35
A micromechanics approach to the study of hydrogen transport and embrittlement A. Taha, P. Sofronis * Department of Theoretical and Applied Mechanics, 216 Talbot Laboratory, University of Illinois at Urbana-Champaign, 104 South Wright Street, Urbana, IL 61801, USA Received 26 January 1999; accepted 15 October 1999 Abstract The mechanisms of hydrogen related fracture are brie¯y outlined. Previous investigations on the physics and treatment of the hydrogen transport processes are reviewed. A hydrogen diusion model based on the interaction of hydrogen induced strain in the lattice with local material elastoplasticity is presented. Finite element studies were carried out to analyze the hydrogen distribution in the neighborhood of a blunting crack tip under small scale yielding conditions and in the neighborhood of a rounded notch in a four-point bend specimen. The calculated hydrogen concentration pro®les and experimental observations of embrittlement in high strength steels are used to make eval- uative statements on the occurrence of the ®rst microcracking event. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hydrogen; Embrittlement; Diusion; Plasticity; Deformation; Fracture 1. Introduction Hydrogen embrittlement is a severe environmental type of failure [1±9]. When hydrogen is present, materials fail at load levels that are very low compared with those that a hydrogen free material can sustain. The result is usually catastrophic fracture which occurs unexpectedly, sometimes after many years of service [10]. Embrittlement can occur due to hydrogen contained in a pressure vessel or arising from chemical reactions. Despite extensive study, the mechanisms) of hydrogen embrittlement has remained unclear. Several candidate mechanisms have evolved, each of which is supported by sets of experimental observations and strong personal views. One reasonable certain aspect of this controversy is that there are several viable mechanisms of hydrogen related failure and that the search for a single mechanism to explain all obser- vations is doomed to failure [1,8,11]. Of the many suggestions, three mechanisms appear to be viable; stress induced hydride formation and cleavage [12±18], hydrogen enhanced localized plasticity HELP) [1,8,19± 25], and hydrogen induced decohesion [26±32]. The ®rst of these has been de®nitively established to be Engineering Fracture Mechanics 68 2001) 803±837 www.elsevier.com/locate/engfracmech * Corresponding author. Tel.: +1-217-333-2636; fax: +1-217-244-5707. E-mail address: [email protected] P. Sofronis). 0013-7944/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0013-794400)00126-0

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