ASTM D790, ISO 178 Flexural Properties Testing

Scope:

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.

Similar to ISO 178, the ASTM D790 standard outlines 3-point flexure testing for both rigid and semi-rigid polymers as well as fiber composites with long-fiber reinforcements. Typical test outcomes consist of – Flexural modulus, specimen break, maximum stress, and yield point stresses and strains.

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.

ASTM D790, ISO 178 Flexural Properties Test Requirements

• The specimens are either mechanically processed from sheets or plates or made via injection molding.
• For plastic molding compounds, specimens with a cross-section of 3.2 mm x 12.7 mm are typically utilized. The support span is 51 mm for the specified thickness/support ratio of 16.
• A thickness/support ratio of 16 might cause unintended shear failure in fiber composites. Larger support spans are utilized, with a ratio of 32, 40, or even 60 to the thickness of the specimen, if the ratio of tensile strength to shear strength is higher than 8.
• Small deflections are taken into account in the flexural stress and strain calculation, although friction at the supports is not. The approach is therefore restricted to a flexural strain of 5%.
• When measuring specimen measurements for a flexure test, extreme caution must be used. The measurement error result is a quadratic function since the specimen thickness is estimated quadratically in the flexural stress. With a nominal specimen height of 3.2 mm, a measurement error of just 0.1 mm results in a flexural stress error of more than 5%.

Video 01: ASTM D790 Plastic 3 point bend flexural test 

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.

Data: 

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.

Conclusion

The calculations done in ASTM D790 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.