How to Perform a Foam Tensile Strength Test by ADMET
This article by ADMET
discusses foam tensile strength testing. A foam tensile strength test can be
found in ASTM D3574 Test E which applies to slab, bonded, and molded flexible
cellular urethane foams. There are many tests in this specification including
tests for density, ball rebound, airflow, etc. However, the most common tests
in the specification are the mechanical tests that can be performed on a universal
testing machine such as indentation force deflection, compression force deflection
and tear resistance. This article discusses Test E, the foam tension test.
The purpose of performing the tension test on polyurethane foam is to measure
the tensile strength and elongation. The tensile strength is obtained by simply
dividing the peak load observed during the test by the cross-sectional area
of the sample.
The foam tensile strength test is similar to testing other polymers, but gripping
and measuring extension require a bit more care. The minimum foam thickness
during testing is 12.5mm and it is common to measure thicker samples. Foams
do not typically have high tensile strengths, so low force (1kN) capacity grips
are fine. Therefore the best grips to use are wide opening (at least 20mm) 1kN
capacity vise or pneumatic grips.
Measuring extension is tricky. Because the sample is "dogbone" shaped,
using grip separation to measure elongation is less than ideal due to the error
caused by the non-uniform specimen width. Despite this, it is still common to
measure elongation using grip separation. The reason for this is because contact
extensometers clip on the foam and this distorts the sample thickness and causes
premature sample breakage. Non contact optical extensometers work well but the
high cost (often times more expensive than the rest of the testing machine)
makes them unpractical.
With the right equipment, the test is actually quite easy. A video of a foam
tensile strength test is shown here:
Run time: 1.09 mins