TestResources 100 Family Test Machine Plastic Bend Test

ASTM D638 is a common method for measuring the strength of unreinforced and reinforced plastics. This testing standard covers the tensile properties of plastics in the form of dumbbell-shaped test specimens. Specimens up to 14 mm, or 0.55 in, in thickness will be tested under conditions of pretreatment, temperature, humidity, and testing machine speed.


ASTM D638 can be used for testing phenolic molded resin or laminated materials. For many materials, there may be a specification that requires the use of this test method, but with some modifications. D638 asks to test for the following measurements:

  • tensile strength
  • elongation at break
  • elongation at yield
  • nominal strain at rupture
  • modulus of elasticity
  • secant modulus
  • Poisson’s Ratio

Testing Procedure

To begin, place your dumbbell-shaped specimen into the grips of the test machine, taking care to align the long axis of the specimen and the grips with an imaginary line joining the points of attachment of the grips to the machine. Tighten the grips evenly and firmly to the degree necessary to prevent slippage of the specimen during the test, but not to the point where the specimen would be crushed. Attach the extension indicator. The change in jaw separation is never to be used for calculating modulus or elongation. Set the speed of testing and start the machine. Record the load-extension curve of the specimen. Record the load and extension at the yield point (if one exists) and the load and extension at the breaking point.

Stress-strain curves for plastics, determined as described in this ASTM D638 test method, almost always show a linear region at low stresses, and a straight line drawn tangent to this portion of the curve permits calculation of an elastic modulus of the usually defined plastic type.

Equipment Required

Universal Test Machines feature constant rate of extension (constant crosshead speed) and constant load rate control (e.g. lb/min). ASTM D638 requires a testing machine of the constant rate- of-crosshead-movement or Constant Rate of extension (CRE) type. By means of our testing expertise and modular product design, we will help find the testing solution that is right for you. Give one of our application engineers a call today for help with creating the best budget and testing plan according to ASTM D638.

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