Overview
EN ISO 13967:2009 - "Thermoplastics fittings - Determination of ring stiffness" is a CEN-endorsed ISO method for measuring the ring stiffness of thermoplastic pipe fittings (bends and branches) intended for use with circular plastic pipes. The standard defines how to compress a fitting across its diameter and calculate the stiffness from the force required to produce a defined diametric deflection (reference: 3% deflection). It applies to bends, equal branches and unequal branches provided the fitting allows at least 4% diametric deflection.
Key topics and requirements
- Test principle: Compress the fitting between two parallel plates at a constant rate of deflection and record force vs. deflection data. Ring stiffness is calculated from the force needed for a 3% diametric deflection.
- Scope of fittings: Bends, equal and unequal branches; fittings with comparable wall construction/material/diameter to a tested pipe may be assigned the same stiffness class without testing (per notes).
- Apparatus:
- Compression testing machine with a specified constant crosshead speed (varies by nominal diameter).
- Flat bearing plates and, when needed, insert plates to distribute load and avoid loading sockets/spigots.
- Accurate dimension and force measuring instruments.
- Test pieces:
- Use three complete fittings (marked “a”, “b”, “c”).
- Small protrusions that affect contact may be removed or accommodated by insert plates.
- Measurement accuracy:
- Inside diameter at the deflection point to within 0.2% or 0.1 mm (whichever is greater).
- Change in inside diameter (deflection) to 0.1 mm or 1% of deflection accuracy (whichever is greater).
- Force measured to within 2% up to 4% deflection.
- Test setup details:
- Load must be applied only to the fitting body (excluding sockets/spigots).
- Calculation length L excludes sockets, spigots and specified transition zones.
Applications
- Quality control and type testing of thermoplastic fittings (elbows, bends, branches).
- Product certification, compliance testing and comparison of fittings for mechanical performance.
- Supporting design and selection of fittings in plastic piping systems where resistance to diametric deflection under service loads is critical (sanitary, stormwater, drainage, buried piping systems).
- Used by manufacturers, independent testing laboratories, certification bodies, piping system designers and standards/regulatory authorities.
Related standards
- ISO 9969 - ring stiffness test method for pipes (provides context for classifying fittings relative to pipe stiffness).
- EN ISO 13967:2009 is an EN-adopted version (CEN endorsement) of ISO 13967:2009.
EN ISO 13967:2009 is the reference method when you need a reproducible, standardized way to quantify the circumferential stiffness of thermoplastic fittings for engineering specification, compliance and product labeling.
