered as the rate controlling factor of hydrogen assisted cracking under sustained or quasi-static loading conditions. The discrepancy in the crack tip zone between the far-field affected diffusion and the asymptotically driven (K-controlled) process is elucidated. The far field (i.e., the stress-strain field which is not K-dominated) is shown to have a minor effect on near-tip hydrogen diffusion. It can only widen the scatter band of crack growth rates in the near-threshold portion of the crack growth kinetics curve.

This paper analyzes the effect of process history on K-dominance condition in hydrogen assisted cracking. The performed study of stress-strain assisted diffusion reveals that hydrogenation of the fracture process zone is not a purely K-driven local autonomous phenomenon. Hydrogenation and crack growth are shown to be coupled processes, one influencing the other. Consequently, particular histories of stress intensity factor K and crack size evolutions affect crack growth rate ν that can occur at the same instantaneous K-value in a given material. Thus the crack growth kinetics curve ν=ν(K) is not unique as an intrinsic material property must be.

The analysis results show that in the presence of hydrogen the nonstationary nature of the thermal effects (heat changes, thermal cycles) in the presence of heterogeneities may on its own be a factor causing a high risk of hydrogen degradation and may be of the implicit type (hidden). The degree of risk depends, in particular, on the kinetics of thermal processes. These defects must be taken into account by estimating the properties of materials after the effect of hydrogen and solving the problems of ensuring safety of structures both in the design stage and controlling efficiency and in determining the optimum (permissible) operating conditions, and also in determining the residual service life. In certain cases the conditions of transition of thermal processes may be of controlling importance for the efficiency of materials in structures.

Hydrogen embrittlement (HE) plays a relevant role in the performance of prestressing steel wires. In this framework, the knowledge of residual stresses and plastic strains in wires due to cold-drawing (manufacturing-induced residual stresses), as well as of wires hydrogenation from harsh environments are the keys to successful predictions of wire lives. This paper advances previous analyses of HE in cold-drawn prestressing wires via numerical modelling, first, of the cold-drawing process to gain the distributions of residual stresses and plastic strains, and next, of the stress-strain assisted hydrogen diffusion in wires towards creation the conditions for HE nucleation. Generated results prove the relevant role of residual stress-and-strain field in hydrogen diffusion in the wires, as well as their possible consequences for HE.

This work is based on previous research on the one-dimensional (1D) analysis of the hydrogen diffusion process, and proposes a numerical approach to simulate the phenomenon in two-dimensional (2D) situations, e.g., near notches. The weighted residual method was used to solve numerically the differential equations set out when the geometry was discretized through the application of the finite element technique. This developed procedure can be a suitable practical tool to analyze hydrogen embrittlement phenomena in structural materials.

Hydrogen degradation represents a problem of major technological concern in the structural integrity of prestressed concrete structures in which cold-drawn wires are the main structural component and can suffer the deleterious effect of hydrogen, with the subsequent risk of catastrophic failure. This paper presents an innovative numerical analysis of hydrogen degradation of cold-drawn prestressing steel wires, focused on the two relevant variables influencing the phenomenon of hydrogen transport by diffusion in the metal: residual stresses generated by manufacture and plastic strains after cold work. To achieve this goal, two real (industrial) drawing processes are analyzed with two main differences, namely, the reduction of cross-sectional area performed at the first drawing stage and the number of drawing steps used in the whole process. Generated results prove the importance of an adequate design of the cold-drawing process with regard to the residual stress-and-strain field and its relevant r...

The authors construct a model for crack propagation in metals under the action of hydrogen. The model takes into account the micromechanisms of deformation and fracture. Their results constitute a closed system of equations which can reproduce the experimentally observed behavior of cracks relative to stress intensity factor, temperature, and hydrogen pressure.

Equations of crack growth caused by hydrogen were constructed and the theoretically calculated results were compared with existing experimental data in order to develop the theory of crack growth in deformed steels under the action of hydrogen for a dislocation model of the micromechanism of failure in such metals in a hydrogen-containing medium. Expressions for the kinetics of hydrogen accumulation in the area of fracture forerunners were derived. The resulting theoretical model was compared with crack growth in metals in hydrogen contact. The relationship of crack growth rate to temperature and hydrogen pressure and to stress intensity factors was also studied. It was found that the theoretical and experimental results agreed with a proposed physicomechanical model of crack growth and the equations calculated on the basis of it.

Commented paper by Tvergaard (2007) addressed the mesh sensitivity in relation to the disagreement between his finite element (FE) simulations of fatigue cracks and those by Levkovitch et al. (2005) which predicted striations-like folded crack profile. Tvergaard (2007) found out no major effect of mesh refinement and attributed the disparity with Levkovitch et al. (2005) to the smoothness of meshes created by distinct remeshing procedures implemented by cited authors. To contrast these data, the results of extensive analysis of mesh sensitivity taking account of the FE size, aspect ratio, type and technology together with the role of constitutive model, are outlined here and accompanied with deductions about its origins. Plane-strain mode I crack under small scale yielding was simulated. The test-piece geometry and loading were the same as in previous analyses (Toribio and Kharin, 1998, 1999, 2000, 2006), where initial crack of the length a had parallel faces and semicircular tip of the width b0 = a/15,000. Within conventional J2 elastoplasticity guided by the power-law stress-strain relation, the Ziegler’s kinematic hardening as well as isotropic one and ‘‘equivalent’’ perfect plasticity were explored as outlined elsewhere (Toribio and Kharin, 2000). The reference mesh M1 was mainly the same as that with which the mesh-convergence was apparently achieved for non-hardening solid (Toribio and Kharin, 1998, 1999). Mesh sensitivity was analysed reducing the near-tip FE size to the half and quarter of the reference one (respective meshes M2 and M3), as well as varying FE aspect ratio, type and technology – interpolation order, shape (quadrilaterals Q and triangles T) and full and reduced integration (FI and RI). Fourand eight-node elements Q4 and Q8, and the crossed linear triangles arrangement Q4T3 were tried without remeshing. Loading route comprised up to 10 cycles. In all cases, prominent mesh sensitivity appeared with the commencement of bifurcating shear localisations, which were revealed using the plastic-strain rate as sensitive detector. For the same material model, finer meshes manifested earlier, sharper and more numerous localisations, whereas with the same mesh, FE technologies giving stiffer elements (e.g., Q4FI vs. Q4RI o Q8FI) reduced the propensity to shear banding. Regarding FE aspect ratio, earlier and sharper banding was observed if deformed elements became more equilateral at approaching the shear instability. For the same mesh, kinematic hardening favoured bifurcations more intensively than isotropic hardening, all bringing strong mesh sensitivity earlier or later along the loading route (Toribio and Kharin, 2000, 2006). Fig. 1 exemplifies solution bifurcations for perfect plasticity and kinematic hardening, where in the former

The stress-strain assisted hydrogen diffusion in metals under variable loading is concerned as a key element of elucidation of hydrogen embrittlement (HE). The suitability of simplified treatments of hydrogen diffusion in notched solids under monotonic loading is addressed comparing various 1D and 2D modeling approaches with the purpose to assess if generated approximate solutions can provide acceptable results along the diffusion depth towards prospective rupture sites, so that quite more expensive simulations may be eluded. For different geometry-and-loading cases, respective time-depth domains are revealed where certain simplified procedures can be fairly suitable to carry out calculations of metal hydrogenation for the purposes of HE analysis and control, while the choice of the optimum strategy for the stress-strain assisted diffusion simulations in notched members is case- and purpose-dependent. Проаналізовано дифузію водню в металах за наявності напружень і деформацій, зумовлених змінними навантаженнями. Шляхом порівняння серії одно- і двовимірних модельних підходів зроблено висновок про можливість застосування спрощених процедур до розв’язування задач про дифузію водню в тілах з вирізами, щоб уникнути громіздких обчислень. Для різних геометрії і навантаження виявлено відповідні області змінних часу і глибини дифузії, за яких деякі спрощені підходи можуть дати прийнятні результати для визначення наводнювання металів. Вибір оптимального шляху вивчення дифузії за впливу напружень і деформацій залежить від конкретної мети моделювання. Проанализирована диффузия водорода в металлах при наличии напряжений и деформаций, вызванных изменяющимися нагружениями. Путем сравнения одно- и двумерных модельных подходов сделан вывод о возможности применения упрощенных процедур к решению задач о диффузии водорода в телах с вырезами, чтобы избежать громоздких вычислений. Для разных геометрии и нагружения выявлены соответствующие области переменных времени и глубины диффузии, когда некоторые упрощенные подходы могут дать приемлемые результаты для определения наводороживания металлов. Выбор оптимального способа изучения диффузии при воздействии напряжений и деформаций зависит от конкретных целей моделирования.

Crack closure effects on hydrogen-assisted cracking (HAC) were studied in terms of the influence of residual stresses produced by load removals at fatigue cycling on stress-assisted hydrogen diffusion towards rupture sites in the crack tip zone. The finite element procedure was applied to cyclic load elastoplastic large-deformation analysis combined with stress-assisted diffusion. Small-scale yielding near the crack tip was addressed and characterized in terms of the stress intensity factor (SIF). The elastoplastic situation near the tip of a blunting-closing crack revealed the cyclically stable stress evolution established after a couple of loading cycles (but this was not attained for strains). Crack closure effects were negligible for sustained-load HAC at SIF above the maximum value at precracking. The test duration for reliable evaluation of the threshold SIF for HAC was estimated. Crack closure affects near-tip hydrogen diffusion, and consequently HAC, at dynamic rising loading after precracking. Modeling was performed for the range of SIF increase rates. It showed that a premature fulfillment of the local fracture criterion in terms of critical combination of the hydrogen concentration and stress-strain parameters can occur at slow dynamic loading if compared with the sustained-load case. The dominating factor--stress or strain--and the scale of the localmore » rupture event are of decisive importance for this. The performed simulations provide more insight into the items of conservatism of HAC testing data and the sources of their uncertainty.« less

Cold-drawn prestressing steel wires are susceptible to hydrogen embrittlement (HE) in aggressive environments, and residual stress-strain states generated in wires by the drawing process play the key role there. Correspondingly, alterations of the stress-strain fields due to peculiarities of the drawing procedure affect the serviceability of wires under HE. On the basis of performed numerical simulations of the process of cold drawing with the use of different drawing dies, the paper addresses the effect of the inlet angle of the die on residual stress-strain fields in wires, and derives their consequences for wires hydrogenation and their susceptibility to HE. The reduction of the die inlet angle is shown to be beneficial for the wires performance. Встановлено, що холоднодеформовані сталеві дроти для арматури попередньо напружених залізобетонів чутливі до водневого окрихчення (ВО) в агресивних середовищах. Залишкові напружено-деформовані стани, які утворюються в цих дротах внаслідок холодного волочіння, відіграють при цьому ключову роль. Зміна цих напружено-деформованих станів через особливості процесу волочіння впливає на роботоздатність дротів під час ВО. На основі числового моделювання холодного волочіння з використанням відмінних фільєрів розглянуто дію вхідного кута фільєра на напружено-деформовані стани в дротах і зроблено висновки про наслідки їх впливів на наводнення дротів в агресивному середовищі і схильність до ВО. Показано, що зменшення вхідного кута фільєра волочіння має позитивний вплив на роботоздатність дротів. Установлено, что холоднодеформированные стальные проволоки для арматуры предварительно напряжeнных железобетонов являются чувствительными к водородному охрупчиванию (ВО) в агрессивных средах. Остаточные напряженно-деформированные состояния, которые создаются в этих проволоках вследствие холодного волочения, играют при этом ключевую роль. Изменение этих напряженно-деформированных состояний вследствие особенностей процесса волочения влияет на работоспособность проволок при ВО. На основе численного моделирования холодного волочения с использованием различных фильеров рассмотрено воздействие входного угла фильера на напряженно-деформированые состояния в проволоках и сделаны выводы относительно последствий их влияния на наводороживание проволок в агрессивной среде и склонность к ВО. Показано, что уменьшение входного угла фильера волочения имеет положительное влияние на работоспособность проволок.