Environmental Engineering Reference
In-Depth Information
3.4 TESTING OF POLYMERS
A wide range of plastics test methods are available and routinely used to
characterize plastics and rubber. Even a basic introduction to the common
tests used is beyond the scope of this chapter. The following introductory
discussion is on several selected test methods that will allow the reader
to better understand the test data presented in other chapters. With most
of these tests, there are an extensive theoretical foundation and numerous
practical details on how to carry them out as well as the relevant standards
thathavebeenleftout.Withmosttestsdescribedhere,theexperience ofthe
operator in sample preparation, carrying out the tests, and analysis of data
is critical to obtain reproducible results.
3.4.1 Tensile Properties
When a plastic film, pipe, or rod is extended along its long axis, it is placed
under tensile stress. Plastic films in shopping bags, plastic ropes, tape, and
fishing gear routinely undergo this type of deformation in use. Therefore,
it is important to test how the plastic behaves under such stress. Tensile
testing is designed to provide this information.
In a typical tensile test, the plastic sample in the shape of a rectangular (or
dog-bone) piece is held at its extremities in a pair of grips and slowly pulled
alongitslongaxis. Thedeformation (ortheincrease inlengthofthesample)
and the force F applied are recorded continuously. The strain increases at a
prescribed rate during the tensile test. Basic quantities involved in the test
where
f
is stress,
F
is force, and
a
is the area of cross section of the plastic
sample prior to deformation. The extension
ε
is defined as the change in
length Δ of the sample to the initial length . Lengths are measured as
the distance between the grips. Both
f
and
ε
vary continuously during
deformation, and the test is completed when the sample snaps. When
plastics deform by extension along one axis, its width and thickness
decrease as shown in the figure.
