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might then be expected, although such a granular body may also be approaching
the sintering threshold. In practice, the temperature and time aspects of pure
particulate flow do not appear to have been much studied.
In modelling cataclastic granular flow, crack propagation within the granules
plays an important role in addition to that of friction at their surfaces and may even
become the rate controlling factor. The temperature and time effects would then be
expected to be similar to those observed in crack propagation studies. While such
effects are still relatively minor compared with similar effects in crystal plasticity
and atom transfer flow, they are nevertheless readily observable and have been
extensively studied in connection with static fatigue and subcritical crack growth
(Anderson and Grew
1977
; Atkinson
1984
; Atkinson and Meredith
1987
; Costin
1987
; Hasselman and Venkataswaran
1983
; Kranz
1979
; Kranz
1980
,
1983
;
Martin
1972
; Ohnaka
1983
; Scholz
1968
,
1972
; Swanson
1984
). However, the
application of such studies in the modelling of cataclastic granular flow has been
less well developed.
The main application of the studies on the kinetics of crack-growth has been to
so-called brittle creep, that is, the small-strain creep that is commonly observed as
a precursor to macroscopic brittle fracture or to gross slippage on an already
existing fault surface (Carter et al.
1981
; Carter and Kirby
1978
; Cruden
1970
,
1974
; Kranz
1980
; Kranz and Scholz
1977
; Lockner and Byerlee
1977a
; Scholz
1968
; Wu and Thomsen
1975
; Yanagidani, et al.,
1985
). One description of
deformation of this type has been as ''elastic creep'', on the grounds that it can be
regarded as resulting from a decrease in the elastic modulus of the material
because of the crack proliferation (for example, Hasselman and Venkataswaran
1983
). However, this description, which seems to imply that the cracks close again
on removal of the deviatoric stress, does not adequately cover the permanent
strains (non-dilational as well as dilational) that may accompany the crack pro-
liferation. There has been considerable progress in the understanding of the phe-
nomenology and of the processes involved in brittle creep, such as its
predominantly transient or primary character, with a relative lack of steady state
behaviour but a strong tendency to a tertiary or failure stage, its relatively low
temperature and strain-rate sensitivity, reflected in a low activation energy if
suitably defined (generally less than 100 kJ mol
-1
), and the importance of crack
interaction; in addition to references already given above, see Price (
1964
),
Robertson (
1964
), Lockner and Byerlee (
1977b
;
1980
), Sasajima and Itô (
1980
),
Ohnaka (
1983
), Segall (
1984
), Costin (
1985
)and Ortiz (
1985
). However, the the-
oretical basis for modelling brittle creep is still not well developed in a fully
integrated way, incorporating both slow crack growth and evolving crack inter-
action aspects.
The other main aspect of cataclastic granular flow, complementary to the small-
strain brittle creep, is the large-strain behaviour, typified in the deformation of fault
gouge or of cataclasites or in phenomena such as hill creep. It seems likely that such
cases will also commonly involve aspects of cohesive granular flow, especially when
clays are present. Thus, whether the nature of the time and temperature dependences
will be similar to those in small-strain brittle creep is questionable. Probably a
