fatigue in composites. Sustained load over a
combination of elastic deformation and viscous
period of time may cause the material to creep.
flow. When the stress is removed after a period
It is important, however, to note that because
of time, the elastic deformation is immediately
of the relative newness of these materials, there
recovered, but the deformation caused by the vis-
is a considerable gap in the definitive durability
cous flow recovers slowly to an asymptotic value,
data of polymeric composites. Systematic inves-
called the recovery strain.
tigations to predict the life of most commonly
In FRP composites, the creep strain depends on
acceptable fiber composites in the civil engineer-
the stress level and temperature and is a function
ing construction environment are rare, and in
of the distribution of stress between the fiber and
many cases, the data available are not relevant to
the matrix. In general, highly cross-linked thermo-
practical applications.
setting polymers exhibit lower creep rates than
Extreme changes in the temperature of com-
thermoplastic polymer composites. With the
posite materials result in several important effects
exception of aramid fibers, commercial reinforc-
(Dutta 1994, Lord and Dutta 1988). Most materi-
ing fibers such as glass and carbon do not creep
als expand when the temperature rises. In fiber-
appreciably at normal loads (Mallick 1988).
reinforced polymer matrix composites, the coef-
ficient of thermal expansion of the matrix is
Fatigue properties
usually an order of magnitude greater than that
The fatigue property of a structural material
of the fibers. A decrease in temperature, due
represents its response to cyclic loading. Repeated
either to cooling during the fabrication process or
cyclic loading usually changes the strength prop-
to low-temperature operating conditions, will
erties of metallic materials. The FRP composites,
cause the matrix to shrink. Contraction of the
on the other hand, are generally well known for
matrix is resisted by relatively stiff fibers through
their excellent fatigue behavior. One of the major
fiber-matrix interface bonding, setting up residual
problems of predicting the fatigue failure limit of
stresses within the material microstructure. How-
FRP composites is the complexity in assessing the
ever, except for a severely cold environment, the
modes of failure (Lorenzo and Hahn 1986). A
induced residual stresses are not of much concern.
unique feature of a fiber composite material in
Most resin matrix materials become stronger and
fatigue testing is that it exhibits a gradual soften-
stiffer, but brittle, as they are cooled. These
ing with increased cycling. Thus, tests are some-
changes can influence the modes of failure.
times done not to a failure represented by the
Unless an FRP composite contains a significant
separation of the specimen, but to a predeter-
percentage of interconnected voids that are filled
mined limit of specimen stiffness or residual
with water, the freeze-thaw effects on the strength,
strength.
within the normal range of temperature (+30C to
20C), (+86F to 4F), are insignificant. Com-
Fire hazards and flammability
mercially available glass fiber composites usually
Although the use of FRP composites in struc-
contain about 0.4% voids, which does not allow
tural construction is increasing at a rapid pace, at
any appreciable frozen moisture to cause any se-
present the coverage of these materials in con-
rious damage.
struction and building codes is not extensive.
The effect of ultraviolet (UV) light on polymeric
Usually the construction authorities, early in the
compounds is well known (Larsson 1988). On
selection process, establish the fire resistive
prolonged exposure to sunlight, the matrix may
requirements of the selected composites that may
harden and discolor. The problem is generally
have to be verified by fire tests. Where fire haz-
overcome by adding a UV-resistant coating to the
ard exists, the fire hazard characteristics (such as
composites. Of major concern is the degradation
the intended use of the structure to be designed,
of reinforcing polymeric fiber such as aramid.
potential ignition sources, potential mode of flame
However, the effect is a self-screening type--that
and smoke spread, and the means for detection,
means only the skin of the composite structure is
suppression and extinguishment) are identified
affected. So, in thicker composites, the degrada-
and the requirements determined. The specific
tion effect is minimal on structural properties.
standards for plastics in a model building code
have been summarized by Heger (1981). A review
of industry literature on FRP composites shows
Creep behavior
Creep is the increase in strain with time at a
that flammability properties are usually specified
constant stress level. Creep occurs because of a
by the manufacturers.
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