|
The basic difference between
ASTM D 790 and ASTM D 6272 is the location of the maximum bending moment
and maximum axial fiber stress. In four point bending the maximum axial fiber
stress is uniformly distributed between the loading noses. In three point
bending the maximum axial fiber stress is located immediately under the loading
nose.
These test methods employ a
four 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 6272-02 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 two points (by means of two loading noses), each an
equal distance from the adjacent support point. The distance between the
loading noses (the load span) is either one third or one half of the support
span. 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 laminated materials.
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 flexural modulus.
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.
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. |
