ISO 178 – Specimen Size

The international standard - ISO 178, specifies a method for determining the flexural properties of rigid and semi-rigid plastics under defined conditions. This method is used to investigate the flexural behavior and for determining the flexural strength, flexural modulus, and other aspects of the flexural stress/strain relationship under the conditions defined.

ISO 178 – Specimen Size

The ISO 178 method is performed using specimens that can be either molded to the specified dimensions, machined from the central section of a standard multipurpose test specimen, or machined from finished or semi-finished products, such as moldings, laminates, or extruded or cast sheets.

Recommended specimen dimensions:

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

Specimens are cut from sheet metal or manufactured by injection molding or stamping according to ISO requirements. ISO 178 provides alternative widths depending on specimen thickness when the recommended specimen dimensions cannot be achieved. For anisotropic materials, testing in both directions is required.

Common Uses of ISO 178

Specimens are cut from sheet metal or manufactured by injection molding or stamping according to ISO requirements. ISO 178 provides alternative widths depending on specimen thickness when the recommended specimen dimensions cannot be achieved. For anisotropic materials, testing in both directions is required.

There are 3 different types of deflection measurement:

• Manual crosshead displacement
• Direct device or strain gauge
• Deflectometer

Limitations of ISO 178

This method is not suitable for the determination of design parameters but can be used in materials testing and as a quality control test.

ISO 178 vs ASTM D790

• ASTM D790 allows only one test speed, whereas ISO 178 allows a second (faster) test speed to be used after the modulus is measured.
  
• ISO 178 requires the use of either a deflectometer or compliance correction in order to determine modulus. ASTM D790 can calculate the modulus from the crosshead displacement alone.
  
• Test speeds may vary between standards, as recommended sample sizes vary and test speeds are dependent on sample depth. The recommended depth for ISO 178 specimens is 4mm and the recommended depth for ASTM D790 specimens is 3.2mm.