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Modeling and simulation of short fatigue crack propagation assisted by internal or external hydrogen in a metastable stainless steel

By Volker Schippl (Universität Siegen, Lehrstuhl für Materialkunde und Werkstoffprüfung )
Co-authors: Sven Brück (Universität Siegen, Lehrstuhl für Materialkunde und Werkstoffprüfung)
Hans-Jürgen Christ (Universität Siegen, Lehrstuhl für Materialkunde und Werkstoffprüfung)
Claus-Peter Fritzen (Universität Siegen, Arbeitsgruppe Technische Mechanik)
Martina Schwarz (Universität Stuttgart, Materialprüfungsanstalt)
Stefan Weihe (Universität Stuttgart, Materialprüfungsanstalt)

A microstructure-based short crack propagation model will be introduced which was developed in order to describe quantitatively the influence of hydrogen on the short fatigue crack growth of hydrogen precharged specimens (internal hydrogen) and hydrogen-free specimens tested in compressed hydrogen (external hydrogen) in a metastable stainless steel. The short crack propagation is governed by the local stresses and displacements at the crack tip which are calculated by means of a two-dimensional dual boundary element method. The diffusion-controlled hydrogen concentration is updated after each crack growth cycle.In the simulation, the polycrystalline microstructure and the elastic anisotropic properties of each grain as well as the crack growth along slip bands and grain or phase boundaries are considered. The deformation-induced phase transformation from γ austenite to α’ martensite is taken into account at the crack tip. In the case of precharged specimens, the redistribution of hydrogen is modelled by a finite element method, while for specimens tested in compressed hydrogen, the hydrogen distribution is calculated by a simplified analytical approach taking the hydrostatic stresses into account. The local hydrogen concentration further influences the activation of slip bands and the corresponding microstructural flow stress which also effects the local phase transformation. The results of simulation calculations are compared with the growth behavior of experimentally observed short fatigue cracks. The comparison shows a reasonable agreement and demonstrates the predictive capability of the modelling approach.

Ⓒ Photos:Toerisme Leuven