ASTM D790 Flexural Test of Plastics & Composites

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ASTM D790 Flexural Test of Plastics & Composites
ASTM D790 Flexural Test of Plastics & Composites

ASTM D790 is used to determine the flexural strength of plastics and composites. Flexural strength is the measure of a material's ability to resist cracking or breaking under bending stress. When designing a product that needs to withstand loading or containing pressure, it is important to know which material or combination of materials will do the best job. ASTM D790 is commonly used to evaluate reinforced and unreinforced plastics, thermoplastics and thermosets, electrical insulation materials, and high modulus composites.

Why Test for Flexural Strength?

Product designers are often faced with the decision of whether or not to use plastic or composites. Flexure strength and flexural modulus are several factors that determine when a material is most appropriate for the application. ASTM D790 is used to measure how strong or stiff a material is. Flexural strength, as well as flexural modulus, are measures of a material's ability to resist cracking or breaking under bending stress. A material with high flexural strength has the ability to resist deformation when force is applied in tension or compression; it inherently will withstand bending, stretching, twisting, and other types of stress. Products that are designed with low flexural strength materials may not survive long-term use in applications like sporting equipment. For this reason, ASTM D790 is used in manufacturing industries like sports equipment, automotive, aerospace engineering, tool design, and construction.

Is ASTM D790 the Best Fit for My Flexural Test Needs?

ISO 178 is similar to ASTM D790 but has a few differences. Materials that require more than 5% strain should then use ASTM D6272 which is a 4-point bend test. If you need to measure tensile strength you should use ASTM D638.

Important ASTM D790 Calculations

ASTM D790 testing is used to determine the following flexural properties:

  • Chord modulus
  • Flexural strength
  • Flexural stress at break - Some specimens break before a yield point, in which cases the flexural strength equals the flexural stress at break.
  • Tangent modulus (or flexural modulus)
  • Secant modulus

What Specimens Are Used in ASTM D790?

ASTM D790 requires the dimensions of a specimen to be determined by its thickness (or depth) of the material.

Molded Materials

Generally 3.2 mm thick (deep), 12.7 mm wide, and 127 mm long (0.12 x 0.5 x 5 in) and use a support span equal to specimen depth times 16.

High-Strength Reinforced Composites

Reference the standard for more information on the dimensions. The dimensions of your specimen should encourage failure to happen in the outer fibers. This will make sure you are testing all aspects of fiber strength and not just an average.

Anisotropic Materials

Specimens should be cut to ensure you test the strongest fiber direction which ensures that failure only occurs due to bending forces.

Cut Sheet Materials

  • The standard sets the rules for testing sheet materials. For sheet materials less than 1.6 mm (0.06 in) in thickness the geometry shall be 50.8 x 12.7 mm (2 x 0.5 in) with a 25.4 mm (1 in) support span
  • Sheet materials with a thickness between 1.6 and 3.2 mm (0.06 and 0.12 in) will have a width of 12.7 mm (0.5 in) and a span defined as specimen thickness times 16
  • Sheet materials thicker than 3.2 mm (0.12 in) will have a width and span proportional to their thickness