Overview
EN ISO 75-3:2025 - Plastics: Determination of temperature of deflection under load, Part 3 - specifies a laboratory method to measure the temperature of deflection under load for high‑strength thermosetting laminates and long‑fibre‑reinforced plastics. Adopted by CEN (EN ISO 75-3:2025) and aligned with ISO 75-3:2025, the standard updates earlier editions (supersedes EN ISO 75-3:2004) and harmonises span calculation with ISO 178.
This test determines the temperature at which a material, under a defined flexural stress, shows a specified increase in flexural strain - a key indicator of heat resistance and thermal deformation behaviour for fibre‑reinforced and laminate plastics.
Key topics and requirements
- Scope: Applies to thermosetting laminates and fibre‑reinforced plastics with fibre length > 7.5 mm (prior to processing). Method has also been validated for certain rigid thermoplastics and long‑fibre compounds.
- Principle: Measures temperature‑dependent reduction in flexural modulus; test load is based on a fraction of the initial flexural modulus to enable comparison across materials.
- Specimens: Defines specimen shape, preferred thicknesses, inspection, annealing and conditioning requirements; test conditions vary with specimen dimensions.
- Apparatus: Specifies means for applying flexural stress, heating equipment, weights, temperature and deflection measuring instruments, micrometers and gauges.
- Test parameters:
- Flexural stress set as a fraction (1/1 000) of the room‑temperature flexural modulus (as described in the standard).
- Flexural‑strain increase for the deflection point was revised to 0.2% for better alignment with ISO 75-2.
- Span calculation updated to follow ISO 178 conventions.
- Procedure & results: Procedures for calculating applied force, initial heating setup, measurement, result expression and reporting, plus precision data (Annex A).
Practical applications & users
- Who uses it:
- Materials testing laboratories and quality control teams
- Manufacturers of thermosetting laminates, long‑fibre‑reinforced plastics and composite components
- OEMs and design engineers specifying materials for elevated‑temperature service
- Certification bodies and standards compliance engineers
- Why use it:
- To compare heat‑resistance and deformation onset of composites under load
- To support material selection for structural, electrical or high‑temperature applications
- To provide standardized data for technical datasheets, product specifications and regulatory compliance
Related standards
- ISO 75 series (general guidance on temperature of deflection under load)
- ISO 75-1:2020 (see Clause 1 for additional information)
- ISO 178 (span calculation and flexural testing methodology)
Keywords: EN ISO 75-3:2025, temperature of deflection under load, thermosetting laminates, long‑fibre‑reinforced plastics, flexural modulus, plastics testing, CEN standard.