How to Perform an ASTM D790 Plastic Flexural 3 Point Bend Test

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Along with ASTM D638, D790 is one of the most commonly used specifications in the plastics industry.  This test measures the flexural strength and flexural modulus of reinforced and unreinforced plastics.  These calculations allow you to choose materials that do not bend when supporting the loads you require for your application.  The test uses a universal testing machine and a three point bend fixture to bend plastic test bars to acquire the data needed to make the calculations.  The calculations and set-up for D790 are more complex and time consuming compared to other tests so please read the entire specification from ASTM before running the test.  ISO-180 is similar in concept to this test although the specimen shapes and some other details are slightly different.  This is a simple summary to help you determine if this specification is right for you and also to call out what testing equipment is needed.

 Test Procedure:

1. Die cut, saw, or injection mold your samples to a shape specified in the ASTM specification.
   1. If you have a standard shape then you do not need to enter the depth, width, and span length for each test.
   2. If you have specimens that vary in width and depth then you will need to adjust the span length for each test as well as enter in the new width, depth, and span length as these values are used in the calculations.
2. Place the test bar on the 3 point bend fixture and begin the test and end after bending to 5% deflection or until the sample breaks.

Tips:

1. Testing speed:  The speed of the test is very slow and variable depending on the support span and depth of beam which are used in the calculation used to determine the speed.
2. Test end: The test ends when the material has acheived 5% deflection or it breaks.  The 5% deflection is determined by a calculation that takes into account the support span and depth of beam.
3. Use a standard size test bar if possible! In tensile tests that have samples that are always different sizes all you have to do is enter the width, thickness, and gage length of a sample.  However, for D790, variable specimen sizes are more burdensome because not only do you need to enter the specimen size, but you also need to adjust the 3 point bend span length and the speed of test.  A standard size allows you to run multiple tests without having to change these parameters.

Equipment Required:

1. Universal testing machine
   1. Capable of constant rate of crosshead speed
   2. Does not deflect more than 1% of the deflection of the test specimen
   3. Meet accuracy of testing machines specified in ASTM E4
2. Deflection measuring devise.
   1. This can be the standard crosshead movement indicator or use an auxiliary deflection measuring devise such as a displacement transducer.
3. Three Point Bend Fixture
   1. There are many different styles of bend fixtures available.  The standard support radii recommended by ASTM is 5mm. See more bend fixtures here.
4. Testing software
   1. Modern software performs all of these calculations automatically after the test.  We recommend ADMET’s MTESTQuattro software.