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In-Depth Information
complex mixture of basic flow mechanisms will be involved, the characterization of
which has not yet been adequately achieved. Therefore, we shall not attempt to define
models for the time and temperature dependence of granular flow in these situations.
For a selection of studies on the time and temperature dependent aspects of flow in
fault gouge and similar materials, see Summers and Byerlee (
1977
), (Bombolakis
et al.
1978
), Wu (
1978
), (Moody and Hundley-Goff
1980
), (Wang et al.
1980
),
(Logan et al.
1981
), Shimamoto and Logan (
1981a
,
b
), Teufel (
1981
), Chu and Wang
(
1982
), (Morrow et al.
1982
), (Moore et al.
1986
), Raleigh and Marone (
1986
) and
Rutter et al. (
1986
).
Closely related to such behaviour are the time and temperature dependent
aspects of friction. These also often involve gouge to a greater or less extent and
have been considerably studied in connection with stick-slip movement between
sliding surfaces and its application to earthquakes. The variations in frictional
resistance with displacement and with velocity have been of particular interest and
may eventually have some application in modelling strain rate and temperature
dependence in friction-controlled granular flow. For a selection of friction studies,
see Dieterich (
1972
,
1978
,
1979
,
1981
), Stesky et al. (
1974
), Stesky (
1978
),
Johnson (
1981
), Dieterich and Conrad (
1984
), Wecks and Tullis (
1985
), Lockner
et al. (
1986
), Tullis and Weeks (
1986
), Rudnicki (
1988
), Tullis (
1988
) and Rundle
(
1989
).
7.3 Granular Flow Controlled by Thermally-Activated
Processes
7.3.1 Introduction: superplasticity
Following the trend in materials science, the term ''superplasticity'' is taken here
to be primarily a phenomenological one rather than a term referring to a particular
deformation mechanism (Gilotti and Hull
1990
; Paterson
1990
). The term was
originally used in metallurgy to refer to the development in tensile tests of very
large elongations, often of the order of ten-fold (natural strains of 2 or more), and it
has been used in the ceramics literature in the same sense (for example, Chen and
Xue
1990
, referring to natural strains in extension of 0.5-1 in many cases). The
essential properties that promote large ductility in high temperature extension tests
are a high strain rate sensitivity of the flow stress, which inhibits necking (Hart
1967
), and the absence of processes such as microcracking and cavitation that lead
to premature fracture. In practice, two other groups of properties are also observed
to be associated with this behaviour:
(1) The flow stress depends strongly on the grain size and the superplastic regime
is limited to grain sizes below a certain level, commonly of the order of
1-10 lm under experimental conditions (hence the terms ''fine-grain super-
plasticity'' and ''grain-size-sensitive flow'')
