Page 95 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GOX/LBD P2: GLQ Final Pages
Encyclopedia of Physical Science and Technology EN012K-587 July 26, 2001 10:35

Plastics Engineering 465

be a successful design in the long term. After a period 2. Impact tests: The ability of a plastic to resist impact
of a month, or six months, or several years, the prod- forces is one of its most practically relevant properties.
uct could fail as a result of the sustained action of the However, this is also one of its least understood proper-
static load. To avoid this, it is essential to design plastic ties. Impact strength is not an inherent material property
articles with a full awareness of the long-term material that can be used in a design calculation. It is simply a
characteristics. quantitative measure of the ability of a material to absorb
impact energy. It is known that this ability is very sensitive
to factors such as geometry, structural variations produced
D. Mechanical Properties of Plastics
by molding, the speed of the impact, and environmental
conditions. In some cases the impact strength of a plas-
1. Short-term tests: The majority of tabular data (e.g.,
tic may be assessed using an arbitrary test that matches
Table I) generated for plastics is based on short-term tests.
quite closely the service conditions of the product. How-
This is clearly because it is quick and easy to obtain such
ever, due to the difficulty of comparing impact test data
information and to a large extent it can be correlated di-
obtained in a wide range of test methods, there has been
rectly with data for other materials (e.g., metals). How-
a move in recent years toward standardization of all im-
ever, from the foregoing comments it should be apparent
pact tests. To a large extent this has been prompted by the
that this type of data is not suitable for most types of de-
desire to have a uniform data presentation format in all
sign calculations for plastic products. The short-term data
computerized databases. The Izod impact test (essentially
are supplied simply to facilitate initial sorting of plas-
flexure of a notched sample) and the tensile impact test are
tics for a particular application and to provide a basis
becoming the most widely used tests. Although it is recog-
for quality control checks. It should also be noted that
nized that the results do not always correlate well with the
the short-term data for plastics are very sensitive to test
behavior of real moldings, the tests are clearly defined and
conditions. Figure 1 illustrates how temperature and test-
are unambiguous. Thus, they provide a useful ranking of
ing speed can alter the stress–strain characteristic for a
materials to indicate which are most likely to be successful
thermoplastic. For this reason it is important to adhere
in a particular application. For research purposes, instru-
to standard, recommended test procedures. The main in-
mented impact tests are preferred in which the load and
ternational standard test methods for plastics are sum-
deformation of the material are recorded simultaneously
marized in Table III. The principal short-term proper-
throughout the brief impact event.
ties quoted for plastics are strength and modulus, using
3. Long-term tests: The two main long-term properties
a tensile and/or flexural test mode. Occasionally, data are
that are relevant for plastics are creep and fatigue. The
quoted for compression or shear behavior, but this is not
former reflects behavior under long-term static loads, and
common.
the latter reflects performance under long-term fluctuat-
ing loads. The creep behavior of plastics arises essentially
from their viscoelastic nature. Thus, when a static load is
applied, there is an almost instantaneous (elastic) increase
in strain followed by a time-dependent (viscous) increase
in strain. The latter is called creep. Similarly, if the static
loadisremovedordecreasedtherewillbeaninstantaneous
decrease in strain followed by a time-dependent decrease
in strain (called recovery). This type of strain response
to an applied stress is illustrated in Fig. 2. In the creep
situation where the strain is increasing at constant stress
(ignoring small changes in cross-sectional area), the most
practical implication is that the modulus of the material
(ratio stress/strain) is decreasing. As most design calcula-
tions involve the use of a modulus for the material, it is
very important that the correct value be used. For metals,
of course, the modulus is usually regarded as a constant,
and the values of the modulus for most common metals
are familiar to designers. However, for plastics one must
choose a modulus that is relevant to the time scale of the
FIGURE 1 Effects of temperature and testing sped on mechani- loading. For example, if the design involves the applica-
cal properties of plastics. tion of a static load for a design life of, say, five years, then