Crack propagation associated with stress-assisted diffusion of impurities under creep conditions / Yan Xu.

Xu, Yan.
xiv, 133 p. : ill. ; 29 cm.
Local subjects:
Penn dissertations -- Mechanical engineering and applied mechanics.
Mechanical engineering and applied mechanics -- Penn dissertations.
This dissertation research involves modeling of the mechanism of brittle fracture whereby diffusion of surface-adsorbed impurities along a grain boundary or interface, which is driven by the concentration gradient as well as the stress field ahead of a crack, reduces the cohesive strength of the boundary as the concentration of impurity builds up, thus causing intergranular decohesion. Such intergranular cracking occurs when material is under high temperature exposure, i.e., about one-third to one-half of the melting temperature of the material. Crack growth is affected by temperature, mechanical loading, and chemical compositions of the material and environment. Elevated temperature cracking associated with intergranular embrittlement arising from diffusion of impurities is studied using a cohesive zone model that couples the nonlinear effects of small-scale creep, diffusion and damage. The constitutive equation for the cohesive zone is coupled with stress-assisted diffusion of impurities into the grain boundary. Two phenomenological models, a Kachanov-type damage model and an additive decomposition of creep deformation and grain boundary decohesion model, are used to describe the effect of impurity concentration on the grain boundary strength. Numerical studies for steady-state cracking reveal the influence of various material parameters and loading conditions on the cracking process. The phenomenological models developed in this research are useful in understanding the physics of the diffusion controlled embrittlement phenomena. These models predict that, in general: (i) the impurity concentration build-up is limited to the very small region in the grain boundary ahead of a crack tip which, typically, is embedded within a zone of enhanced creep; (ii) the applied load versus cracking speed correlation is dependent upon diffusivity, creep resistance of the material, cracking speed, and the influence of impurities on grain boundary cohesion; and (iii) there is a transition in cracking behavior between the low-load regime where the velocity increases with increasing stress intensity factor to the high-load regime where the velocity decreases with increasing stress intensity factor.
Supervisor: John L. Bassani.
Thesis (Ph.D. in Mechanical Engineering and Applied Mechanics) -- University of Pennsylvania, 1999.
Includes bibliographical references.
Local notes:
University Microfilms order no.: 99-26218.
Bassani, John L., advisor.
University of Pennsylvania.
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