Abstract

This thesis presents an investigation into the mechanical properties of ductile
polyphase materials, which were studied by a number of different techniques.
The first approach was to do creep tests and transparent deformation cell
experiments with two-phase composites of organic crystalline rock-analogues,
camphor and octachloropropane (OCP). A deformation apparatus to deform these
materials in simple
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shear to very high bulk shear strain (up to )'=185) was developed
and built during this study. The method for the analysis of progressive deformation
in the transparent deformation cell, using small marker particles, was improved and a
computer program was written to perform this analysis. In the search for new rock
analogues, norcamphor was found to be a promising material. First observations on
its microstructural behaviour and rheological properties are presented.
The rheological properties of OCP and camphor at 28°C were determined: both are
power-law creep materials with stress exponents of 4.5 and 3.3 respectively. The
strain rate difference is two to three orders of magnitude within the experimental
range of stresses (0.2-1.2 MPa). Mixtures of OCP and camphor are also power-law
creep materials at these stresses. The soft phase OCP forms a matrix around the
harder camphor inclusions and can accommodate most of the deformation. The
properties of these mixtures can be described with matrix-rigid inclusion models,
even at small volume fractions of OCP, because the camphor is virtually rigid
compared to the OCP.
Simple shear experiments allowed the observation of microstructural developments
to high finite shear strain. Isotropic composites of camphor inclusions in OCP
showed the development of a foliation of camphor lenses and layers and finally
(y>50-75) folding of the foliation. Folding commenced with buckling of the camphor
layers. Axial planes were initially at a high angle (45-90°) to the flow plane, and
rotated towards it as the folds tightened. Experiments were done at a constant shear
stress and the bulk shear strain rate response to these developments was an initial
shear strain rate increase (softening) as the foliation formed and a subsequent shear
strain rate decrease (hardening) at the onset of folding. Localisation of deformation
into a narrow shear zone was observed in a composite of OCP inclusions in camphor
at Y<1. The shear zone was formed by linking up of the OCP inclusions, which
caused softening of the composite as a whole.Numerical modelling was the second approach. This was used to investigate the
effect of anisotropy of the phase distribution on bulk properties of a composite. It
was observed that, for a given pair of phases, the bulk bulk properties of a composite
are strongly related to the percolation fraction of the stronger phase. This is the
fraction at which, with increasing concentration, this phase starts to form a connected
framework, which is dependent on the geometrical distribution of the phases. The
percolation fraction determines the horizontal distance of the composite properties
between the Voigt or constant strain/strain rate bound and the Reuss or constant
stress bound. Thus one can make an estimate of the composite properties if the
percolation fraction is known, as well as the properties of the constituent phases and
their volume fractions.
Grain growth and its effects on microstructure were also studied. Grain growth
during deformation may suppress foliation development in a polycrystalline
aggregate. Deformation induced neighbour switching was found to be an important
mechanism for this. A computer model was written to model grain growth. With this
model one can outline microstructural and topological features, such as grain shape,
grain boundary curvature and the frequency distribution of size and number of sides
of grains, which are indicative of the grain growth history of the grain aggregate.
Drag by second phase particles and deformation during grain growth are incorporated
in the model.
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