Testing standard
Thermoplastics pipes — Determination of tensile properties — Part 1: General test method; Part 3: Polyolefin pipes
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
The ISO 6259 series determines the tensile properties of thermoplastics pipes — stress at yield and elongation at break — on test pieces taken longitudinally from the pipe wall and tested at full wall thickness. Part 1 carries the general method for all thermoplastics pipe; Part 3 covers polyolefin pipe, meaning polyethylene, cross-linked polyethylene, polypropylene and polybutene. Both current editions are dated 2015.
Test pieces of specified shape are taken from the wall of a finished pipe in the longitudinal direction, by cutting or machining, and pulled in a tensile machine under specified conditions. Part 1 carries the general method — definitions, apparatus, sampling, conditioning and expression of results — and the material parts add what changes for that polymer family. Part 3 covers polyethylene, cross-linked polyethylene, polypropylene and polybutene. The properties determined are stress at yield and elongation at break; Part 1 applies to all thermoplastics pipe.
The material as the extruder actually delivered it, in the form it will be used in. The series was drawn up on the basis of ISO 527, and Part 3's introduction says why it is separate: ISO 527 is written for sheet, with test pieces a few millimetres thick, whereas a pipe wall can exceed 50 mm and the pipe is deliberately tested as supplied, without reducing the thickness. Everything different about ISO 6259 follows from that.
Yield stress from a pipe wall is a process check as much as a material check — it responds to the compound, to how the melt was cooled and to degradation in the extruder. Elongation at break on a polyolefin separates a sound pipe from an embrittled one, and is often the figure a product standard sets a minimum against. Neither substitutes for long-term performance: the standard says plainly that the results serve as a material or process control test and are not a quantitative assessment of long-term pipe behaviour, which is the province of internal-pressure, creep and fatigue testing.
The pipe is tested as supplied, without reducing the wall thickness. Everything that separates ISO 6259 from ISO 527 follows from that one decision — a pipe wall can exceed 50 mm where ISO 527 assumes a few millimetres of sheet.
Quoting a figure without naming the part and the test piece type it came from makes it uninterpretable. Three geometries with different gauge lengths sit under one series designation, and an elongation at break is only meaningful against the gauge it was measured over.
Speed is not a free choice and is not one number. It follows from the constituent material and the wall thickness, and is set by the relevant material part rather than by the operator.
Both reported properties come from the initial dimensions of the test piece and the marked gauge length, not from anything measured after the specimen has drawn.
A = b × h
Defined in Part 1 as the product of initial width and thickness. Everything downstream uses this area.
σy = F / A
Yield is defined as the transition from elastic to plastic deformation, usually shown by a decrease or a shoulder in the stress–strain curve.
εb = (L − L₀) / L₀ × 100
Calculated from the gauge length at break, so it belongs to the marked gauge and not to the distance between the grips.
A break at the grip is the dominant failure on a thick, tough polyolefin test piece, and the instinctive fix — tightening the jaws — makes it worse. The standard's own answer is to widen the ends in proportion to the wall thickness.
Part 1 requires a grip that maintains or increases pressure on the test piece as the force rises — a description of self-tightening wedge action. Sliding wedges do exactly that, and hold a thin-wall test piece without the operator having to guess at a clamping force. Thicker walls need jaw faces built to the section.
Specifications
Polyolefin pipe elongates enormously before it breaks and a clip-on gauge runs out of stroke long before the test does. The adjustable 15 to 50 mm gauge covers the 50 mm gauge of the Type 1 test piece, and the gripper arms release at break rather than being dragged into the failure.
SpecificationsA tensile machine complying with ISO 5893, the specification for constant-rate-of-traverse rubber and plastics test equipment. The load indicator must be essentially free from inertia lag at the rate used and indicate load to within 1 % of the actual value. ISO 5893 is the normative requirement here; ISO 7500-1 is drawn to the reader's attention rather than imposed.
The grip requirement is written as behaviour, not as a part number. Grips must hold the test piece so its major axis coincides with the direction of pull through the centreline, prevent slip as far as possible, and be of a type that maintains or increases pressure as the force rises — a description of a self-tightening wedge or a vice action closed under pressure. The clamping system must not cause premature fracture at the grips, and pre-stressing to settle the test piece is permitted.
Test speed depends on the constituent material and the wall thickness, and comes from the relevant part rather than being chosen. Polyolefin pipe elongates enormously before breaking, so elongation at break needs a long-travel or non-contact device; a clip-on gauge runs out of stroke first.
Grip breaks, the dominant failure on a thick, tough test piece, usually solved by widening the ends rather than tightening the jaws. Die cutting a wall that should have been machined, leaving non-parallel sides and a low result. Machining that heats the face being tested. Taking elongation from crosshead travel, which includes slip and machine compliance. And quoting a figure without saying which part and test piece type produced it.
All four pull a polymer in tension. Only one of them tests the pipe wall at full thickness, and only one of them tests across a fused joint.
| ISO 6259-1 / -3 | ISO 527-1 / -2 | ISO 13953 | ASTM D638 | |
|---|---|---|---|---|
| Subject | Thermoplastics pipe, as supplied | Plastics in sheet or moulded form | A butt-fused PE joint | Rigid and semi-rigid plastics |
| Specimen | Taken longitudinally from the pipe wall, full thickness | Moulded or machined dumbbells a few millimetres thick | Waisted piece machined across the joint | Five dumbbell types |
| Reports | Stress at yield, elongation at break | The full tensile property set including modulus | Tensile strength and ductile-or-brittle failure mode | Strength at yield and break, elongation, modulus |
| Answers | Was the pipe extruded from sound material | How does the material rank | Was the fusion sound | How does the grade rank |
ISO 6259 exists because ISO 527's geometries do not suit pipe, and the two are not interchangeable on a certificate. ASTM D638 is a different method again and is not an equivalent — a supplier certifying to one and a customer testing to another will disagree on a compliant material.
It is the ISO series for determining the tensile properties of thermoplastics pipes — stress at yield and elongation at break. Part 1 is the general test method and applies to all types of thermoplastics pipe regardless of intended use; Part 2 covers the PVC family and Part 3 covers polyolefin pipe. Test pieces are taken longitudinally from the pipe wall and tested at the full wall thickness.
ISO 6259-1:2015 and ISO 6259-3:2015, both second editions, both published in 2015 and both confirmed as current on the ISO catalogue. Each cancelled and replaced its 1997 first edition after technical revision, and those 1997 editions are withdrawn. Part 2 is ISO 6259-2:2020. The series is maintained by ISO/TC 138/SC 5.
Because ISO 527 is written for materials in sheet form with test pieces a few millimetres thick, and a pipe wall can exceed 50 mm. ISO 6259 was drawn up on the basis of ISO 527 but takes the deliberate decision to test the pipe as supplied, without reducing the thickness, and the different test piece geometries follow from that. For greater detail on the underlying principles the series points the reader back to ISO 527.
Polyolefin pipes: polyethylene, cross-linked polyethylene, polypropylene and polybutene. It determines stress at yield and elongation at break, and carries the corresponding basic specifications in informative annexes for information purposes only.
The choice depends on the wall thickness of the pipe the piece was taken from. Type 1 is the ISO 527-2 Type 1B geometry, 150 mm long with a 10 mm narrow portion and a 50 mm gauge. Type 2 is the smaller geometry shared with the PVC part. Type 3 is the Type B test piece of ISO 13953, the thick-wall geometry. Below about 12 mm of wall the piece is preferably die cut; above it, machined.
A tensile machine complying with ISO 5893, the specification for constant-rate-of-traverse rubber and plastics test equipment, with a load indicator essentially free from inertia lag at the rate used and indicating load to within 1 % of the actual value. ISO 5893 is the normative requirement; Part 1 draws attention to ISO 7500-1 rather than imposing it. Computer-controlled machines in accordance with ISO 527-1 are an option.
It describes behaviour rather than a part number. The grips must hold the test piece so its major axis coincides with the direction of pull through the centreline, prevent slip as far as possible, and be of a type that maintains or increases pressure as the applied force rises — which describes a self-tightening wedge or a vice action closed under pressure. The clamping system must not cause premature fracture at the grips.
Almost always because the wall is thick and tough enough that the clamped ends reach their limit before the gauge section yields. The standard's own remedy is to increase the width of the ends in proportion to the wall thickness, not to tighten the jaws — extra clamping force on a polyolefin simply moves the damage forward. Non-parallel sides left by a die used on too thick a wall have the same effect.
No, and the standard says so directly. The results are principally a test of the material in the form of pipe and can be useful as a material or process control test, but they are not a quantitative assessment of long-term pipe performance. Applications where the conditions of force application differ considerably need the appropriate impact, creep and fatigue tests instead.
Directly, and by design. ISO 13953 determines the tensile strength and failure mode of a test piece taken across a butt-fused polyethylene joint, and its Type B test piece is the same geometry as the Type 3 test piece of ISO 6259-3. One measures the pipe material, the other measures the joint made in it, and a pipeline specification frequently calls for both.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Set by the test piece section, which carries the full wall thickness — modest on a thin-walled Type 2 piece and substantial on a thick-wall Type 3. Size the frame from the wall area and the material, not from the pipe diameter. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ISO 5893; the load indicator shall be essentially free from inertia lag at the rate used and indicate load to within 1 % of the actual value. ISO 7500-1 is drawn to the reader's attention in Part 1, not imposed | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Strain measurement | An extensometer to not specified — Part 1 assigns no extensometer class; elongation at break is calculated from the gauge length at break, so the device must follow the marked gauge and not the grips, gauge length 50 (Type 1); 25 (Type 2); Type 3 is the ISO 13953 Type B geometry | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Grips that keep the major axis of the test piece on the direction of pull, prevent slip as far as possible, and maintain or increase pressure as the force rises | Our self-tightening serrated wedge grips, with V-jaws for round specimens or vice-action grips, built to the specimen |
| Environment | Conditioning as the relevant part requires, with the test run in the same conditions | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
