ISO 178:2019 PDF

Overview - ISO 178:2019 (Plastics - Determination of flexural properties)

ISO 178:2019 specifies a laboratory method for determining the flexural properties of rigid and semi‑rigid plastics using a three‑point bending (freely supported beam loaded at midspan) test. The standard defines preferred test specimen dimensions, acceptable alternative sizes, a range of test speeds, and measurement/recording requirements to determine flexural strength, flexural modulus, flexural strain and other aspects of the stress–strain/deflection relationship. ISO 178 is intended for materials testing and quality control rather than as a direct design specification.

Key technical topics and requirements

• Test principle: Three‑point loading of a freely supported beam (midspan load) to produce flexural stress/strain curves.
• Specimens: Preferred specimen geometry is specified; specimens may be molded or machined from multipurpose bars, finished mouldings, laminates, extruded/cast sheet (see ISO 20753). Tests on non‑standard dimensions can give non‑comparable results.
• Materials covered: Thermoplastic and thermosetting moulding, extrusion and casting materials, including filled/reinforced compounds; rigid thermoplastic and thermosetting sheets. Applies to fibre‑reinforced compounds with fibre lengths ≤ 7.5 mm prior to processing. Long‑fibre laminates (>7.5 mm) are covered by ISO 14125.
• Measurement systems: Requirements for test machines, supports, loading edge, force and deflection measuring (including deflectometers), and calibration/verification differentiated by test type.
• Test conditions: Specimen conditioning, test speed (range provided), and dimensional measurement influence results; conventional deflection and flexural modulus calculation methods are specified (flexural modulus is determined between defined strain points).
• Limitations: Not normally suitable for rigid cellular materials or sandwich structures (see ISO 1209‑1 / ISO 1209‑2). Not intended to provide design parameters.

Practical applications - who uses ISO 178

• Materials scientists and R&D labs evaluating new thermoplastic/thermosetting formulations and reinforced compounds.
• Quality control teams in plastics manufacturing for batch acceptance and process control.
• Compounders and converters validating effects of fillers, reinforcements and processing (e.g., injection‑moulding skin layer effects).
• Test laboratories and certification bodies performing standardized mechanical property reporting for product datasheets and material comparison.

Related standards (commonly referenced)

• ISO 14125 - fibre‑reinforced plastics (four‑point bending, laminates)
• ISO 10350‑1 / ISO 10350‑2 - material data presentation (fibre‑reinforced compounds)
• ISO 1209‑1 / ISO 1209‑2 - testing of cellular/sandwich plastics
• ISO 20753, ISO 291, ISO 293, ISO 294‑1, ISO 295, ISO 16012, ISO 2818, ISO 7500‑1, ISO 9513

Using ISO 178 ensures repeatable, comparable measurement of flexural strength and modulus for plastics testing, supporting material selection, quality assurance and regulatory reporting.