Overview
ISO 13967:2009 - Thermoplastics fittings - Determination of ring stiffness - specifies a laboratory test method for measuring the ring stiffness of thermoplastic bends and branches used with circular plastics pipes. The method determines the resistance of a fitting to diametric deflection by compressing the fitting between parallel plates and recording force versus deflection. The test is applicable to bends, equal and unequal branches that permit at least 4% diametric deflection and defines how to report stiffness referenced at a 3% deflection.
Key topics and technical requirements
- Scope and applicability: Applies to thermoplastic fittings (bends and branches) for circular-section pipes; relates to pipe stiffness testing (see ISO 9969) and gives conditions when fittings can be assigned pipe stiffness without testing.
- Test principle: Compress the fitting at a constant rate of deflection between parallel bearing plates and generate force–deflection data to calculate ring stiffness from the force required for a 3% diametric deflection.
- Apparatus: Compression testing machine with controlled crosshead movement, flat bearing plates and (where needed) insert plates to distribute load, accurate dimension- and force-measuring instruments.
- Specimen preparation: Complete fittings (including attachments); small protrusions that contact plates may be removed or accommodated with insert plates to improve linearity.
- Dimensions and calculation length (L): Rules for measuring inside diameter, wall height and the calculation length for bends and branches (depends on bending radius and geometry).
- Test conditions:
- Number of test pieces: three samples (labelled “a”, “b”, “c”).
- Age/conditioning: specimens aged at least 24 h before test; type testing and disputes use (21 ± 2) days. Condition at test temperature for ≥24 h.
- Test temperature: normally (23 ± 2) °C (or (27 ± 2) °C where standard laboratory temperature is 27 °C).
- Results and reporting: Calculation of ring stiffness from measured force at 3% deflection; required items for the test report and informative guidance on interpreting results (Annex A).
Applications and who uses this standard
- Manufacturers of thermoplastic pipe fittings for product development, quality control and type approval.
- Independent testing laboratories performing compliance and performance verification.
- Engineers and specifiers evaluating structural behaviour of pipe systems under external loads.
- Regulators and procurement bodies requiring documented mechanical properties for pipelines and sewer systems.
Related standards
- ISO 9969 - Stiffness of plastics pipes (uses ring-stiffness concept for pipes); ISO 13967 is aligned to emphasize comparability between pipe and fitting stiffness.
