ISO 178 Flexure Testing of Plastics

ISO 178 is a test method for the determination of flexural properties of rigid and semi-rigid plastics.  These properties include flexural modulus and flexural strength. Instron’s 2810-400 bend fixture is fully compliant with the fixture requirements.  This bend fixture is adaptable for use with 2.0 mm radius anvils for testing specimens up to 3.0 mm thick, or 5.0 mm radius anvils for thicker specimens.  The support span requirement is based on a ratio of the specimen thickness, which varies depending on the rigidity of your material.  The 2810-400 bend fixture has an adjustable span from 10 to 200 mm.

In a 2010 update to the standard, there was a significant change to the accuracy requirements for deflection measurement.  Previously, the error of this measurement could not exceed 1% of the value.  However, in the 2010 update, operators must now use a Class 1 deflectometer per ISO 9513, or use software capable of removing machine compliance from the results.

The challenges of testing to this standard are:
  • Flexural strain measurement
  • Repeatability of calculations
  • Reporting calculations in compliance with the standard

Instron's Solution:

  • Flexural strain measurement –  ISO 178 requires that the method used to measure strain is not influenced by the compliance of the machine. Advanced strain solutions include non-contacting extensometers, such as the Advanced Video Extensometer (AVE2) and automatic contacting extensometers, such as the AutoX 750.  These solutions require little influence from the operator, resulting in improved repeatability.  Clip-on extensometers such as 2630-107 or 2630-110 attached to a deflectometer plunger also meet the measurement accuracy stated.  In general, measurement of crosshead displacement is not suitable for modulus determination unless a compliance correction is applied.
  • Repeatability of calculations –  When setting up the test, having an adequate preload is highly recommended to ensure accurate and consistent strain measurements.  How much force is applied to the specimen prior to starting the test directly impacts the repeatability of calculations such as flexural modulus. Alternatively, a slack correction can be used to ensure that the calculations are performed on the appropriate section of the testing data.
  • Reporting calculations in compliance with the standard – Instron Bluehill® Universal testing software has advanced pre-built methods available with pre-configured plastic specific calculations helping to ensure compliance.

For labs with a high volume testing Instron offers both robotic 6-axis AT6, and non-robotic 3-axis AT3 automated testing solutions.

It is important to review ISO 178:2010 in order to fully understand the test setup, procedure, and results requirements.

For more information please visit our Definitive Guide to ISO 178.

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