ABSTRACT REF NO: ICF100889OR THE EFFECT OF HYDROGEN-INDUCED SOFTENING ON THE DEFORMATION AT A CRACK TIP: IMPLICATIONS FOR FRACTURE Y. M. Liang and P. Sofronis Department of Theoretical and Applied Mechanics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA ABSTRACT The hydrogen concentrations in equilibrium with local stress and plastic straining are studied in conjunction with large elastic-plastic deformation in the neighborhood of a blunting crack tip. The hydrogen effect on material behavior is modeled through the hydrogen-induced volume dilatation and the reduction in the local flow stress. Plane strain finite element analysis shows that stress relaxation due to solute hydrogen decreases the extend of plastic yielding whereas hydrogen-induced material softening causes the deformation to center in the region directly ahead of the crack tip. The consequence of this result on the hydrogen-induced fracture processes is discussed. KEYWORDS Hydrogen, diffusion, plasticity, softening, fracture INTRODUCTION Of the many suggestions for the explanation of the hydrogen related failures, the mechanism of hydrogenenhanced localized plasticity (HELP) appears to be a viable one [1]. Arguments in support of the HELP mechanism are based on experimental observations [2] and theoretical calculations [3] that in a range of temperatures and strain rates, the presence of hydrogen in solid solution decreases the barriers to dislocation motion, thereby increasing the amount of deformation that occurs in a localized region adjacent to the fracture surface [4]. The underlying principle in the HELP mechanism is the shielding of the elastic interactions between dislocations and obstacles by the hydrogen solutes [2, 3]. Reduction of the interaction energies between elastic stress centers results in enhanced dislocation mobility, i.e., decreased material local flow stress [4]. Nonetheless, significant issues remain to be resolved. Among these are: i) what is the detailed mechanism by which the enhanced dislocation mobility causes fracture in bulk specimens? ii) does the fracture occur by microvoid coalescense, formation of a Stroh crack or some other mechanism? iii) can the contribution of the plastic deformation to the fracture energy be established as a function of the hydrogen concentration? In view
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