|These test methods employ a three
point loading system applied to a simply supported
beam. And can be used to determine the flexural properties of reinforced and
unreinforced plastics, including high-modulus composites and electrical
insulating materials. The test specimens are in the form of rectangular bars
which may be molded in that shape specifically for testing purposes, or be cut
from molded shapes, plates, or sheets.
The ASTM D 790 specifications apply to both rigid and semi-rigid materials. Flexural strength however, cannot be determined for
materials that do not break or do not fail in the outer surface of the test
specimen within the 5.0 % strain limit of these test methods.
In these test methods, both
Procedures A and B, a rectangular specimen rests
upon two supports and is loaded midway between the supports. The test specimen
is deflected until rupture occurs in the outer surface of the specimen or until
a maximum strain of 5.0% is reached, whichever happens first. The standard
support span-to-depth ratio of 16:1 is employed unless there is reason to
believe that a larger span-to-depth ratio will be needed, such as, with certain
For High Strength Reinforced
Composites, Including Highly Orthotropic Laminates the span-to-depth ratio is chosen to ensure that failure occurs in the outer
fibers of the test specimens and is a result only of the bending moment. For
these types of materials a ratio of 32:1 or 40:1 is recommended. Although when
modulus data is required, a span-to-depth ratio of 60:1 is recommended because
shear deformation can noticeably influence modulus measurements even at ratios
as high as 40:1.
Procedure A, is designed primarily for materials that break at
relatively small deflections. A strain rate of 0.01 mm/mm/min is employed, this
is the preferred test procedure. Unless the material specifications state
otherwise, procedure A is used for determining flexural properties, especially
Procedure B, employs a strain rate of 0.10 mm/mm/min and is designed for materials that
experience relatively large deflections while testing. Procedure B is used only
for determination of flexural strength.
Flexural properties as measured by
these test methods are particularly useful for
quality control and specification purposes.
If materials do not fail under the
maximum strain for these methods then they should be
tested using a 4-point bend test such as ASTM D
The specification of the
material being tested should be referenced before proceeding with these
test methods. Any testing parameters, specimen preparation, dimensions,
conditioning, or combination thereof, covered in the materials specification
shall take precedence over those mentioned in these test methods. If there are
no material specifications, then the default conditions apply. Table 1 in Classification System D 4000 lists the ASTM materials standards that
currently exist for plastics.