Micrometre-scale plasticity size effects in metals and ceramics: theory and experiment.
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This thesis comprises studies of size effects in the plasticity of metals and
ceramics at length scales of the order of micrometres and includes both experimental
work and theoretical development. Experimental results are presented for foil flexure
(nickel and copper)and nanoindentation (ceramics and hard metals).These studies
were conducted because existing data does not cover a range broad enough or with
sufficient precision to test various theories.
With the developed bending technique more accurate data is obtained covering
a wide range of strain, especially around the key region of the elastic-plastic
transition. Moreover, the interaction between grain and thickness size effect is
successfully studied by varying the ratio of grain size over thickness of the foils.
After carefully calibrating the indenters, the macroscopic indentation yield
strength for ceramics and high strength metals is determined in a direct way by using
spherical nanoindentation. The magnitude of size effect is significantly different
between metals and ceramics. By comparing the Berkovich and spherical indentation
size effect, the results implies that the contact size, a, is the most fundamental length
scale in the indentation size effect, independent of the indenter shape. The
indentation strength is found to be inversely scaled with the square root of a.
The slip-distance theory (based on (Conrad et al, 1967)) with an effective
length scale reconciling intrinsic and extrinsic size effects appears able to account
for the size effects in all contexts, without requiring strain gradient plasticity theory
or an implicit characteristic length.
Authors
Zhu, TingtingCollections
- Theses [3831]