Chemistry Reference
In-Depth Information
4.9
StressStrain Tests
Stress-strain tests were mentioned in Section 1.8 and in
Fig. 4.14
. In such a ten-
sile test a parallel-sided strip is held in two clamps that are separated at a constant
speed, and the force needed to effect this is recorded as a function of clamp sepa-
ration. The test specimens are usually dogbone shaped to promote deformation
between the clamps and deter flow in the clamped portions of the material. The
load-elongation data are converted to a stress
strain curve using the relations
mentioned in Section 1.8. These are probably the most widely used of all mechan-
ical tests on polymers. They provide useful information on the behavior of isotro-
pic specimens, but their relation to the use of articles fabricated from the same
polymer as the test specimens is generally not straightforward. This is because
such articles are anisotropic, their properties may depend strongly on the fabrica-
tion history, and the use conditions may vary from those in the tensile test.
Stress
strain tests are discussed here, with the above cautions, because workers
in the field often develop an intuitive feeling for the value of such data with par-
ticular polymers and because they provide useful general examples of the effects
of testing rate and temperature in mechanical testing.
Dynamic mechanical measurements are performed at very small strains in
order to ensure that linear viscoelasticity relations can be applied to the data.
Stress
strain data involve large strain behavior and are accumulated in the non-
linear region. In other words, the tensile test itself alters the structure of the test
specimen, which usually cannot be cycled back to its initial state. (Similarly,
dynamic deformations at large strains test the fatigue resistance of the material.)
Figure 1.2 records some typical stress
strain curves for different polymer
types. Some polymers exhibit a yield maximum in the nominal stress, as shown in
part (c) of the figure. At stresses lower than the yield value, the sample deforms
homogeneously. It begins to neck down at the yield stress, however, as sketched in
Fig. 4.22
. The necked region in some polymers stabilizes at a particular reduced
diameter, and deformation continues at a more or less constant nominal stress until
the neck has propagated across the whole gauge length. The cross-section of the
necking portion of the specimen decreases with increasing extension, so the true
stress may be increasing while the total force and the nominal stress (Section 1.8)
are constant or even decreasing. The process described is variously called yielding,
necking, cold flow, and cold-drawing. It is involved in the orientation processes
used to confer high strengths on thermoplastic fibers. Tough plastics always exhibit
significant amounts of yielding when they are deformed. This process absorbs
impact energy without causing fracture of the article. Brittle plastics have a
stress
strain curve like that in Fig. 1.2b and do not cold flow to any noticeable
extent under impact conditions. Many partially crystalline plastics yield in tensile
tests at room temperature but this behavior is not confined to such materials.
The yield stress of amorphous polymers is found to decrease linearly with
temperature until it becomes almost zero near
T
g
. Similarly, the yield stress of
