ISO 13968 Plastics pipe ring flexibility test | UnitedTest - Ring Flexibility & Ring Stiffness Testing Machine

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Material strength is a course that most engineering fields are required to study in university education. This interesting course is very practical and makes use of testing equipment in the materials laboratory.

Mechanical testing of industrial products and components is essential to ensure their load-bearing, fatigue resistance, and impact resistance, directly preventing production accidents and ensuring stable operation of equipment. UnitedTest can cover a full range of tests including tensile, hardness, and fatigue, complying with national and international standards, and provides customized industrial mechanical testing solutions.

Mechanical testing in the plastic pipe industry determines ring stiffness, impact resistance, and tensile performance, directly preventing pipe bursts and deformation risks. UnitedTest can cover full-dimensional mechanical testing of pipes, complies with ISO,ASTM,EN et.c, international standards, and provides customized testing solutions.

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Mechanical testing in the food and beverage industry involves the compressive and tensile performance of packaging and the safety of food texture, directly ensuring storage, transportation safety, and the eating experience. UnitedTest can cover all-dimensional mechanical testing for packaging and food texture, comply with national and international standards, and provide customized testing solutions.

Mechanical testing in the packaging industry determines the packaging's resistance to compression, impact, and tension, directly ensuring the safety of contents during storage and transportation. UnitedTest can cover comprehensive mechanical testing for packaging, complying with ISO, ASTM Standards, and provides customized testing solutions for compression, drop, tear, and more.

Mechanical testing in the transportation industry concerns the load-bearing and fatigue resistance performance of core components such as vehicles, tracks, and containers, directly ensuring transport safety and equipment durability. UnitedTest can cover mechanical testing for all types of transportation materials, complying with national and international standards, and provides customized testing solutions including tensile, fatigue, and impact tests.

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Mechanical testing in the glass-ceramics industry determines core properties such as flexural strength, hardness, and fracture toughness, directly helping to avoid brittleness and failure risks. UnitedTest can cover comprehensive testing for flexural strength, hardness, and fracture toughness, compatible with national and ISO/ASTM standards, and provides customized testing solutions.

Tensile test evaluates material resistance to stretching, measuring strength/elongation critical for quality control. UnitedTest offers universal tensile testers (10-600KN) compliant with ASTM/ISO standards, providing tailored solutions for metals, polymers, medical & industrial materials.

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Fatigue Test assesses material durability under cyclic loading, critical for predicting service life of industrial components. UnitedTest offers fatigue testing systems (axial/bending/torsion) compliant with ASTM E466/ISO 12106, providing tailored solutions for automotive, aerospace and manufacturing sectors.

Peel Test measures material adhesion strength by evaluating resistance to peeling forces, critical for adhesives, films, packaging. UnitedTest offers peel testers compliant with ASTM D903/ISO 8510, covering 90°/180° peel tests, and tailored solutions for packaging, automotive, textile industries.

Impact Test evaluates material resistance to sudden shock loads, critical for assessing toughness and fracture risk. UnitedTest offers impact testers (Charpy/Izod/fall weight) compliant with ASTM E23/ISO 148, providing tailored solutions for metals, plastics, glass and industrial components.

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ISO 13968 Ring Flexibility Tester for Thermoplastic Pipes | UnitedTest

UnitedTest manufactures ISO 13968 ring flexibility testing machine for circular thermoplastics pipes. This test evaluates pipe resistance to diametric deflection and structural integrity under radial compression for plastic piping and ducting systems.

ISO 13968 Plastics piping and ducting systems — Thermoplastics pipes — Determination of ring flexibility. ISO 13968 specifies a laboratory compression test to measure ring flexibility of thermoplastics pipe with circular cross-section, assessing the pipe’s capability to withstand diametric ring deflection without cracking, buckling or loss of structural integrity. The test records compression force and deflection behaviour, widely used for quality control and material validation of solid-wall and structured-wall plastic drainage and sewer pipes.

Test Principle

A ring section cut from the pipe is compressed diametrically at a constant speed. Force and deflection are recorded continuously until at least the specified deflection is reached, or the test piece fractures first.

Every test piece is monitored during the test and inspected afterwards for several specific, named types of mechanical failure. The two measured variables are:

change in inside diameter or outside diameter (either is acceptable), and
the corresponding compressive force.

The essential idea: ring flexibility is not "how much load can it carry" (that's ring stiffness) — it is "how far can it be squeezed before it is damaged."

Ring flexibility vs. ring stiffness — why you need both

Ring stiffness (ISO 9969) is measured at only 3 % deflection and is reported as an SN class (SN4, SN8, SN16) in kN/m². It tells you the pipe is stiff enough.

Ring flexibility (EN 1446 / ISO 13968) pushes the pipe to 30 % — ten times further — and tells you the pipe is damage tolerant enough. This matters because modern lightweight, structured-wall, foam-core and spirally wound pipes get their stiffness from geometry, and geometry can fail by local buckling or delamination long before the polymer itself reaches its limit. Both failure modes are completely invisible at 3 % deflection.

That is why EN 1446 is standard practice for buried non-pressure drainage and sewerage, trenchless and ploughed conduit installation, and every multi-layer or lined pipe construction where the liner or bond line is the weak link.

Test Machine Required of ISO 13968 Plastics pipe ring flexibility test

Ring Stiffness Compression Testing Machine

Complies with requirements from ISO 9969. It must deliver the required constant test speed and achieve the full specified diametric deflection of the specimen.

In practice this means a universal/compression testing machine fitted with two parallel flat compression plates;

a calibrated load cell, and a deflection transducer (or crosshead displacement) with adequate range — because a standard ring-stiffness test normally stops at ~3 % deflection,

whereas ring flexibility runs to 30 %.

ISO 9969 Ring stiffness testing for plastic pipe

Dimensional‑ and force‑measuring devices

Conforming to ISO 9969, but capable of measuring diametric deflections up to at least the specified deflection and the corresponding compressive forces.

Test Specimen Information

Quantity: three test pieces.
Source: all three from a single pipe (so the result characterises that pipe, not a batch).
Geometry: prepared as specified in ISO 9969 (cut ring with ends square/parallel to the axis).
Length override: the referring standard may specify longer test pieces than ISO 9969 defines. (Longer pieces are common for structured-wall pipes so that a representative number of ribs/corrugations is included.)

Key Test Parameters

Default target diametric deflection: 30 % of pipe outside diameter, unless modified by the referenced product standard.

Compression speed: follows speed values defined in ISO 9969 (depends on pipe diameter).

Test temperature: documented in the final test report, governed by referenced standards.

Measured outputs: compression force, inside‑ or outside‑diameter deflection, force‑deflection curve, failure mode and deflection‑force values at damage onset.

ISO 13968 Plastics pipe ring flexibility Test Procedures:

Prepare three ring specimens from one pipe, assign identifiers a, b, c; apply conditioning as ISO 9969 specifies.
Mount specimen on compression tester. Perform compression following ISO 9969 procedure, keep compressing and record diameter change (inner or outer diameter) continuously.
Stop compression when reaching specified deflection OR mechanical failure occurs first. Further compression may be performed optionally.
Generate force‑deflection graph for every specimen. Inspect each specimen and record failure type, failure location, plus corresponding force and deflection values at each damage event.
Compile complete test report covering pipe identification, specimen length, test temperature, force‑deflection plots, failure data, maximum‑point force‑deflection, influencing factors and test date.

Related Test Standard:

ASTM D2412	Standard Test Method for Determination of External Loading Characteristics of Plastic Pipe by Parallel-Plate Loading
ISO 9969	Thermoplastics pipes — Determination of ring stiffness
ISO 9967	Thermoplastics pipes - Determination of creep ratio
GB/T 9647	China standard: Thermoplastics pipes—Determination of ring stiffness
ISO 13967	Thermoplastics fittings - Determination of ring stiffness
ISO 13268	Thermoplastics piping systems for non-pressure underground drainage and sewerage — Thermoplastics shafts or risers for inspection chambers and manholes — Determination of ring stiffness
GB/T 39385	Plastics piping and ducting systems—Thermoplastics pipes—Determination of ring flexibility
ASTM F2433	Standard Test Method for Determining Thermoplastic Pipe Wall Stiffness
ISO 13968	Plastics piping and ducting systems. Thermoplastics pipes. Determination of ring flexibility
DIN 16961	Thermoplastics pipes and fittings with profiled wall and smooth pipe inside
ISO 7685	Glass-reinforced thermosetting plastics (GRP) pipes — Determination of initial ring stiffness
EN 1446	Plastics piping and ducting systems - Thermoplastics pipes - Determination of ring flexibility
EN 1228	Ring stiffness for glass-reinforced (GRP) pipes — same test geometry, different material family.
AS/NZS 1462.22	Methods of test for plastics pipes and fittings Method 22: Thermoplastics pipes – Determination of ring stiffness

Industry Applications

Ring flexibility is a required characteristic mainly in non-pressure, buried thermoplastics piping, where the pipe must survive installation and ground movement rather than internal pressure:

Municipal drainage & sewerage	Structured-wall (corrugated, spirally wound, ribbed) PVC-U, PP, PE pipes
Stormwater / surface water	Large-diameter PE/PP structured-wall pipes, retention/detention systems
Buried cable & ducting	Telecom/power ducts, conduits
Land & agricultural drainage	Corrugated PE/PP drainage pipe
Trenchless installation	Pipe used in horizontal directional drilling, pipe bursting, slip-lining
Building & industrial	Non-pressure soil/waste piping, chemical-resistant drainage (where the material is compatible)
Certification & QC labs, procurement	Type testing, batch acceptance, third-party certification, material/formulation comparison

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pipe ring stiffness test machine is used to determine the ring stiffness of circular cross-section thermoplastic pipes, according customer’s request, it can also execute compression, ring stiffness, ring softness and creep ratio test.

Q1. What is ISO 13968?

ISO 13968 is an International Standard titled "Plastics piping and ducting systems — Thermoplastics pipes — Determination of ring flexibility." The current version is ISO 13968:2008 (2nd edition, published 15 October 2008), which cancelled and technically revised the 1997 edition. It was prepared by ISO/TC 138/SC 5 and was last reviewed and confirmed in 2023, so the 2008 version remains current. It is only 4 pages long and is a test-method standard, not a product standard.

Q2. What does "ring flexibility" actually mean?

Clause 3.1 defines it as "the ability of a pipe to resist ring diametric deflection without loss of structural integrity" (definition sourced from EN 13476-1:2007). In plain language: how far a pipe can be squeezed oval before it is damaged.

Q3. What does the test determine?

Two things, measured together: (a) the deflection and (b) the force at which physical damage — if any — occurs, within a specified diametric deflection. For each specimen you also get a full force/deflection curve.

Q4. Why this test matters for the material?

Ring flexibility is the complement to ring stiffness, and the two answer opposite questions:

Ring stiffness (ISO 9969): "How much load does it take to deform it a little (3 %)?" → resistance to deformation.

Ring flexibility (ISO 13968): "How far can it be deformed before it is damaged?" → tolerance of deformation, i.e. ductility and structural integrity at large strain.

Concretely, the test matters because:

Installation survivability. Pipes are stacked, dragged, lifted, bedded and backfilled. Point loads, poor haunching or careless compaction impose large local diametric deflection. A pipe with good ring flexibility absorbs it; a brittle or poorly fused one cracks.

Ground movement tolerance. Flexible thermoplastic pipes are designed to deflect and share load with the surrounding soil. Differential settlement, soft soils, seismic zones and reclaimed/coastal land all demand a pipe that can ovalise substantially without cracking or buckling — this is the core advantage of plastics over rigid materials.

Structured-wall integrity. For corrugated, spirally wound or multi-layer/foam-core walls, the failure modes that matter are delamination between layers, cracking at rib roots, and permanent buckling of the profile — none of which are revealed by a 3 %-deflection stiffness test. ISO 13968 targets exactly these modes.

A sensitive indicator of material and process quality. Failure at 30 % deflection usually signals something fixable upstream: wrong or degraded raw material, bad formulation, excessive filler/regrind, poor melt homogenisation, incomplete welding of the spiral seam, or high residual stress from cooling. That makes the test a practical "process barometer" on the production line.

Design and specification basis. Engineers specifying pipe for difficult geology, deep burial with poor compaction, or trenchless methods need evidence of flexibility, not just an SN class. Because ISO 13968 produces a force/deflection curve plus the exact force and deflection at each damage event, it gives designers quantitative data on the damage threshold, not a simple pass/fail.

Objective, comparable, certifiable. Because the method fixes specimen count (3 from one pipe), speed, endpoint (30 % unless overridden), observation categories and the exclusion of whitening, results are reproducible across laboratories and can be used for certification, acceptance and supplier comparison.

Q5. Can one machine do both ISO 9969 (ring stiffness) and ISO 13968 (ring flexibility)?

Yes — and this is the normal configuration. ISO 13968 was written to run on ISO 9969 equipment; the only extra requirements are greater platen travel / deflection range and software that keeps recording past 3% deflection out to 30% (or more), with curve plotting and event marking.

Q6. Is the 30% deflection always the requirement?

No — 30% is only the default ("unless otherwise specified in the referring standard"). Product standards do vary; for example some EN 13476 applications cite an alternative 20% class (RF20). Always read the referring standard first.

Q7. What counts as failure?

For each specimen you inspect and record the type, position, force and deflection of:

(a) non-compliance of the force/deflection graph with the referring standard;
(b) cracking or crazing anywhere in the wall structure;
(c) wall delamination (layer separation — critical for structured/multi-layer walls);
(d) permanent buckling anywhere in the wall structure;
(e) any other observation required by the referring standard.

Q8. Is stress whitening a failure?

No — and this is explicitly stated in the standard. "Whitening of the pipe shall not be considered as an indication of one of the above-mentioned mechanical failures." This prevents false rejections, especially with PP and PE, where whitening is a common optical effect at high strain.

Q9. Why does it matter in real installations?

Pipes are stacked, dragged, lifted, bedded and backfilled. Point loads, poor haunching, over-compaction or rock inclusions impose large local diametric deflection. Good ring flexibility lets the pipe absorb this. In service, flexible thermoplastic pipes are designed to ovalise and share load with the surrounding soil — so differential settlement, soft soils, seismic activity, coastal/reclaimed land and trenchless installation (HDD, pipe bursting, slip-lining) all depend on the pipe surviving large deflection without cracking, delaminating or buckling.

Q10. Why is it especially important for structured-wall pipes?

For corrugated, spirally wound, ribbed or foam-core/multi-layer walls, the dominant failure modes — delamination between layers, cracking at rib roots, permanent buckling of the profile, weld/seam separation — simply do not appear at the 3% deflection used in a stiffness test. They only show up at large deflection. That is precisely the region ISO 13968 covers. (Pipelines made from PE structured-wall pipe are known to find 30% deflection without irreversible deformation the hardest requirement to meet, because of PE's low modulus and tall ribs.)

Q11. What does a failure tell the manufacturer?

Failure at the specified deflection is usually a process or material signal: wrong or degraded raw material, bad formulation, excessive filler or regrind, poor melt homogenisation, incomplete welding of the spiral seam, or high residual stress from cooling. Because of this, routinely running the test is a practical "production barometer," not just a compliance checkbox.

Q12. Which industries and products use it?

Non-pressure buried thermoplastics piping: municipal drainage and sewerage, stormwater / surface water and retention systems, buried cable and telecom ducting, land/agricultural drainage, non-pressure soil & waste piping, and trenchless installation. Users are pipe manufacturers (R&D, QC, production verification), independent testing and certification laboratories, design engineers and specifiers, and procurement/regulatory bodies.

Q13. Who needs to buy or use a ring flexibility testing machine?

Pipe manufacturers running type tests, batch acceptance and process control
Third-party testing, inspection and certification laboratories
Quality-control departments of construction/material groups
Universities and research institutes working on pipe materials and structures

Q14. Why choose ISO 13968 Ring Flexibility Testing Machine from UnitedTest?

UnitedTest supplies ISO 13968 ring‑flexibility testing machines for thermoplastic structured‑wall drainage pipes. Combined ISO 9969 ring‑stiffness & ISO 13968 ring‑flexibility test solution for PVC‑U, PP, HDPE corrugated sewer pipes. High‑accuracy force‑deflection recording, factory calibration & global support.

UnitedTest is a professional manufacturer for plastic pipe mechanical testing equipment. Our ISO 13968 ring‑flexibility tester is designed fully complying with ISO 13968:2008 standard for thermoplastics piping and ducting systems, widely used for quality‑control labs, plastic pipe factories, third‑party inspection institutions and municipal‑engineering research institutes worldwide.

This integrated test platform supports ISO 13968 ring‑flexibility test and ISO 9969 ring‑stiffness test on one single machine. It applies parallel‑plate diametric compression to ring‑shaped pipe specimens, precisely controls compression speed, continuously captures compression force and diametric deflection, and auto‑generates force‑deflection curves for each specimen.

✅ Key machine capabilities for ISO 13968 compliance

Sufficient large displacement travel to realize default 30 % outer‑diameter deflection for ring‑flexibility assessment
High‑precision load cell and displacement sensor, meets measurement requirements defined in ISO 13968 & ISO 9969
Software preset ISO 13968 test workflow: stop condition set for target deflection or specimen mechanical failure
Supports observation & record for cracking, crazing, delamination, permanent buckling and other failure modes
Complete test‑report output covering all mandatory fields required by ISO 13968 clause 9
Compatible with structured‑wall pipes: double‑wall corrugated pipe, spiral wound pipe, PVC‑U, PP, PE non‑pressure sewer & drainage pipes

✅ Why choose UnitedTest ISO 13968 ring‑flexibility tester

Dual‑standard solution: run ring‑stiffness (ISO 9969) and ring‑flexibility (ISO 13968) without purchasing separate machines.
Customizable: adjust test frame size, platen dimension, load capacity for small‑to‑large‑diameter pipe samples.
Factory calibration before shipment, ensures test traceability for lab accreditation.
Full‑package service: equipment delivery, installation guidance, standard‑method training, 1‑year hardware warranty, lifetime free software upgrade, remote technical support for global customers.

If you are looking for reliable ISO 13968 ring‑flexibility test equipment for your plastic‑pipe QC laboratory, contact UnitedTest for detailed quotation and technical proposal.

We offer customization to meet your specific needs. Our expert team will collaborate with you to develop the perfect product for you