Mechanics and Mechanisms of Creep and Ductile Fracture Page: 2
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Srivastava, Ankit, Mechanics and mechanisms of creep and ductile fracture.
Doctor of Philosophy (Materials Science and Engineering), August 2013, 154 pp., 3
tables, 72 figures, bibliography, 147 titles.
The main aim of this dissertation is to relate measurable and hopefully
controllable features of a material's microstructure to its observed failure modes to
provide a basis for designing better materials. The understanding of creep in materials
used at high temperatures is of prime engineering importance. Single crystal Ni-based
superalloys used in turbine aerofoils of jet engines are exposed to long dwell times at
very high temperatures. In contrast to current theories, creep tests on Ni-based
superalloy specimens have shown size dependent creep response termed as the thickness
debit effect. To investigate the mechanism of the thickness debit effect, isothermal creep
tests were performed on uncoated Ni-based single crystal superalloy sheet specimens
with two thicknesses and under two test conditions: a low temperature high stress
condition and a high temperature low stress condition. At the high temperature, surface
oxidation induced microstructural changes near the free surface forming a layered
microstructure. Finite element calculations showed that this layered microstructure gave
rise to local changes in the stress state. The specimens also contained nonuniform
distribution of initial voids formed during the solidification and homogenization
processes. The experiments showed that porosity evolution could play a significant role
in the thickness debit effect. This motivated a basic mechanics study of porosity
evolution in single crystals subjected to creep for a range of stress states. The study was
performed using three-dimensional finite deformation finite element analysis of unit cells
containing a single initially spherical void in a single crystal matrix. The materials are
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Srivastava, Ankit. Mechanics and Mechanisms of Creep and Ductile Fracture, dissertation, August 2013; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc283799/m1/2/?rotate=180: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .