Flexural Testing of Plastics

ASTM D 790: Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials

ISO 178: Plastics - Determination of flexural properties

3 point flexural jig with deflection gage

3 point jig diagram - ASTM D790

Testing the properties of plastics can be done in many different test methods, flexural testing aka bend test is often used to determine the flexural properties of plastics. Plastics are some of the most widely used materials in todays world and the flexural test is critical in determining the flexural strength, flexural modulus, stress, strain, flexural stress at break etc.

The ASTM D790 and ISO 178 are two different standards that use very similar methods on a Universal Testing Machine. Both standards lay out the size and shapes and also the span distance (L) of the jigs, size of rolers (support and loading nose) and the speed of the test. Although similar in its testing methods and specimen shapes it is to be noted that these two standards are not considered equivilents as they do have points that are different from one another.  

The "Preferred specimen type" for ISO 178 are (L)80 ± 2mm , (W)10 ± 0.2, (Thickness)4 ± 0.2 mm

There are other shapes or dimensions that can be tested under the ISO 178 if the preferred specimen type cannot be used. (Check 6.1.3 Other test specimens)

For ASTM D790 the specimen size is dependent on the material or category as it covers a much wider range of plastics and related materials. 

Bending/flexural (3 point) test method

Test parameter inputs for 3 point flexural/bend test (Span set at 16h (4mm x 16 = 64mm in this case)

ISO 178 Fig 2 of flexural test typical curves

When testing under the ISO 178 standard the test speed of the machine should be maintained during the test between 1~500mm/min. For ASTM D790 Procedure A requires strain rate control and Procedure B uses a set mm/min speed.