ASTM D790 Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials

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 D790 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 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.

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 6272-02.

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.

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