ASTM D790, ISO 178 Flexural Properties Testing

ASTM D790, ISO 178 Flexural Properties Testing

Essential testing of flexural properties of Polymers and Plastics is done by using ASTM D790 and ISO 178 to ensure suitability under various end-use conditions. The flexural modulus and flexural strength of reinforced and unreinforced plastic materials are determined by this method.

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    Flexural Properties Testing by ASTM D790, a commonly used test in the plastics industry, measures the flexural modulus and flexural strength of reinforced and unreinforced plastic materials. The results obtained indicate the stiffness of the test material, enabling the choice of materials that do not bend when supporting the loads needed for a required application. 

    Flexural Properties Testing by ASTM D790 test provides a way to measure the impact required to bend a beam under the three-point loading machine. There is great importance in selecting precise materials for parts and joints that support the load without bending. Flexural modulus gauges a material’s toughness and resistance when flexed. With the temperature change, the physical properties of several materials change. Sometimes it is appropriate to test the materials at optimal temperatures simulating the end-use environment because the physical properties of several materials change with change in temperature. 

    Test Procedure: 

    ASTM D790 test requires and works on the parameters like support span, loading speed, and max deflection. Parameters are based on the sample thickness. After placing the sample on the span of the universal testing machine, an impact at the center is set up by the loading nose causing three-point bending. As per ASTM D790, the test is ceased if the sample reaches 5% deflection or breaks before reaching 5%. As per ISO 178, the test is ceased when the sample ultimately breaks. If the sample does not break, the test is continued, and the stress measured at 3.5% (conventional deflection) is recorded.

    Elevated or reduced temperature test procedure:

    The universal test machine has an installed thermal machine. The chamber allows the test mounts from the base as well as the crosshead of the machine. It is done so that test mounts pass through the top & bottom of the chamber. Set test fixtures are installed inside the chamber. Testing is conducted inside an ambient temperature. The chamber has built-in heaters for elevating the temperatures. For reduced temperatures, the chamber uses CO2 as a coolant.

    Specimen size:

    A wide variety of shapes are allowed in this test; however, the general sample size for ASTM is 3.2 mm × 12.7 mm × 125 mm (approx 0.125 inch × 0.5 inch × 5.0 inch) and for ISO is 10 mm × 4 mm × 80 mm.


    Flexural stress at yield. 

    Flexural strain at yield.

    Flexural stress at break.

    flexural strain at break.

    flexural stress at 3.5% (ISO) or 5.0% (ASTM) deflection.

    flexural modulus. 

    Stress/Strain curves and raw data.


    These calculations relate to your material’s stiffness, allowing you to select materials that will not bend when bearing the loads you need for your application. To get the data needed to do the calculations, the test employs a universal testing machine and a three point bend fixture to bend plastic test bars.


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