Dear Milos, in some cases it would be hard to divide them. There are not so much possibilities to define which is dominant at this moment. I would suggest fractography analysis. In my investigations i observe in most cases two types of structures simultaneously. It seems to me that these processes could work together. Or am I mistaking?

Thanks dear Anton my observation also paralleled yours. Modern studies indicate this fact. In addition, it is necessary to implement appropriate critical experiments that can unequivocally point to the simultaneous action of both hydrogen embrittlement mechanisms in steel. SEM and TEM analysis are mandatory. I wonder how you are conducting research.

Dierk thanks a lot for comment and link.

It seems to me that you are not an absolute advocate of hydrogen-enhanced localized plasticity, or to be more precise: hydrogen–dislocation interactions,but also you does not neglected some effects on hydrogen embrittlement. Observed cracking at the triplets of grain boundaries (strong hydrogen trapping site) also suggested to me on a possible action of HEDE mechanism, because of favorable conditions for reaching a critical concentration of hydrogen. These observations are in line with my research on plain carbon steels (0.2% C, 0.4% Mn and Si from 0.15% to 0.40%) which indicate on simultaneously active HE micro-mechanisms and transition from dominant HELP to dominant HEDE mechanism (depending on hydrogen concentration). I think that the single out of the dominant mechanism/mechanisms is a extremely difficult problem.

But then, on the other side, your paper proposed a very interesting mechanism that emphasizes the importance of hydrogen trapping at the deformation twin boundaries which seems very reasonable to me.

Also, thanks for multi step understanding of hydrogen degradation process.

Dierk, It would be my pleasure if you join my HE group on linkedin. Some prestigious experts in the field of materials science and hydrogen degradation of materials are already present in this group.

Rgd. Milos Djukic

http://www.linkedin.com/groups/Hydrogen-Embrittlement-4991909/about?trk=anet_ug_grppro

Dear colleagues,

It is my pleasure and honor to introduce you Network of Excellence (NoE) in Hydrogen Embrittlement. For some more details about NoE branches, please see attached link.

http://milosdjukichydrogen.wordpress.com/2014/05/14/the-network-of-excellence-noe-in-hydrogen-embrittlement/

"The Network of Excellence (NoE) in Hydrogen Embrittlement aims to strengthen scientific and technological excellence by developing an integrated and interdisciplinary scientific understanding of hydrogen degradation of engineering materials and their co-evolution with science, materials science, industry and society, and also by addressing the fragmentation of European and Worldwide research in this area".

Regards

Milos

Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials.

M.B.Djukic, V.Sijacki Zeravcic, G. Bakic, A. Sedmak, B. Rajicic, Hydrogen Embrittlement of Low Carbon Steel, Procedia Materials Science, Volume 3, 2014, pages 1167-1172, DOI: 10.1016/j.mspro.2014.06.190

http://www.sciencedirect.com/science/article/pii/S2211812814001916

http://www.sciencedirect.com/science/article/pii/S2211812814001916

Very interesting new article in which the phenomenon of hydrogen-induced intergranular failure of iron is attributed to a hydrogen-enhanced plasticity (HELP)-mediated decohesion process.

Link to the Article: S. Wang, M.L. Martin, P. Sofronis, S. Ohnuki, N. Hashimoto, I.M. Robertson, Hydrogen-induced intergranular failure of iron, Acta Materialia, Vol. 69, May 2014, pp. 275–282, DOI: 10.1016/j.actamat.2014.01.060

http://www.sciencedirect.com/science/article/pii/S1359645414000871

http://www.sciencedirect.com/science/article/pii/S1359645414000871

Dear colleagues, who work with hydrogen and hydrogen embrittlement. Playing with this teoretical mechanisms we often forget the role of grain boundaries and the role of precipitates in this phenomenon. The surface layer and grain- and the other type of boundaries will be always the first place, where the hydrogen will be. Only after that something can happen inside the grains with dislocations and so on (This is of course temperature dependent). But will it be not too late at this moment? Will the crack go through the grain boundaries before hydrogen will be trapped by... some dislocations inside the grains. Of course I am not telling that these mechanisms do not work, they are. But what about the impact? The question was - "what is the dominant mechanism?" In my opinion, "HELP - localised plasticity lead to embrittlement" - sounds not very good, as it is the direct opposition to fracture. I perfectly know who and how has it been discovered. But in this case the overall stress should be lower in the hydrogenated material. Which i did not see until now. May be i am mistaking and someone could give me a reference on it. As far as I saw, It was always higher. In this case - decohesion make sence, as it introduces stresses, in particular, the tensile stresses inside the material due to redistribution of electron density and repulsion force of the additional hydrogen atom. Such "prestressed" state have no other choise as to brake and the falure occurs preliminary, unexpectedly. This is called embrittlement. After that, of course, the fracture surface should be thoroughly investigated. But here is the question. Will this mechanism be always the same for the different hydrogenation degree and material type? This is very interesting and I continue my investigation on the hydride forming zirconium materials. May be one day i switch to non-hydride-forming materials.

All the best.

Thanks a lot Anton Pshenichnikov!

Dear Anton Pshenichnikov, in relation to your question please take a look at our new article:

Djukic, M.B., Sijacki Zeravcic, V., Bakic, G.M., Sedmak, A., Rajicic, B., Hydrogen

damage of steels: A case study and hydrogen embrittlement model, Engineering Failure Analysis (2015), http://dx.doi.org/10.1016/j.engfailanal.2015.05.017

https://www.researchgate.net/publication/278035151_Hydrogen_damage_of_steels_A_case_study_and_hydrogen_embrittlement_model

www.researchgate.net/publication/278035151_Hydrogen_damage_of_steels_A_case_study_and_hydrogen_embrittlement_model

Article Hydrogen damage of steels: A case study and hydrogen embritt...

Greetings and first thanks for the interesting discussion.

As I see, there are lot of misconceptions and acronyms thrown in complicating the problem. HELP is not really a help in aiding the crack propagation. There is a reduction in the energy for dislocation nucleation at the crack tip due to reduction in the surface energy - hence glide occurs - However it cannot continue since dislocation transport is hindered by the H-atmosphere - liking dragging a heavy load. Hence slip cannot continue and back stress reduces further slip and makes the crack to extend by cleavage mode until a next dislocation emission again becomes easier - this alternating glide-cleavage takes place causing embrittlement - Because the reduction of surface energy also helps the cleavage mode. With increasing H-pressure, the material becomes more brittle since cleavage becomes easier than glide. We have analyzed this growing crack problem in H-steels - Excellent work by Vehoff and Roth on Fe-Si alloy validates our analysis. Some of the discrete dislocation analysis is presented in the paper published in Corrosion Review that can be accessed as it was down loaded. Dr. K. Solanki has done H-simulation.

Dear Colleagues and friends, Check our brand new and the most comprehensive critical review paper (June 2019) published in Engineering Fracture Mechanics journal (244 citations): "The synergistic action and interplay of hydrogen embrittlement mechanisms in steels and iron: Localized plasticity and decohesion"

The particular emphasis is given to the proposal of the novel and unified HELP + HEDE model based on the specific microstructural mapping of the dominant hydrogen embrittlement (HE) mechanisms with implications on the fracture process and resulting #hydrogen-assisted fracture modes. Most up-to-date experimental and modeling approaches, current trends and future challenges in the investigation of the synergistic interplay of HE mechanisms in different grades of steel, including the most advanced, and iron, are also included and critically discussed.

Best regards,

Milos Djukic

Article The synergistic action and interplay of hydrogen embrittleme...