Testing standard
Standard Test Method for Determination of External Loading Characteristics of Plastic Pipe by Parallel-Plate Loading
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D2412 measures how a plastic pipe resists being squashed. A ring of pipe is laid on its side between two flat parallel plates and compressed at 12.5 mm/min. Load and vertical deflection give pipe stiffness and stiffness factor, normally reported at 5 % deflection of the inside diameter, along with any cracking or wall buckling seen along the way.
A ring cut from the pipe, at least 150 mm long, is laid on its side on a flat steel plate. A second flat plate, parallel to the first and long enough to overhang the specimen at both ends, is brought down onto it at 12.5 ± 0.5 mm/min. The ring is squashed vertically and force is recorded against the reduction in vertical diameter. The inside diameter is measured before loading, because every reported deflection is expressed as a percentage of it. The load at 5 % deflection is the usual reporting point, and the test frequently continues well beyond it to look for cracking, wall buckling or layer separation.
The primary outputs are pipe stiffness and stiffness factor. Pipe stiffness is what a designer needs when a buried pipe has to survive the weight of soil and traffic above it: a flexible pipe carries external load partly through its own ring stiffness and partly by deflecting into the surrounding soil, and the balance between the two decides the installation. Stiffness factor converts the measurement into a quantity proportional to the wall's bending stiffness, which — unlike pipe stiffness — can be compared between diameters and wall constructions. For structured-wall, ribbed and multi-layer pipe the test is also the practical way to verify that a complicated wall behaves as designed, since no simple calculation captures it.
The specimen is a slice of the product rather than a machined coupon, so it carries the pipe's real wall structure — including any ribs, layers or foam core.
Pipe stiffness is a property of the pipe, not of the plastic. It combines material modulus with wall geometry, so it cannot be transferred between diameters or wall constructions.
PS = F / Δy
Force per unit length, so the specimen length divides out and specimens of different lengths are comparable.
SF = 0.149 r³ × PS
Recovers a quantity proportional to the wall's bending stiffness, EI, which unlike pipe stiffness can be compared across diameters.
Direct compression platens carrying flat parallel plates. What the method demands is that the plates overhang the specimen at both ends and stay flat under load — a plate that bows unloads the middle of the ring and the pipe reads soft.
SpecificationsForce measurement to ASTM E4 over a very wide range — a thin-wall small-diameter pipe may reach 5 % deflection under a few hundred newtons, while heavy-wall sections need tens of kilonewtons — and a crosshead holding 12.5 mm/min. The demanding part is the plates rather than the frame: they must be flat, parallel, longer than the specimen at both ends, and stiff enough not to bow under the test load. Plate bowing is the commonest apparatus fault in this test and it biases results low, because a flexing plate unloads the middle of the specimen relative to its ends. The frame also needs enough daylight and platen width to take the pipe diameters being tested.
Two systematic errors dominate, and both bias in one direction rather than adding scatter. Plates that bow under load unload the centre of the specimen and make every pipe read soft. Using a nominal rather than a measured inside diameter shifts every deflection percentage, and therefore every reported load. Beyond those, the frequent reporting mistake is treating a D2412 pipe stiffness figure and an ISO 9969 SN rating as interchangeable; they are different quantities, at different reference deflections, with different formulae. Finally, on structured-wall pipe a specimen that does not contain whole profile repeats is not representative of the wall, and no amount of care elsewhere recovers that.
| ASTM D2412 | ISO 9969 | |
|---|---|---|
| Output | Pipe stiffness PS and stiffness factor SF | Ring stiffness S, in kN/m² |
| Reference diameter | Inside diameter | Inside diameter |
| Usual reporting deflection | 5 % of inside diameter | 3 % of inside diameter |
| Formula | PS = F/Δy; SF = 0.149 r³ PS | S includes a deflection-dependent correction term |
These give different numbers with different units and different reporting deflections, and neither converts cleanly into the other. A pipe specified as SN8 to ISO 9969 has not been shown to meet a D2412 pipe stiffness requirement, or the reverse.
It is the ASTM test method for the external loading characteristics of plastic pipe by parallel-plate loading. A ring of pipe is laid on its side between two flat parallel plates and compressed at 12.5 mm/min. The result is pipe stiffness and stiffness factor, normally quoted at 5 % deflection of the inside diameter, together with observations of any cracking or buckling.
Pipe stiffness, PS, is force per unit length divided by deflection — a direct measure of how much this pipe resists being squashed. Stiffness factor, SF, multiplies it by 0.149 r³ to recover a quantity proportional to the wall's bending stiffness. PS describes the article and cannot be transferred between diameters; SF describes the wall and can.
Not directly. They report different quantities in different units and at different reference deflections — D2412 at 5 % of the inside diameter, ISO 9969 at 3 % — and ISO 9969's formula carries a deflection-dependent correction that D2412 does not. A pipe qualified as SN8 has not thereby been shown to meet a D2412 requirement, or the reverse. Where both are required, both are run.
The most frequent apparatus cause is plates that bow under load. If the plates flex, the middle of the specimen is unloaded relative to its ends and the pipe reads softer than it is. The next most frequent is using a nominal inside diameter rather than a measured one, since every deflection is a percentage of that dimension. Both produce a consistent bias rather than scatter, which makes them easy to overlook.
Not necessarily. At 5 % deflection most pipes are still within their elastic response and recover. Where the specification calls for the test to continue to 10 %, 15 % or further, it is usually looking for cracking, wall buckling or layer separation — and there the observation, not the load, is the acceptance criterion. Recording the deflection at which anything visible first happens is part of the result.
At least 150 mm, and longer where the wall is profiled. Structured-wall and ribbed pipes have a repeating profile, and a specimen that cuts through the middle of a rib rather than including whole repeats does not represent the wall. For those products the specimen length is chosen around the profile pitch rather than a flat minimum.
Because extruded pipe is rarely perfectly round and its wall thickness is rarely perfectly uniform. Loading a thin part of the wall at the crown gives a different answer from loading a thick part there. Marking the orientation and keeping it consistent across a set does not make the pipe uniform, but it does stop that non-uniformity appearing as random scatter.
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 | Wide — from a few hundred newtons on thin-wall small-diameter pipe to tens of kilonewtons on heavy-wall sections | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Two flat parallel steel plates, longer than the specimen and stiff enough not to bow under load | Our compression anvils, built to the specimen |
| Environment | 23 ± 2 °C; temperature matters because pipe stiffness is temperature-dependent | 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.
