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ISO 178 | Plastics | Bend Testing

ISO 178 is used to determine the flexural properties of plastics. This test measures the flexural stress, strain, and modulus of rigid and semi-rigid plastics. 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. Two methods are outlined in the standard: method A uses a strain rate of 1%/min, method B uses two different strain rates: 1%/min for determination of the flexural modulus and %5/min or 50%/min, depending on the ductility of material, for the determination of the remainder of the flexural stress-strain graph.

Prior to conducting ISO 178, it is important to read the entire specification in the relevant ISO publication.

Click here to read the equivalent ASTM standard method, ASTM D790.

ISO 178 | Plastics | Bend Testing

To run the test, begin by die cutting, sawing, or injection molding your samples to a shape specified in the ASTM specification.

  1. Prepare specimens and measure the width and the thickness at mid-length.
  2. Adjust the support span length that you find in the standard determined by your sample size.
  3. Place the test bar on the 3 point bend fixture and begin the test at the rate specified rates in the standard.
  4. Record the force and the corresponding deflection.


  • Flexural Stress
  • Flexural Strain
  • Flexural Modulus

ISO 178 | Plastics | Bend Testing

ISO 178 | Plastics | Bend Testing

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