dε/dt = F(σ,T)
(11)
damental damage" mechanism. Recently, Mossa-
lam and Bank (1991), and Mossalam and Cham-
where ε = strain
bers (1995) presented a simplified and efficient
t = time
design procedure to predict the deflection of
F(σ,T) = function of stress σ and temperature T.
pultruded composites under sustained load, and
a laboratory procedure for determining the creep
In the case of composites, F is a function of the
coefficients. Thus, while a large volume of infor-
stresses produced in all the components, since
mation is available on the creep characteristics of
the net creep resistance will depend on the creep
FRP materials in general, the specific information
resistance of each of the components. If the two
on whether FRP rebars will creep under sustained
components have two different creep resistances,
loading is very scant.
the creep of the low-resistance component will be
In this investigation, the scope of the creep
checked by the high-resistance material, owing to
study was limited to determining whether the
adhesion between them. Thus, with a higher bond
commercially available FRP rebars would creep
strength between the components, a creep resis-
under a sustained tensile load over a wide range
of temperatures: low temperature (23C, 10F),
tance even greater than that of its components
room temperature (21C, 70F), and high tempera-
should result.
ture (49C, 120F). Because these rebars had fibers
Creep in polymeric composites has been the
subject of investigation for a long time (Glaster
generally oriented in the longitudinal direction,
et al. 1983, 1984). Tunik and Tomashevskii (1974)
the load was carried primarily by the fibers.
discussed creep and the long-time strength of glass
FRP in interlaminar shear. Weidmann and
Test description
Ogorkiewicz (1974) studied the tensile creep of a
Commercially available fiberglass composite
rebars (Fig. 56) made with 5- to 10-m E-glass
unidirectional glass fiber epoxy laminate. The
creep strength of discontinuous fiber composite
fibers in a polyester resin matrix were selected
has also been studied by Bocker-Pedersen (1974).
for this creep study. The mechanical characteris
The power law approach to modeling the creep
behavior of plastics and FRP is primarily due to
the original work by Findley (1960), which he
again updated in 1987. Numerous other projects
about creep behavior of FRP in general have also
been reported in composites literature. These
include the work on creep in FRP beams by
Holmes and Rahman (1980). Brinson et al. (1980),
Hiel and Brinson (1983), and Dillard and Brinson
(1983) used numerical methods of predicting creep
and delayed failures. Transverse creep and the ten-
sile behavior of composite laminates were stud-
ied by Eggleston (1994), and Huang and Gibson
(1990) performed both theoretical and experi-
mental studies on sandwich beams with linear
viscoelastic cores. The creep behavior of Kevlar/
epoxy composites was studied by Beckwith (1984),
who concluded that the creep behavior in the lami-
nate composites was primarily "fiber-dominated"
and independent of resin modulus. Krish-
naswamy (1991) presented the results of a finite-
element model of the ductile behavior of poly-
mers. The creep effects in composite columns were
studied by Chen and Lottman (1991), Ueng (1991),
and Vinogradov (1989). Slattery (1994) developed
Figure 56. Examples of com-
the procedure for predicting the accelerated fail-
mercially available glass fiber
ure rate by extrapolating short-term data and by
reinforced composite rebars.
taking into consideration the "progression of fun-
42
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