ISO 9969 is the test method; the SN class is what a product standard does with the number it produces. ISO 9969:2016 specifies how to compress a ring cut from a thermoplastic pipe between two parallel plates at a diameter-dependent speed, and convert the force at 3 % diametric deflection into ring stiffness, S, reported in kN/m² [1]. The method itself neither passes nor fails a pipe. Product standards such as EN 1401-1 and the EN 13476 series take the measured value and compare it with a declared minimum — SN4, SN8 or SN16 [4][5]. If you need a ring stiffness test pipe result that a third-party laboratory will reproduce, the two variables that decide your answer are the measured inside diameter and the rigidity of the plates.
What ISO 9969 Actually Determines
ISO 9969:2016 is titled Thermoplastics pipes — Determination of ring stiffness and applies to pipes with a circular cross section [1]. Clause 4 fixes the principle: a cut length of pipe is supported horizontally, compressed vertically between two parallel flat plates, and a force–deflection plot is produced. Ring stiffness is then a function of the force needed to reach a 3 % diametric deflection [2].
Two consequences are routinely missed. The reported figure is a secant property, not a tangent stiffness and not a modulus — it describes the ring at one specific deflection. And the standard sets no acceptance threshold; that is supplied by the referring product standard, as Clause 8.1 says explicitly [2].
That is why a report can carry a valid ring stiffness value that still fails a purchase specification, and why this page sits one level below our plastic pipe testing standards overview.
Apparatus: The Tolerances That Decide Your Result
Clause 5 sets four apparatus requirements, and each one is a measurable acceptance criterion for a laboratory rather than a purchasing detail [2].
| Apparatus element | Requirement in ISO 9969:2016 | What failure looks like |
| Compressive testing machine (5.1) | Constant rate of crosshead movement, sufficient force and travel for the specified deflection | Speed drifts under load; the force–deflection plot is no longer at constant deflection rate |
| Parallel plates (5.2) | Hard and rigid, flat, smooth, clean; length ≥ test piece length; width ≥ contact width under load + 25 mm | Plates bow, the specimen centre is unloaded, and every result reads soft |
| Dimensional measuring devices (5.3) | Length ± 1 mm; inside diameter ± 0.5 %; change of inside diameter 0.1 mm or 1 % of deflection, whichever is greater | The 3 % deflection target is computed from the wrong reference |
| Force measuring device (5.4) | Accuracy ± 2 % of the force needed for a deflection of up to 4 % of the test piece | Bias in every reported stiffness, invisible in the plot |
The plate requirement is the one that costs money to get wrong: the standard states it functionally, so demonstrating compliance takes a stiffness calculation or a load–compliance test of the frame [2].
Deflection Speed: One Table, and the 2016 Change
Deflection speed is keyed to the inside diameter, dᵢ, of the test piece — not to the pipe’s nominal size [2].
| Inside diameter dᵢ (mm) | Deflection speed (mm/min) |
| dᵢ ≤ 100 | 2 ± 0.1 |
| 100 < dᵢ ≤ 200 | 5 ± 0.25 |
| 200 < dᵢ ≤ 400 | 10 ± 0.5 |
| 400 < dᵢ ≤ 710 | 20 ± 1 |
| dᵢ > 710 | 0.03 × dᵢ ± 5 % |
The 2007 to 2016 change
ISO 9969:2007 indexed the same five bands to nominal diameter, dₙ [3]. The 2016 third edition re-indexed them to measured inside diameter and added the pre-load force F₀ as a defined symbol [1][2][3]. If your laboratory still runs a 2007-era recipe, that is the first thing to change.
Test Pieces: Length, Numbering and Structured Walls
Three test pieces — a, b and c — are taken from one pipe marked along a generatrix for its full length, with ends cut perpendicular to the pipe axis [2]. Length follows four different rules depending on construction.
| Pipe construction | Required average test piece length |
| dₙ ≤ 1 500 mm, solid wall | (300 ± 10) mm |
| dₙ > 1 500 mm | at least 0.2 dₙ |
| Perpendicular ribs, corrugations or other regular structures | the minimum whole number of ribs giving ≥ 290 mm (or ≥ 0.2 dₙ above 1 500 mm), cut at the mid-point between ribs |
| Helical ribs, corrugations or other regular structures | equal to the inside diameter ± 20 mm, but not less than 290 mm and not more than 1 000 mm |
Length is measured three to six times around the perimeter — 3 measurements to dₙ 200, 4 between dₙ 200 and 500, 6 at dₙ 500 and above — then averaged, with the smallest not less than 0.9 times the largest [2]. Inside diameters dᵢₐ, dᵢᵦ and dᵢ𝚌 come from ISO 3126, either as the average of four or more mid-length measurements to ± 0.5 % or by π tape; dᵢ is their arithmetic mean [2].
These rules exist because wall stiffness comes from the profile: a specimen cut through the middle of a rib is not the wall the pipe is made of, and the result is not reproducible.
Conditioning, Temperature and Specimen Age
ISO 9969 runs on two clocks, and both are minimums rather than targets [2].
| Condition | ISO 9969:2016 requirement |
| Test temperature (8.1) | (23 ± 2) °C; or (27 ± 2) °C where 27 °C is the national standard laboratory temperature. Disputes: (23 ± 2) °C |
| Conditioning (Clause 7) | In air at the test temperature for at least 24 h immediately before testing |
| Age of test pieces (6.4) | At least 24 h; for type testing and in case of dispute, (21 ± 2) days |
The age requirement is the one laboratories underestimate. A thick-wall pipe stored unheated for three weeks has the right age but the wrong temperature, and ring stiffness falls as thermoplastics warm. Conditioning is through-wall, so the time must be long enough for the middle of the wall to reach equilibrium.
The Procedure, Step by Step
Clause 8 is short and specific, and most reproducibility problems trace back to the first two steps [2].
- Position. If the orientation of lowest ring stiffness can be determined, piece a goes in that position; otherwise piece a is placed with the marking line or split line against the upper plate. Pieces b and c are rotated 120° and 240° relative to piece a.
- Alignment. Attach the deflection gauge, check the angular position against the upper plate, set the longitudinal axis parallel to the plates, and put the specimen’s middle point vertically under the load cell centre-line.
- Pre-load. Lower the plate until it touches the specimen, then apply the pre-load F₀ and zero the deflection gauge and load cell.
| Condition | Pre-load F₀ |
| dᵢ ≤ 100 mm | 7.5 N |
| dᵢ > 100 mm | F₀ = 250 × 10⁻⁶ × dᵢ × L, in newtons, rounded up to the next newton — dᵢ and L in millimetres |
The applied pre-load must fall between 95 % and 105 % of the calculated value, measured with the load cell used for the test [2].
- Load and record. Compress at the Table 1 speed, recording force and deflection continuously until the deflection reaches at least 0.03 dᵢ. Where the wall construction height deforms by more than 5 %, the force–deflection plot must be built from measured inside-diameter change rather than crosshead travel [2].
Step 4 is where a machine earns its price: crosshead travel includes frame and plate compliance, an internal extensometer inside the bore does not.
Calculating Ring Stiffness: The Correction Term
Ring stiffness is calculated from the force at 3 % diametric deflection:
with F in kN, L and y in metres, giving S in kN/m². At the reporting point, y / dᵢ = 0.03, so the bracketed correction term evaluates to 0.01935 [2].
That correction is why an ISO 9969 ring stiffness result is not numerically interchangeable with a North American parallel-plate result. ASTM D2412 reports pipe stiffness PS and stiffness factor SF from the same physical arrangement but references 5 % deflection and omits the correction term entirely [10]. Different reference deflection, different formula, different quantity — a D2412 result cannot be converted into an SN classification.
Zero-point correction
The reported ring stiffness is the mean of the three test pieces. Where the curve suggests the zero point is wrong, the standard’s remedy is to extrapolate the initial straight portion back to the horizontal axis and use that intersection as the corrected zero [2].
Reading an ISO 9969 Test Report
A report that satisfies Clause 10 lets a second laboratory repeat the test. Use it as a checklist rather than a summary [1][2].
| Report item | Why it matters |
| Reference to ISO 9969 and the edition used | 2007 and 2016 give different speeds and pre-load treatment |
| Pipe identification: manufacturer, material, dimensions, wall construction | The stiffness value is meaningless without the profile |
| Measured inside diameter dᵢₐ, dᵢᵦ, dᵢ𝚌 and mean dᵢ | The 3 % target is derived from it |
| Measured test piece lengths and the angular positions used | Reproducibility depends on both |
| Conditioning temperature and duration; test temperature | The largest single source of laboratory-to-laboratory spread |
| Force at 3 % deflection for each piece, and each individual ring stiffness | Lets you see scatter, not just the mean |
| Mean ring stiffness in kN/m² | The number the product standard compares |
| The SN class the result satisfies, when claimed | Distinguishes a measured value from a declared class |
| Cracking, buckling or delamination observed | Relevant even though ISO 9969 stops near 3 % deflection |
Where the SN Class Comes From
SN is a classification, defined in a product standard, of the minimum ring stiffness a pipe or fitting must achieve [12]. EN 1401-1 for PVC-U non-pressure underground drainage lists SN2, SN4, SN8 and SN16, with the measured value required to meet or exceed the class rather than round up to it [4]. Structured-wall systems of PVC-U, PP and PE take their stiffness requirements from the EN 13476 series instead [5]. Which class a route actually needs is a burial-design question governed by installation practice such as ASTM D2321 [11], and the external-loading companion to this method is the ISO 1167 hydrostatic pressure test.
| Ring stiffness class | Minimum ring stiffness | Typical duty in a buried, non-pressure system |
| SN2 | 2 kN/m² | Very light load, designed bedding, no traffic |
| SN4 | 4 kN/m² | Normal burial depth, non-trafficked ground |
| SN8 | 8 kN/m² | Deeper cover, traffic loading, less controllable bedding |
| SN16 | 16 kN/m² | Heavy or very deep loading, engineer-specified |
Three neighbouring standards answer different questions. ISO 13967 applies the ring-stiffness concept to bends and branches, provided the fitting permits at least 4 % diametric deflection, referencing the result at 3 % so fittings and pipe compare directly [6]. ISO 13968 determines ring flexibility, not a stiffness at a reporting point [7]. ISO 9967 determines the creep ratio from a constant compressive force held for 1 008 h (42 days), extrapolated to two years [8][9].
The Five Errors That Bias an ISO 9969 Result
| Error | Direction of bias | Correction |
| Plates that deflect or bow under load | Result reads soft | Verify frame and plate rigidity; the plate must not deform enough to affect results |
| Nominal diameter used instead of measured inside diameter | Every recorded force shifts in the same direction | Measure dᵢ per ISO 3126 and use it for both the speed band and the 3 % target |
| Incomplete through-wall conditioning | Result reads stiff | Condition in air at test temperature ≥ 24 h; verify a large pipe through the wall |
| Structured-wall specimen length that ignores the profile pitch | Non-reproducible | Whole ribs only, cut at the mid-point between ribs |
| Zero point left uncorrected | Offset in F at 3 % | Extrapolate the initial linear portion to the horizontal axis |
ISO 9969 Acceptance and OQ/PQ Checklist
| Parameter | Reference value | Source |
| Deflection speed | Table 1 band for the measured dᵢ, ± the stated tolerance | ISO 9969:2016, 5.1 |
| Plate length / width | ≥ test piece length / ≥ contact width + 25 mm | ISO 9969:2016, 5.2 |
| Length measurement accuracy | ± 1 mm | ISO 9969:2016, 5.3 |
| Inside diameter measurement accuracy | ± 0.5 %, or π tape per ISO 3126 | ISO 9969:2016, 5.3 / 6.3 |
| Deflection measurement accuracy | 0.1 mm or 1 % of deflection, whichever is greater | ISO 9969:2016, 5.3 |
| Force measurement accuracy | ± 2 % for deflection up to 4 % of the test piece | ISO 9969:2016, 5.4 |
| Test temperature | (23 ± 2) °C, or (27 ± 2) °C where applicable | ISO 9969:2016, 8.1 |
| Conditioning | ≥ 24 h in air at test temperature | ISO 9969:2016, Clause 7 |
| Test piece age | ≥ 24 h; (21 ± 2) days for type testing or dispute | ISO 9969:2016, 6.4 |
| Pre-load F₀ applied | 95 % to 105 % of the calculated value | ISO 9969:2016, 8.4 |
| Number of test pieces | 3 (a, b, c), rotated 120° apart | ISO 9969:2016, 6.1 / 8.2 |
Questions worth putting in writing to a supplier or a contract laboratory:
| Question to put in writing | Why it matters |
| Which load cell and deflection gauge calibrations are traceable, and what is the last verification date? | ISO 9969:2016 requires ± 2 % on force and 0.1 mm or 1 % on deflection change |
| How is the plate assembly shown not to deform at the maximum test force? | Clause 5.2 is a functional requirement; the answer is a stiffness calculation or a load–compliance test |
| Is inside-diameter change measured directly or inferred from crosshead travel? | Crosshead travel includes frame and plate compliance |
| What is the total conditioning time, including transit to the laboratory? | Through-wall equilibrium at (23 ± 2) °C is the largest single source of laboratory-to-laboratory spread |
| How is the zero point determined? | The standard’s remedy is extrapolating the initial linear portion back to the horizontal axis |
| Will the report state each individual test piece result alongside the mean? | Scatter across pieces a, b and c is the fastest diagnostic for wall variation or plate misalignment |
Equipment for ISO 9969 Testing
Ring stiffness work is low-force work. Worked from the formula above, an SN8 pipe at an inside diameter of 500 mm needs roughly 1.9 kN at the 3 % reporting point, so capacity is driven by the rest of the programme: flattening and ring flexibility runs to 20 % or 30 % diametric deflection, where force rises far above the stiffness figure.
| Model | Max force | Force accuracy | Compression plate | Inside-diameter extensometer | Programme |
| WD-100H pipe ring stiffness & flexibility testing machine | 100 kN | Class 0.5 (± 0.5 %) | 1000 × 400 mm rigid plate | 180–1500 mm internal | Ring stiffness, flexibility, flattening, creep ratio (42 d) |
| WD-50H pipe ring stiffness & flexibility testing machine | 50 kN | Class 0.5 (± 0.5 %) | 1000 × 400 mm rigid plate | 180–1500 mm internal | Ring stiffness, flexibility, flattening, creep ratio (42 d) |
Choosing between 50 kN and 100 kN
Confirm whether the programme reaches only the 3 % reporting point or continues to flattening and flexibility deflection; calculate the force at the highest deflection you will actually run, not the ring stiffness load. Step up to 100 kN when large-diameter or metal-reinforced profiles and flexibility to 30 % deflection are routine; stay at 50 kN for ISO 9969 verification and routine quality control. Both cover every Table 1 speed band, from 0.01 to 500 mm/min.
Frequently Asked Questions
What is the ISO 9969 ring stiffness test used for?
It measures how much a thermoplastic pipe resists radial compression. A ring is squeezed between two parallel plates at a controlled speed, and the force at 3 % diametric deflection is converted into ring stiffness in kN/m². Product standards such as EN 1401-1 and the EN 13476 series use that measured value to verify a declared SN class [2][4].
How long should an ISO 9969 test piece be?
For pipes up to dₙ 1500 the average length is (300 ± 10) mm, and above dₙ 1500 it is at least 0.2 dₙ. Structured-wall pipe follows the profile instead: perpendicular ribs require a whole number of ribs giving at least 290 mm, and helical ribs require a length equal to the inside diameter ± 20 mm, between 290 mm and 1 000 mm [2].
Why are three test pieces tested in ISO 9969?
Extruded pipe is not perfectly uniform around its circumference, and ring stiffness depends on where the thin and thick sections sit relative to the load. ISO 9969 takes three pieces from one marked pipe, rotates them 120° apart, and reports the mean — so the figure describes the pipe rather than one arbitrary orientation [2].
What deflection speed does ISO 9969 require?
The speed scales with measured inside diameter: 2 ± 0.1 mm/min to 100 mm, 5 ± 0.25 mm/min to 200 mm, 10 ± 0.5 mm/min to 400 mm, and 20 ± 1 mm/min to 710 mm. Above 710 mm the rate is 0.03 × dᵢ ± 5 %. The 2016 edition indexed these bands to inside diameter; the 2007 edition used nominal diameter [2][3].
What pre-load does ISO 9969 apply before the test?
The plate is lowered until it touches the specimen, then a pre-load seats the ring before the gauge and load cell are zeroed. At or below 100 mm inside diameter it is 7.5 N; above that it is calculated from 250 × 10⁻⁶ × dᵢ × L and rounded up to the next newton. The applied force must be within 95 % to 105 % of that value [2].
At what temperature is the ISO 9969 ring stiffness test run?
At (23 ± 2) °C, or (27 ± 2) °C in countries where 27 °C is the standard laboratory temperature; disputes use 23 °C. Specimens are conditioned in air at that temperature for at least 24 hours. Test piece age must also be at least 24 hours, and for type testing or disputes it must be (21 ± 2) days [2].
Why do ISO 9969 ring stiffness results come out too low?
Check the compression plates first. If they bend under load, the middle of the specimen is unloaded relative to its ends and every result is biased low. The second cause is using a nominal inside diameter instead of a measured one: the 3 % deflection target shifts, and so does every recorded force [2].
Does an SN class apply to fittings as well as pipe?
Pipe stiffness classes come from the pipe product standard, but ISO 13967 defines how ring stiffness is determined for bends and branches, provided the fitting permits at least 4 % diametric deflection. It reports a stiffness referenced at 3 % deflection, so a fitting figure and a pipe figure can be compared on the same basis [6].
Conclusion — How to Read an ISO 9969 Ring Stiffness Result
Read a ring stiffness report in this order:
- Check the edition — 2007 speeds and pre-load treatment are not current [3].
- Check that the inside diameter is measured, and that the 3 % target came from it.
- Check temperature, conditioning time and age — 24 hours, or the (21 ± 2) days a dispute requires.
- Read the three individual results before the mean; scatter means wall variation or a misaligned plate.
- Compare the mean with the declared class — the value must meet or exceed SN4, SN8 or SN16, not round up to it [4].
- Ask separately whether a D2412 figure is also needed — different quantity, different reference deflection [10].
Send us your diameter range, wall construction and stiffness classes, and we will confirm the capacity and the deflection measurement method that fits — talk to our pipe testing team, or see the full plastic pipe testing equipment range.
Shenyang Baoruitong Automation Equipment Co., Ltd. — ring stiffness, hydrostatic pressure and melt flow testing systems for plastic pipe laboratories. Checked against ISO 9969:2016, ISO 9967:2016 and ASTM D2412-26.
