ICAMS / Interdisciplinary Centre for Advanced Materials Simulation


Analysis of hydrogen assisted fracture in martensitic steels

Date: 17.06.2013
Time: 5:00 p.m.
Place: 13th International Conference on Fracture, Beijing, China

Philipp Schwittek
Alexander Hartmaier

Hydrogen embrittlement has been investigated for more than 50 years and our current understanding is that a multitude of material specific mechanisms can be the origin of this embrittlement. Moreover, recent studies revealed that both the mechanical loading situation and the local hydrogen concentration might change the damage or degradation mechanism that occurs within a specific region in a specific material. In this work we develop and apply a model that is capable of describing stress-assisted hydrogen diffusion and trapping as well as hydrogen interaction with local plasticity and cohesive failure along grain boundaries or through grains. This model is applied to fracture in martensitic steels as a prominent example for structural materials that suffer from hydrogen embrittlement. For this class of materials a parametric study is conducted to explore the conditions that lead to a change in the failure mechanism. Thus, a map of the different failure mechanisms is constructed over the domain of different mechanical and chemical loading situations.

For martensitic steels with their fine-grained microstructure, a particular focus of the parametric study is laid on the role of the different trap sites that hydrogen can occupy. Fine-grained microstructures with a large number of different lattice defects differ mainly in the trap binding energies and the number of trap sites. Furthermore plastic deformation at crack tips can severely change the local dislocation density and hence the number of trap sites. In consequence the hydrogen concentration in the tri-axial stress field around crack tips is a strongly non-linear function of plastic deformation. On the one hand side plastic deformation reduces the stresses and therefore the driving force for stress-assisted diffusion; on the other hand side plastic deformation increases the trapped hydrogen concentration, which again interacts strongly with the ability of the material to undergo further plastic deformation and work hardening.

Supporting information:

Schwittek_13th International conference on Fracture_ 201307.pdf
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