ISO 75-3:2025

Overview

ISO 75-3:2025 - "Plastics - Determination of temperature of deflection under load - Part 3" is the ISO test method for determining the temperature at which high‑strength thermosetting laminates and long‑fibre‑reinforced plastics deform under a specified bending load. It specifies procedures, apparatus and specimen requirements to measure the temperature‑dependent decrease in flexural modulus (commonly referred to as HDT or temperature of deflection under load) for materials with fibre length greater than 7.5 mm prior to processing. The method is part of the ISO 75 series and complements ISO 75‑1 (general method) and ISO 75‑2 (thermoplastics).

Key topics

• Scope: Applies to thermosetting laminates and long‑fibre‑reinforced plastics (fibre length > 7.5 mm); method has also been validated for certain thermoplastics.
• Principle: Measure the temperature at which a predefined flexural strain/deflection is reached while a constant flexural stress is applied; tracks reduction in flexural modulus with temperature.
• Test load definition: Flexural stress is set as a fraction of the initial (room‑temperature) flexural modulus (not a fixed load) - the method uses 1/1 000 of the initial flexural modulus.
• Standard deflection: Based on a flexural strain increase of 0.2% (to align with ISO 75‑2), span and specimen thickness.
• Span calculation: Span is tied to specimen thickness (conventionally span ≈ 30 × thickness) - machines commonly use spans of 64 mm or 100 mm, driving preferred thicknesses.
• Specimen dimensions: Width 9.8–12.8 mm; thickness 2.0–7.0 mm; preferred thicknesses 2.1 mm and 3.3 mm; length at least 10 mm longer than span.
• Apparatus & measurement: Heating equipment, deflection and temperature measuring instruments, weights and micrometers per ISO 75‑1 requirements.
• Preparation & conditioning: References ISO 295, ISO 10724‑1, ISO 1268, ISO 2818 and other specimen preparation standards.

Applications

• Material selection and comparison based on service temperature (HDT).
• Quality control and batch verification for composite laminates and long‑fibre compounds.
• Design guidance for components expected to carry flexural load at elevated temperatures.
• Regulatory and procurement specifications where standardized HDT values are required.
• Research and development of high‑strength thermosets and reinforced plastics.

Who uses this standard

• Materials and composite engineers
• Test and certification laboratories
• OEMs in automotive, aerospace, electrical and industrial sectors
• Quality assurance and procurement professionals specifying mechanical performance at temperature

Related standards

• ISO 75‑1:2020 - Plastics - Determination of temperature of deflection under load - Part 1: General test method
• ISO 75‑2 - Thermoplastics (see for thermoplastic specimen preparation)
• ISO 295, ISO 1268, ISO 2818, ISO 10724‑1, ISO 14125, ISO 16012 - referenced standards for specimen production, machining and flexural property measurement

Keywords: ISO 75-3:2025, temperature of deflection under load, HDT testing, thermosetting laminates, long‑fibre‑reinforced plastics, flexural modulus, test specimen dimensions, composite testing.