How to Perform an ASTM D638 Plastic Tensile Strength Test

  

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ASTM D638 ( D 638 ) is one of the most common plastic strength specifications and covers the tensile properties of unreinforced and reinforced plastics.  This test method uses standard "dumbell" or "dogbone" shaped specimens under 14mm of thickness.  A universal testing machine (tensile testing machine) is needed to perform this test.  If you are going to perform this test, you should read the entire specification from ASTM. This is a quick summary to decide if this test is right for you, and to point out what equipment you need to perform the test.

First off, do not perform this test if you have films or elastomers.  If you have film under 1mm in thickness use ASTM D882.  If you have an elastomer use ASTM D412.

Test Procedure:

1. Cut or injection mold your material into one of the five "dumbbell" shapes.  The exact shape you use is dependent upon your material's rigidity and thickness.
2. Load the specimen into tensile grips.
3. Attach the extensometer to the sample
4. Begin the test by separating the tensile grips at a constant rate of speed.  Speed depends on specimen shape and can range from 0.05 - 20 inches per minute.  The target time from start of test to break should be from 30 seconds to 5 minutes.
5. End the test after sample break (rupture)

Analysis obtained: 

1. Tensile Strength
2. Percent Elongation at Yield
3. Percent Elongation at Break
4. Nominal Strain at Break (Grip Separation)
5. Modulus of Elasticity
6. Secant Modulus
7. Poisson's Ratio (Requires Transverse Extensometer)

Equipment required:

1. Universal testing machine (tensile testing machine)
   
   1. Needs to be servo controlled to keep a constant rate of speed.
   2. Capacity needs to be enough for your materials.  Usually a low force 2,000 pound dual column machine is enough for most common materials.  A high capacity 10,000 pound (or more) model is required for larger samples and/or stronger materials such as reinforced plastics or composites.
2. Extensometer
   1. Required when measuring modulus, yield, and modulus. Why?  For two reasons: 1) The linear region of plastics is very small and happens suddenly so grip separation is just not accurate enough.  2) Dumbbell specimens do not have uniform widths so there will be errors when both the wide and narrow sections of the dumbbell shaped specimen elongate at different rates. 
3. Data Acquisition
   1. Software or suitable electronics are required to operate the machine and to take the measurements. Basic systems will provide the raw data, and stress-strain charts. Using these sources of data, you can determine and calculate all of the analysis listed above. However, fully PC based systems have the capability to calculate all of these automatically. For example, our MTESTQuattro testing software has built in support for ASTM D638 and all of these calculations are provided immediately after performing the test.
4. Tensile Grips
   
   1. Any grip with serrated faces is usually adequate for this test.  You can use wedge, pneumatic, vise, or other self tightening grips such as eccentric roller or scissor grips.