ISO 178 – Flexural Properties of Plastics

Fixtures & span selection

Use a three-point bending fixture with two supports and a central loading nose. Support and nose radii should suit the material to minimize stress concentrations and surface damage.

• Span-to-depth ratio: Select according to ISO 178 (commonly 16:1 or 20:1). Example: 4 mm thick ? span – 64 mm (16×).
• Alignment: Ensure the loading nose contacts at midspan and supports are level and parallel; verify with a gauge block.
• Low-friction supports: Help avoid shear from support drag, especially for thin or ductile materials.

Deflection & instrumentation

For modulus, measure midspan deflection directly with a deflectometer or high-precision displacement transducer. Video extensometry can also be used to track markers at midspan. Crosshead displacement alone may include system compliance and is less precise for modulus.

• Load cell: Choose so test forces fall within 10–90% of rated capacity.
• Deflection sensor: Mount per fixture instructions; zero under minimal preload.
• Sampling: Capture data at a sufficient rate to resolve the linear elastic region and any yield or break events.

Strain rates & speeds

ISO 178 defines strain rate categories and corresponding crosshead speeds calculated from span, depth, and target strain rate. Program the machine to maintain the prescribed rate within tolerance.

• Verify span and thickness inputs are correct before speed calculation.
• Use a short preloading segment to seat the specimen on supports.
• Confirm rate using a trial specimen and recorded deflection vs. time.

Quick setup checklist

Procedure

1. Measure specimen width and thickness; compute average values.
2. Set support span according to the chosen span-to-depth ratio.
3. Install deflection sensor (or video setup) and verify zero.
4. Apply preload to seat the specimen; zero load and deflection as required.
5. Run at the programmed speed; record load and midspan deflection continuously.
6. Stop at break or at the specified strain/deflection limit; save raw data and curve.

Calculations

Ensure you use the correct dimensions (width b, thickness h) and support span L. When computing modulus, select a strain interval entirely within the linear elastic region. Document the exact calculation method and any software options used.

Reporting

• Specimen dimensions, conditioning, span, nose/support radii.
• Deflection measurement method and location.
• Programmed speed/strain rate, preload, and compliance corrections (if any).
• Results: modulus, strength, strain, curves; n, mean, SD; outlier policy.
• Instrument IDs, calibration dates, operator, and software version.

QA, verification & uncertainty

Establish a verification schedule for load, deflection, and speed. Use traceable references and document uncertainties. Run periodic proficiency checks and maintain locked test methods to prevent inadvertent changes.

Recommended Testometric setup

Model suggestions for ISO 178

Specs vary by configuration; contact FITCO for a calibrated ISO 178 method and accessory kit.

FAQs

Industries & materials

ISO 178 flexural testing is widely used in automotive, appliances, electronics, packaging, building products, and R&D labs to compare grades and verify performance. Materials include PP, PC, ABS, PA, POM, filled/FR variants, and composites. Select nose/support radii to avoid surface imprinting on softer grades.

Common pitfalls

• Incorrect span-to-depth ratio causing non-representative stress/strain results.
• Using crosshead displacement for modulus without accounting for system compliance.
• Misalignment at midspan leading to torsion or uneven loading.
• Support/nose radii too small, inducing surface damage.

Span planner (quick reference)

Pick a common thickness and see example spans for 16× and 20× rules of thumb. Always use the exact ratio required by your control plan, and report your actual L and h.