ISO 178 Flexural Testing of Plastics

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ISO 178 Flexural Test Machine, Bend Fixtures and Accessories
ISO 178 Flexural Testing of Plastics

ISO 178 standard defines the test method for flexural strength of plastics. Flexural strength is a material property, defined as the stress in a material just before it yields in a flexure test. ISO 178 helps determine the flexural properties of rigid and semi-rigid plastics.

How to Perform a Flexural Test on Plastic per ISO 178?

Plastics have become an essential part of many different industries due to their wide usability, being low-cost, lightweight, corrosion-resistant, and adaptable in their usage. One of the most important properties of plastic is how it reacts when you apply stress or bend it. Knowing the physical properties of plastics is very important when determining how they will perform in the real world.

What Does ISO 178 Measure?

A full ISO 178 report includes flexural properties that include flexural stress, strain, modulus, strength, flexural stress at break and at 5% flexural strain. ISO 178 must be carried out either by bending the sample with a three-point bending machine or by applying loads to the top or bottom edges of a rectangular plastic test specimen. The force applied is measured using a load cell and the resulting deflection is monitored by either the crosshead displacement value or via a direct strain measurement.

ISO 178 Specimens

Recommended specimen dimensions:

  • Thickness: 4 mm (± 0.2)
  • Length: 80 mm (± 2)
  • Width: 10 mm (± 0.2)

Specimens are either cut from sheets or created by injection molding or compression per ISO requirements. ISO 178 offers alternative widths that depend on the specimen's thickness if the recommended specimen dimensions are not achievable. Testing in both directions is necessary when the material is anisotropic.

ISO 178 Test Machines

When testing plastics per ISO 178, you can use a lower force and less expensive universal test machine. Our universal test machines maintain a constant speed of 1-500 mm/min, which falls within the standard specifications. We offer a variety of load cells that are designed to meet the requirements of the ISO 7500-1 standard, which ensures that our products report accurate force measurements. With a high level of accuracy and a long lifespan, our load cells are perfect for a wide range of applications. One of the most important things to remember when purchasing a load cell, is to select a load cell with an appropriate force capacity and verification range.

Specimen Measurement

ISO 178 requires that the thickness and width measurements are taken at the center of the specimen’s length. At a minimum, three measurements should be taken for both thickness and width. The mean is then used for both values.

ISO 178 Test Method

The experimental conditions are dependent on the type of material being tested - for example, if it's low temperature or high temperature. The ISO 291 standard can provide guidance on the environmental conditions that are needed for safe and efficient operation of a product. Our fully-integrated environmental chambers accommodate non-ambient testing.

ISO 178 specifies two methods for setting the crosshead's speed: Method A and Method B. Method A has a single speed for the entire test. Method B uses a slow test during the modulus test and then speeds up the test. These constant crosshead speeds are estimated strain rates and provide a means of quantifying the material deformation and fracture.

The test specimen is cycled through the preload stage, where it is deflected until it reaches 5% flexural strain or it breaks. Throughout the test, force and deflection data is recorded at a high data acquisition rate to accurately measure the test specimen's performance. To get accurate and reliable results, we highly recommend you preload the test. The amount of force used on the specimen before the test impacts its repeatability. A higher application of force will provide more consistent testing results, while a lower application of force will result in more variable testing results.