Near Tip Mechanics of Stress Induced Microcracking in Brittle Materials
Charalambides, Panayiotis Gabriel
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https://hdl.handle.net/2142/71687
Description
Title
Near Tip Mechanics of Stress Induced Microcracking in Brittle Materials
Author(s)
Charalambides, Panayiotis Gabriel
Issue Date
1986
Department of Study
Theoretical and Applied Mechanics
Discipline
Theoretical and Applied Mechanics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Applied Mechanics
Abstract
Typically ceramics are processed at high temperatures and cooled to ambient. This causes thermal stresses dominated by expansion mismatches in multiphase ceramics and thermal expansion anisotropies in single phase systems. If the material microstructure (grain size in single phase systems or particle size in multiphase ceramics) is sufficiently large, spontaneous microcracking can occur on cooling after processing. This damage can be avoided by controlling the grain or particle size.
In this work, a continuum mechanics description of the phenomenon of stress induced microcracking at facets is presented. The continuum modulus reduction model was used in finite element calculations to predict microcrack formation around the crack tip of a major mode I crack. Small scale microcracking zones were obtained and their shape and size with respect to microstructure are discussed. The stress and strain fields are also presented. Furthermore, the modulus reduction effects on the material toughness were studied both theoretically and numerically. Substantial toughening was predicted. The trends in toughness with respect to grain size ratio were consistent with experimental observations. During finite element simulation of crack propagation R-curve characteristics were present. The obtained extended microcrack wake zones were substantially larger than the initial microcrack zones.
The coupled effects of modulus reduction and residual strains due to microcracking were also considered. Stronger R-curves than those predicted by the modulus reduction model were obtained. However, the trends in toughness with respect to grain size ratio were similar whereas the microcracking was confined to a smaller region around the crack tip. Initially expanding microcrack wake zones were predicted.
Microcracking is also known to be associated wtih dilatant transformation of second phase particles. When in a severe stress environment, zirconia particles in alumina undergo martensitic transformation. The volume expansion during the transformation in combination with thermal residual stresses cause radial and circumferential microcracks to develop around the particle. These microcracks further shield the crack from the applied stress. Microcrack densities and microcrack residual strains were related to particle size and particle volume fraction which provided the basis for a continuum mechanics description of the above phenomenon. (Abstract shortened with permission of author.)
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