rial can be calculated for the material's
response at various frequencies and tem-
a
peratures. Because the responses of poly-
mers can have a viscous component, the
b
moduli that are calculated are complex
quantities. Figure 3 depicts the real part
of the complex Young's modulus plot-
ted against the temperature of the mate-
c
loading for the synthetic rubber Buna N.
The data were originally published by
Nolle (1950). This figure illustrates that
d
the glass transition occurs primarily be-
tween 0 and 20C for lower frequency
loading, but that the transition occurs at
Extension
h i g h e r t e m p e r a t u re s f o r h i g h e r f re -
Figure 2. Schematic depiction of load and extension responses of a
quency loading. These features, i.e., that
typical polymer tested in simple tension tests at different tempera-
glass-like behavior can occur both at low
tures, from temperature well below the glass transition tempera-
temperatures and at high fr equency
ture, i.e., curve a, to temperature above the glass transition tem-
loading rates, are typical of elastomers.
perature, i.e., curve d. (a) brittle fracture, (b) ductile failure, (c) cold-
The distinction between rubbery and
drawing, and (d) rubber-like behavior. (After Ward 1983.)
glassy behavior is made here to clearly il-
lustrate the impact that temperature can
have on the behavior of sealants. A distinction be-
The glass transition temperature is often
tween rubbery and viscous behavior should be
quantified by the measurement of the load and
made as well, however, since the effects of the vis-
deformation response of a material specimen
cous response of predominantly rubber-like ma-
during low strain amplitude harmonic loading
terials can be more than subtle, and since com-
experiments, creep tests and stress-relaxation
pounding of elastomers with additives can yield
tests. From the dynamic measurements of har-
monic loading tests, elastic moduli of the mate-
overly viscous formulations that behave with ex-
cessive creep and stress relaxation for sealant ap-
plications. ASTM (1991b) provides a definition for
rubbery behavior by suggesting that a specimen
made from rubber, when stretched to twice its
length for a duration of one minute, will return
upon release to 1.5 times its original length within
one minute. This definition implies that a distinc-
tion can be made between predominantly rubbery
103
and viscoelastic behaviors, while recognizing that
102
rubber behavior incorporates some viscoelasticity.
Failure of elastomers is typified by brittle frac-
101
ture that occurs only after large, nonlinear elastic
100
deformations, with any plastic deformation being
8
restricted to a very small volume of the material
10
4
0
6
around the fracture (Williams 1984). Bueche and
10
2
0
4
Berry (1959), however, have suggested that, for pre-
10
Te
mp 0
z)
2
10 cy (H
era
dictions of tensile failure, a critical stress criterion
2
tur
e( 0
0
n
ue
0
C
rather than a fracture mechanics-based criterion
40 2 1
q
Fre
)
0
is preferable. For simple tension specimen configu-
1
rations, Smith and Stedry (1960) have made mea-
Figure 3. Real part of the complex Young's modulus
surements that demonstrate the influence that
plotted against the temperature of the material and the
strain rate and temperature have on tensile fail-
frequency of the harmonic loading in a three-axis plot
ure, and have shown that a tensile failure enve-
for the synthetic rubber Buna N. (After Hearle 1982 and
lope connecting rupture points of the stressstrain
Nolle 1950.)
5
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