ISO 178 Flexural Testing: Specimen Size, Geometry & Test Conditions
What Is ISO 178?
ISO 178 is the international standard for the determination of flexural properties of rigid and semi-rigid plastics. It defines test specimen geometry, preparation, conditioning, test equipment, and calculation procedures for flexural modulus, flexural strength, and flexural strain at maximum stress or at break.
ISO 178 is the international equivalent to ASTM D790 and is widely used in Europe, Asia, and globally for material qualification, product development, and regulatory compliance in the automotive, electronics, and packaging industries.
Specimen Geometry in ISO 178
Specimen dimensions in ISO 178 are critical because the flexural test is sensitive to specimen size—particularly the ratio of span length to specimen depth (support span-to-depth ratio), which determines whether the test measures primarily bending stress or is confounded by shear stress.
Standard ISO 178 Specimen Dimensions
The preferred specimen per ISO 178 is an 80 mm × 10 mm × 4 mm bar (length × width × depth).
Support Span
ISO 178 specifies a support span-to-depth ratio of 16:1. For the standard 4 mm thick specimen, this gives a support span of 64 mm.
Specimen Preparation Methods
Specimens are prepared by:
- Injection molding — to produce specimens directly in the standard geometry (preferred for quality control)
- Machining — cutting specimens from sheet, plate, or finished parts
- Compression molding — for thermoset materials
Key Specimen Parameters and Their Effects
Parameter | Standard Value | Effect of Deviation |
Length | 80 mm | Affects the overhang at supports |
Width | 10 mm | Affects the bending moment calculation |
Depth (thickness) | 4 mm | Most critical; directly affects stress and modulus |
Span/depth ratio | 16:1 | Low ratio → shear-dominated; reduces apparent modulus |
Surface finish | Smooth | Roughness affects failure initiation in flexure |
Test Procedure
The specimen is supported on two rounded supports and loaded centrally by a single loading nose (three-point bending) at a specified test speed:
- Test speed for modulus: 2 mm/min
- Test speed for strength: 2 mm/min (most materials); alternative speeds defined for materials showing excessive creep
Flexural stress (σ) and strain (ε) are calculated from the load-deflection curve using beam theory formulas:
- Flexural stress: σ = 3FL / 2bh²
- Flexural strain: ε = 6dh / L²
- Flexural modulus: E = σ/ε (from initial linear slope)
Where F = applied force, L = support span, b = width, h = depth, and d = deflection.
Differences Between ISO 178 and ASTM D790
Feature | ISO 178 | ASTM D790 |
Preferred span/depth ratio | 16:1 | 16:1 (Method I) |
Standard specimen | 80×10×4 mm | 127×12.7×3.2 mm |
Test speed | 2 mm/min | Calculated from span and depth |
Modulus strain range | 0.05%–0.25% | Tangent at origin |
Results from the two standards are broadly comparable for most materials but should not be directly interchanged without cross-validation.
Conclusion
ISO 178 is a fundamental international standard for evaluating the flexural properties of plastics, providing critical data on stiffness, strength, and deformation under bending loads. Defining precise specimen geometry, test conditions, and calculation methods, it ensures consistent and comparable results across laboratories and industries.
Used widely for material selection, quality control, and product design, ISO 178 helps engineers understand how plastics will perform in real-world applications involving bending stresses. Ultimately, it plays a key role in ensuring reliable, high-performance plastic components across global markets.
Why Choose Infinita Lab for ISO 178 Flexural Testing?
Infinita Lab offers ISO 178 and ASTM D790 flexural testing services with precision test frames, calibrated fixtures, and accredited test methods. Our team provides expert specimen preparation, testing, and reporting for material qualification and product development programs.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
Why is specimen thickness so critical in ISO 178 flexural testing? Flexural stress is inversely proportional to the square of specimen depth (h²). A small deviation in thickness causes a larger-than-proportional error in calculated stress and modulus. Specimen thickness must be measured accurately (to 0.01 mm) and must be uniform across the gage length.
Can ISO 178 be used for fibre-reinforced plastics? Yes, but ISO 14125 is the preferred standard for unidirectional fibre-reinforced polymer composites. ISO 178 is used for short-fibre-reinforced thermoplastics and general-purpose plastics. For long-fiber composites, the span/depth ratio may need to be increased to avoid shear-dominated failure.
What conditioning is required before ISO 178 testing? ISO 291 specifies standard conditioning at 23°C ± 2°C and 50% ± 5% RH for a minimum of 16 hours (with 88 hours recommended for hygroscopic materials). Correct conditioning is essential for reproducible results, particularly for polyamides and other moisture-sensitive polymers.
What is the difference between flexural modulus and tensile modulus for plastics? Flexural modulus (from ISO 178/ASTM D790) and tensile modulus (from ISO 527/ASTM D638) theoretically measure the same property (Young's modulus in the elastic range) but may differ slightly due to differences in strain distribution (inhomogeneous in bending vs. uniform in tension), specimen geometry, and residual stresses. For most homogeneous plastics, the two values are within 5–10% of each other.
Can flexural specimens be cut from injection-molded finished parts? Yes, but the specimen orientation relative to the flow direction and gate location must be documented, as fiber orientation and residual stress from molding can cause significant differences in flexural properties compared to standard injection-molded test bars.
