🔩 Ring stiffness S (kN/m²) of thermoplastics pipes of circular cross-section — compression of three pipe test pieces between parallel plates at a constant speed depending on the diameter, force at a deflection of 3 % of the diameter; PN-EN ISO 9969

Materials and NDT PN-EN ISO 9969

Overview

WHAT THE STANDARD COVERS. ISO 9969:2016 (clause 1) specifies a method for testing the ring stiffness of thermoplastics pipes of circular cross-section. Normative reference: ISO 3126 (dimensions of piping system components). Symbols (clause 3): dn — nominal diameter, di — internal diameter of the test piece, ec — construction height, F — force, F0 — preload, L — length of the test piece, p — pitch of the ribs or of the helix, S — ring stiffness in kN/m², y — vertical deflection. The "Scope" field of the PKN card of PN-EN ISO 9969:2016-02 (Polish version): "This International Standard specifies a method for determining the ring stiffness of thermoplastics pipes having a circular cross-section" (our translation).

PRINCIPLE AND APPARATUS (clauses 4–5). Ring stiffness is determined from the measurement of force and deflection while the pipe is deflected at a constant speed: a horizontally supported pipe test piece is compressed vertically between two parallel flat plates moved at a constant speed depending on the diameter, a plot of force against deflection is recorded and the stiffness is calculated as a function of the force needed to deflect the diameter by 3 %; the test temperature may be set by the referring standard. Deflection speed (Table 1) depending on the internal diameter: up to 100 mm — 2 ± 0.1 mm/min; over 100 up to 200 mm — 5 ± 0.25; over 200 up to 400 mm — 10 ± 0.5; over 400 up to 710 mm — 20 ± 1; above 710 mm — 0.03 · di ± 5 %. Plates: hard, rigid, smooth and clean, not deflecting, with a length at least equal to the length of the test piece and a width not less than the contact width under load plus 25 mm. Measuring instruments: length of the test piece to within ±1 mm, internal diameter — ±0.5 %, change in diameter in the direction of loading — 0.1 mm or 1 % of the deflection (whichever is greater); the force needed for a deflection of up to 4 % — to an accuracy of ±2 %.

TEST PIECES (clauses 6–7). The pipe is marked with a line along one generatrix, and three test pieces a, b, c are taken with ends perpendicular to the axis. The length of a test piece is the mean of 3, 4 or 6 measurements spread around the circumference (dn up to 200 mm, over 200 to below 500 mm, from 500 mm), each to within 1 mm, and the smallest may not be less than 0.9 of the largest. Mean length: (300 ± 10) mm for dn up to 1500 mm, at least 0.2 dn for larger diameters; structured-wall pipes with perpendicular ribs or corrugations are cut midway between the ribs so that the test piece has a whole number of ribs and a length of at least 290 mm (or 0.2 dn), and pipes with helical ribs — to a length equal to the internal diameter ± 20 mm, not less than 290 and not more than 1000 mm. The internal diameter of each test piece is determined as the mean of at least four measurements in the cross-section at mid-length (according to ISO 3126) or with a π circumference tape, and di as the mean of the three test pieces. At the time of testing the test pieces are at least 24 h old, and in type tests and in case of dispute — (21 ± 2) days; before testing they are conditioned in air at the test temperature for at least 24 h.

PROCEDURE (clause 8, up to point 8.4). Unless otherwise specified by the referring standard, the test is carried out at (23 ± 2) °C (in countries with a reference temperature of 27 °C — at (27 ± 2) °C; in case of dispute 23 °C; according to a note, temperature probably affects the result). If it is known in which position the stiffness is lowest, the first test piece is placed in that position, otherwise with the marking line in contact with the upper plate; test pieces b and c are rotated by 120° and 240° relative to the first one. The test piece is placed with its axis parallel to the plates and its centre under the axis of the force sensor. The plate is lowered until it makes contact and a preload F0 is applied: 7.5 N for di up to 100 mm, and for larger diameters F0 = 250 × 10⁻⁶ · di · L1 (in newtons, rounded up), with a tolerance of 95–105 %; then the deflection and force sensors are zeroed (in case of dispute, the zeroing method of point 8.6).

STATUS (PKN card as of 27.10.2026). The card of PN-EN ISO 9969:2016-02 (Polish version) has no withdrawal label: publication 23-08-2017, 14 pages, Chemistry Sector, KT 140 Plastics Pipes, Fittings and Valves, "Adopts: EN ISO 9969:2016 [IDT], ISO 9969:2016 [IDT]", "Replaces: PN-EN ISO 9969:2008 - Polish version", ICS 23.040.20. PKN search, search term "ISO 9969" (27.10.2026): there is also the English version of PN-EN ISO 9969:2016-02; the editions PN-EN ISO 9969:1997 and 2008 are withdrawn. The third edition ISO 9969:2016 (ISO/TC 138/SC 5) replaced ISO 9969:2007. In the iTeh catalogue (read on 27.10.2026) ISO 9969:2016 has the status "Published" and the stage "9093 – International Standard confirmed", and EN ISO 9969:2016 — "Published".

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database refreshed from the production server on 27.10.2026 and current PCA documents downloaded on that day). Number 9969 appears in 8 records at 5 Polish laboratories: AB 005 (two entries), AB 163, AB 237, AB 1106, AB 1256 (three entries). In all 5 current PCA documents the standard is in the same 8 rows; no entry is suspended. Forms: "PN-EN ISO 9969:2016-02" (7 records) and "PN-EN 9969:2016" (AB 005, pipes with an internal diameter of 1200–4000 mm) — the PKN catalogue has no standard with such a designation (without "ISO"). In the same entries there are standards for pipes of other materials or for other applications: PN-EN 1228:1999 (AB 005, AB 163), PN-EN ISO 13967:2011 (AB 005, AB 163), ISO 7685:2026 (AB 163), PN-EN ISO 13968:2009 (AB 1256, ring flexibility) and PN-EN ISO 13268:2023-07 (AB 1256, shaft pipes). The "Laboratories" tab ("PN-EN ISO 9969") shows 7 laboratories: these 5 Polish ones and 2 Czech ones (tab query on labcoda.pl, 27.10.2026).

WHAT THE LABORATORIES TEST (database copy and PCA documents). Ring stiffness of plastics pipes and fittings: up to an external diameter of 950 mm at a load of up to 300 kN (AB 005, compression test), thermoplastics pipes with an internal diameter of 1200–4000 mm by the force-loading method (AB 005), pipes, fittings and other products up to 100 kN (AB 163), pipes of circular cross-section up to a diameter of 1100 mm, static compression test up to 100 kN (AB 237), thermoplastics pipes in the range 0.1–150 kN (AB 1106), pipes up to a diameter of 1200 mm and shaft pipes of 200–1000 mm, load 25–30 000 N (AB 1256; there also ring flexibility).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — the age of the test pieces: at least 24 h after production, and in type tests and disputes 21 ± 2 days; a result on younger test pieces does not meet the requirements of the standard. Second — the deflection speed depends on the internal diameter (Table 1); for pipes above 710 mm it is calculated as 0.03 · di. Third — three positions: the test pieces are rotated by 120° and 240°, and the first one is placed in the position of the lowest stiffness if it can be established (point 8.2); three measurements of one test piece in one position are not this test. Fourth — structured-wall pipes: the test piece shall have a whole number of ribs or corrugations, and the cut falls midway between them (point 6.2.4); pipes with helical ribs have their own length rule (point 6.2.5). Fifth — temperature and conditioning: 23 ± 2 °C and at least 24 h at the test temperature; a note in the standard says that temperature probably affects the stiffness. Sixth — preload and zeroing: F0 according to formula (2) with a tolerance of 95–105 %, zeroing after F0 has been applied; in case of dispute — the method of point 8.6. Seventh — scope: the standard concerns thermoplastics pipes; laboratories have the initial ring stiffness of glass-reinforced thermosetting plastics (GRP) pipes in their scopes according to PN-EN 1228 (AB 1640, AB 1808), and AB 005 and AB 163 — both standards in one entry.

WHAT WE DO NOT GIVE. The fragment of ISO 9969:2016 we read ends at point 8.4. We do not give the further course of the compression (points 8.5–8.6), the formula for ring stiffness, the expression of the result or the requirements for the test report.

Method principle

Three pipe test pieces (usually 300 mm each), rotated by 120° relative to each other, are compressed one after another vertically between two rigid, parallel plates moved at a constant speed depending on the internal diameter (2–20 mm/min, above 710 mm — 0.03 · di), after the preload F0 and zeroing of the sensors; from the force–deflection plot the ring stiffness S (kN/m²) is calculated as a function of the force needed to deflect the diameter by 3 % (ISO 9969:2016, clauses 4–8). WE DO NOT GIVE the formula.

Applications

• Thermoplastics pipes of circular cross-section, including structured-wall pipes with perpendicular or helical ribs (ISO 9969:2016, clauses 1 and 6)
• Type tests (age of the test pieces 21 ± 2 days) and routine tests (age at least 24 h)
• In accreditation scopes: 5 Polish laboratories — pipes and fittings, diameters up to 950, 1100, 1200 and 4000 mm, forces up to 300 kN

Key parameters

Standard

Standard number

PN-EN ISO 9969:2016-02 (wersja polska i wersja angielska; wprowadza EN ISO 9969:2016 i ISO 9969:2016 [IDT])

Title (PL)

Rury z tworzyw termoplastycznych — Oznaczanie sztywności obwodowej

Title (EN)

Thermoplastics pipes — Determination of ring stiffness

Step-by-step procedure

1. Test pieces

   Marking the pipe with a line along a generatrix, taking three test pieces with ends perpendicular to the axis and a length according to point 6.2; measuring the internal diameter of each (point 6.3).

2. Age and conditioning

   At least 24 h after production (type tests, disputes: 21 ± 2 days), conditioning in air at the test temperature ≥ 24 h (point 6.4, clause 7).

3. Positioning

   Temperature (23 ± 2) °C; the first test piece in the position of the lowest stiffness or with the marking line at the upper plate, the next ones rotated by 120° and 240°; centre under the axis of the force sensor (points 8.1–8.3).

4. Preload and zeroing

   F0 = 7.5 N or according to formula (2), within 95–105 %; zeroing of the sensors (point 8.4).

5. Compression and result

   Compression at the constant speed from Table 1 and calculation of S from the force at a deflection of 3 % (clause 4). WE DO NOT GIVE the further course or the formula. PN-EN ISO 9969:2016-02 — current (PKN card as of 27.10.2026).

Required equipment and apparatus

Health and safety (OHS)

• TECHNICAL NOTE: the age of the test pieces matters — in type tests and disputes 21 ± 2 days (point 6.4)
• TECHNICAL NOTE: three test pieces in three positions 120° apart, not three measurements in one position (point 8.2)

Frequently asked questions