Geoscience Reference
In-Depth Information
An extrapolation method, based on a partial rheological model, has been
presented by McGown et al. (1984). A related approach uses the extrapolation of
isochronous stiffness and time correlation curves (Andrawes et al. 1986). In a
more recent paper, Sawicki and Kazimierowicz-Frankowska (1998) have shown
that, within sufficient engineering accuracy, a standard rheological model can
describe the creep response of many geosynthetics under constant and step-
increasing loads.
Another class of models, admittedly more complex than rheological
models, is that based on integral techniques. For example, the multiple integral
technique suggested by Onaran and Findley (1965) has been found useful in
representing the nonlinear viscoelastic behavior of a range of geotextiles and
geogrids (Kabir, 1988). According to this technique, for uniaxial creep at some
load p
p 2
p 3
1
¼
R
ð
t
Þ
p
þ
M
ð
t
Þ
þ
N
ð
t
Þ
ð
5
Þ
For constant loading of geotextiles and polymers, the kernal functions R,
M, and N are expected to take on the following form:
v 1 t n
R ð t Þ¼
m 1 þ
ð
6
Þ
M ð t Þ¼ m 2 þ v 2 t n
ð 7 Þ
v 3 t n
N
ð
t
Þ¼
m 3 þ
ð
8
Þ
where m 1 , m 2 , m 3 , v 1 , v 2 , v 3 represent temperature-dependent material functions
and n is a function of the material that may or may not be a function of
temperature. Equations (6)-(8) are then substituted into Eq. (5) to give a single
expression for strain. The seven parameters associated with this model are
determined by fitting the results of creep tests for at least three different loads
(Kabir, 1988).
A related approach has been proposed by Findley et al. (1976), who
represented the creep behavior of nonlinear viscoelastic materials by a series of
“multiple integrals.” However, to effectively use the model, the magnitude of the
loading must be known a priori. Thus, if tertiary creep is to be predicted, creep
tests to failure must be performed. Using the model of Findley et al. (1976),
Helwany and Wu (1992) were able to simulate the creep response of a
polypropylene composite, heat-bonded geotextile and a polypropylene
nonwoven, heat-bonded geotextile. Stress levels used in the creep tests were
not high enough to result in tertiary creep, however, so the assessment of the
model was incomplete.
Perkins presented a more rational constitutive model for geosynthetics
(Perkins, 2000). In this model, the elastoplastic response combines orthotropic
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