ISO 178 specifies a method for determining the flexural properties of rigid and semi-rigid plastics under defined conditions. A standard test specimen is defined, but parameters are included for alternative specimen sizes for use where appropriate. A range of test speeds is included.

The method is used to investigate the flexural behaviour of the test specimens and to determine the flexural strength, flexural modulus and other aspects of the flexural stress/strain relationship under the conditions defined. It applies to a freely supported beam, loaded at midspan (three-point loading test).

The method is suitable for use with the following range of materials:

thermoplastic moulding, extrusion and casting materials, including filled and reinforced compounds in addition to unfilled types; rigid thermoplastics sheets;
thermosetting moulding materials, including filled and reinforced compounds; thermosetting sheets.
ISO 178:2010 applies to fibre-reinforced compounds with fibre lengths less than or equal to 7,5 mm prior to processing. For long-fibre-reinforced materials (laminates) with fibre lengths greater than 7,5 mm, see ISO 14125.