Transmission, Distribution & Storage

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.

This paper deals with notch-induced anisotropic fracture behavior of progressively cold drawn pearlitic steels on the basis of their microstructural evolution during manufacturing by multi-step cold drawing that produces slenderizing and orientation of the pearlitic colonies together with densification and orientation of the Fe/Fe3C lamellae reviewing previous research by the author. Results of fracture test using notched specimens of cold drawn pearlitic steels with different degrees of cold drawing (distinct levels of strain hardening) in air and hydrogen environment shows: (i) the key impact of the colonies and lamellae alignment and orientation on notch-induced fracture producing anisotropic fracture behavior with its related crack path deflection (or fracture path deviation); (ii) the necessity of both stress triaxiality (constraint) and microstructural orientation (colonies/lamellae) alignment to produce fracture path deflection; (iii) hydrogen presence (the circumstance) promotes crack path deviation in addition to the inherent microstructural anisotropy created by cold drawing; (iv) the anisotropic fracture path with a stepped profile in cold drawn pearlitic steel consisting of deflections and deviations from the initial transverse fracture path in mode I resembles Masaccio’s Tribute Money painting with its mountains at the background so that the present paper can be considered as a Tribute to Masaccio.

With the rapid growth in demand for effective and renewable energy the hydrogen era has begun. To meet commercial requirements efficient hydrogen storage techniques are required. So far four techniques have been suggested for hydrogen storage: compressed storage hydrogen liquefaction chemical absorption and physical adsorption. Currently high-pressure compressed tanks are used in the industry; however certain limitations such as high costs safety concerns undesirable amounts of occupied space and low storage capacities are still challenges. Physical hydrogen adsorption is one of the most promising techniques; it uses porous adsorbents which have material benefits such as low costs high storage densities and fast charging–discharging kinetics. During adsorption on material surfaces hydrogen molecules weakly adsorb at the surface of adsorbents via long-range dispersion forces. The largest challenge in the hydrogen era is the development of progressive materials for efficient hydrogen storage. In designing efficient adsorbents understanding interfacial interactions between hydrogen molecules and porous material surfaces is important. In this review we briefly summarize a hydrogen storage technique based on US DOE classifications and examine hydrogen storage targets for feasible commercialization. We also address recent trends in the development of hydrogen storage materials. Lastly we propose spillover mechanisms for efficient hydrogen storage using solid-state adsorbents.

Reinforced concrete (RC) structures are long-term operated objects with service life of 50–100 years. During their operation they subject to continuous ambient effects (cyclic temperature changes acid rains de-icing salts) and service loads (e.g. traffic) which effect on structural integrity of the composite and lead to worsening of structures serviceability. One of the reasons for strength loss of RC members is bond degradation between rebar and concrete. It could be caused by two different factors: overprotection of RC and reinforcement corrosion. These effects were simulated in the laboratory conditions by the electrochemical methods applying of impressed cathodic current and accelerated corrosion tests respectively. It was shown that applied anode polarization causes not only concrete cracking due to internal pressure of corrosion products at the interface but also due to their expansion far from rebar for a distance comparative with a specimen thickness evidently into preliminary formed cracks. Since intensive corrosion of steel reinforcement decreases its diameter and corrosion products can migrate from the rebar surface into a depth of concrete these factors could weaken bond in RC installations up to a total loss of cohesion between rebar and concrete. The influence of cathodic polarization of steel embedded in concrete is commonly seemed to consist in its possible hydrogen embrittlement and ions redistribution in concrete matrix. In this paper the effect of hydrogen recombined at the rebar–concrete interface on bond weakening and concrete cracking is considered.

Stress corrosion cracking (SCC) of steels can reduce the structural integrity of gas pipelines. To simulate in-service degradation of pipeline steels in laboratory the method of accelerated degradation consisted in subjecting specimens to electrolytic hydrogenation to loading up the certain plastic deformation and heating of specimen at 250°C was recently developed. The purpose of this paper was to analyse mechanical and SCC behaviour of in-service and in-laboratory degraded gas pipeline steels and to reveal some fractographic features of SCC. Three pipeline steels of the different strength (17H1S which is equivalent of API X52 API X60 and API X70) were investigated. The characteristics of the as-received pipeline steels with different strength were compared with the properties of pipeline steels after in-service and in-laboratory degradation. An influence of the NS4 solution on SCC resistance of 17H1S and API X60 steels in the as-received state and after the accelerated degradation using slow strain rate tension method was analysed. The noticeable decrease of plasticity for 17H1S and API X60 steels after long-term operation was shown. Deep microdelaminations revealed in the central part of fracture surfaces for the operated steels can be considered as the signs of dissipated damaging in the metal caused by texture and hydrogen absorbed by metal. Comparison of the SCC tests results showed that the characteristics of both steels in the as-received state were insignificantly changed under the influence of the environment. At the same time the degraded steels were characterized by a high sensitivity to SCC. It was shown fractographically that it associated with cracking along interfaces of ferrite and pearlite grains with secondary deep intergranular cracks formation and also by delamination between ferrite and cementite inside pearlite grains. The similar fracture mechanism at SCC tests was revealed for near the outer surface of the specimens and in the central part of the fracture surfaces of in-laboratory degraded specimens. These results demonstrated the key role of hydrogen during SCC and in-bulk cracking as well.

In this paper we propose and verify a novel method to simulate crack propagation without propagating a crack by finite element method. We propose this method for elastoplastic analysis coupled with convection-diffusion. In the previous study we succeeded in performing elastoplastic analysis coupled with convection-diffusion of hydrogen for a material with a crack under tensile loading. This research extends the successful method to fatigue crack propagation. In convection-diffusion analysis in order to simulate the invasion and release of elements through the free surface the crack tip is expressed by using a notch with a sufficiently small radius. Therefore the node release method conventionally used to simulate crack propagation cannot be applied. Hence instead of crack propagation based on an analytical model we propose a novel method that can reproduce the influence of the vicinity of the crack tip on a crack. We moved the stress field near the crack tip in the direction opposite to that of crack propagation by an amount corresponding to the crack propagation length. When we extend the previous method to fatigue crack propagation simulation we must consider the difference in strain due to loading and unloading. This problem was solved by considering the strain due to loading as a displacement. Instead of moving the strain due to loading we moved the displacement. First we performed a simple tensile load analysis on the model and output the displacement of all the nodes of the model at maximum load. Then the displacement was moved in the direction opposite to that of crack propagation. Finally the stress field was reproduced by forcibly moving all the nodes by the displacement amount. The strain due to unloading was reproduced by removing the displacement. Furthermore we verified the equivalence of the crack propagation simulation and the proposed method.

Strategic networks of hydrocarbon pipelines in long time service are adversely affected by the action of aggressive chemicals transported with the fluids and dissolved in the environment. Material degradation phenomena are amplified in the presence of hydrogen and water elements that increase the material brittleness and reduce the safety margins. The risk of failure during operation of these infrastructures can be reduced if not prevented by the continuous monitoring of the integrity of the pipe surfaces and by the tracking of the relevant bulk properties. A fast and potentially non-destructive diagnostic tool of material degradation which may be exploited in this context is based on the instrumented indentation tests that can be performed on metals at different scales. Preliminary validation studies of the significance of this methodology for the assessment of pipeline integrity have been carried out with the aid of interpretation models of the experiments. The main results of this ongoing activity are illustrated in this contribution.

Regularities of fatigue crack growth for pipeline steels of different strength are presented and the changes in fatigue behavior of these steels after long term operation are analyzed. Threshold values of stress intensity factor range are lower for operated steels comparing to the corresponding values for as received ones. During the testing in the simulated soil solution NS4 a barely noticeable tendency to increase the threshold values of SIF was traced. It was explained by the appearance of intergranular fracture elements on the backgrownd of the typical flat fatigue relief already in the near-threshold region of fatigue crack growth curves in the soil solution. A higher relief of intergranular facets provided favorable conditions for occurrence of crack closure effect.<br/>Fatigue testing was performed using steel specimens after in-laboratory and in-service degradation and it was shown that results for both degraded steels are very close to each other proving the validity of the method of in-laboratory degradation. A new methodic approach to fatigue testing of pipe steels is presented which allows simulating working conditions of gas pipelines namely the hydrogen diffusion through the pipe wall to its external surface and estimating its possible effect on SCC. It consists in evaluation of the influence of hydrogen reached the crack tip only due to its diffusion on the crack growth. It is found that hydrogen absorbed by metal during the test providing such conditions causes a leap of crack growth rate in the Paris region of the fatigue crack growth curve of the tested 17H1S steel. Intergranular mechanism of fracture detected on the specimen fracture surface is suggested as a clear evidence of embrittlement of grain boundaries as a result of its hydrogenation.

For prohibiting a global warming fuel-cell systems without carbon dioxide emissions are a one of the promising technique. In case of a fuel-cell vehicle (FCV) high-pressure H₂ gas is indispensable for a long running range. Although there are lot of paper for studying a hydrogen embrittlement (HE) there are few paper referred to the effect of high-pressure H₂on the HE phenomenon.

In this study an effect of high-pressure H₂ gas on tensile & fatigue properties of stainless steel SUS316L were investigated by means of the internal high-pressure H₂ gas technique. Main findings of this study are as follows;

• Although there are almost no hydrogen embrittlement effect on the 0.2 % proof stress and tensile strength elongation and reduction of area decrease in H₂ gas environment
• For case of low Ni_eq material fatigue life and fatigue limit decrease in H₂ gas environment
• For case of low Ni_eq material not a few α’ martensitic phase generated on the fatigue fractured specimen.

The paper addresses the sensitivity to hydrogen embrittlement of heavily cold-drawn wires made of the new generation of lower alloyed duplex stainless steels often referred to as lean duplex grades. It includes comparisons with similar data corresponding to cold-drawn eutectoid and duplex stainless steels. For this purpose fracture tests under constant load were carried out with wires in the as-received condition and fatigue-precracked in air and exposed to ammonium thiocyanate solution. Microstructure and fractographic observations were essential means for the cracking analysis. The effect of hydrogen-assisted embrittlement on the damage tolerance of lean duplex steels was assessed regarding two macro-mechanical damage models that provide the upper bounds of damage tolerance and accurately approximate the failure behavior of the eutectoid and duplex stainless steels wires.