ISO 178

ISO 178 is a three-point bend test where a force is applied on the midpoint of a sample, which is supported at either end.

Last Updated: September 2nd, 2022 First Published :


ISO 178 Testing Standard

ISO 178 is a three-point bend testing procedure used to test the flexural property of semi-rigid and rigid plastic on a universal testing system. 

The test sample is placed on two parallel anvils and bent using a compression die without breaking the piece. The applied force is measured via a load cell, and the deflection is measured either by a direct strain measurement device or the system’s crosshead displacement. The specimen’s thickness is a function of the span of the three-point bend fixture. 

The ISO 178 test is stopped if the specimen breaks. If the specimen does not break, the test is continued and the stress at 3.5% (conventional deflection) is reported. 

A variety of sample shapes can be used for this test but generally, a sample with dimensions – 10 mm* 4 mm* 80 mm is used. This method is usually used for determining the flexural modulus, flexural strength, and flexural strain/stress relationship under defined conditions.

Image 01: Three-point bend test setup for plastics as per ASTM D790

The method is suitable for:

  • Thermosetting molding material, including reinforced and filled compounds
  • Thermosetting sheets
  • Thermoplastic mold casting and extrusion material, including reinforced and filled compounds 
  • Rigid thermoplastic sheet

Common Uses of ISO 178 testing

  • Determine flexural properties of rigid and semi-rigid plastic
  • Determine flexural properties of polymers

Advantages of ISO 178 testing

  • Sample preparation is easy 
  • As compared to the tension test, gripping of sample is easy

Limitation of ISO 178 testing

  • It is a destructive test
  • Displacement range and force applied are limited 
  • Different test environments require different specialized equipment accessories 

Industrial Application of ISO 178 testing

  • Aerodynamics 
  • Plastic and polymer industry
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