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Flexibility or flattening test

What does it means and why should a pipe fulfil the requirements?
Today the requirements for a sustainable pipe system have changed, not always the cheapest solution is the best for a country. More and more designers and engineers dare to search for the technically best product for a project, at very reasonable costs. Also the cost aspect changed from single costs to the „lifetime cost“– from cradle to crave, including all.

So, which support for a designer or a quality control department does the so called flexibility test or flattening test mean? How is the test-procedure and how should the result be valued? In several international and national standards like EN13476, DIN16961, ISO 21138 or the ASTM F-894 a link to these tests is given – but why?

Basically the test will check how flexible a pipe can be, without having any irreversible damages on the pipe. This pipe property will give safety to the designers against overloading of the pipe or ground-movements occurred by earthquakes or unexpected soil settlement.

The standards for the test: The flexibility test is ruled in ISO 13968 and the flattening test is ruled in ASTM F894 8.6.

Deflection at 30%
Picture 1: Deflection at 30%

What is the difference ?
ISO 13968 : Determination of ring flexibility: The ring flexibility of a pipe is tested by measuring the force and the deflection while deflecting a ring section from the pipe diametrically at a constant speed until at least the specified deflection is achieved or prior fracture is occurred. The test should be in accordance to with ISO9969, but continue compression while measuring the change in either inside or outside diameter until the specified deflection has been achieved, unless otherwise specified the referring standard, the deflection shall be 30% of the outside diameter. The test report should include the applied forces. Usually the standard is only for pipes and not for socket/spigot or fittings.

ASTM F894: The Flattening test: Flatten three specimens of pipes (also with bell and spigot), in a suitable press until the internal diameter has been reduced by 40% of the original inside diameter of the pipe – so the deflection is 60% (double of the ring flexibility test) –The rate of loading shall be uniform and at 2in./min. (50,8 mm/min),


Which test equipment is needed?
For both tests a compression testing machine, as specified for ISO 9969, is needed, but capable of producing at least the specified diametric deflection of the test piece at the applicable speed.

What will be checked during and after the test?
During the compression, look out for signs of failure until the specified deflection has been reached. The checking should be done under normal light and with unaided eye.

  1. Inspect the force/deflection graph for compliance to the requirements in the standard
  2. Check for any cracking or crazing in any part of the wall structure
  3. Check for any wall delamination or separation
  4. Check for any permanent buckling in any part of the structured wall
  5. Check bracing of profiles.
  6. Any other observation as specified in the referring standard

What is the he meaning of a memory effect?
Usually PE-Pipes will almost go back to the prior shape after the load is taken away and so far the strain at yield is not exceeded. This material hysteresis is a very good property of thermoplastic pipes, so the pipe will still work with nearly the same hydraulic capacity.

Why a designer should consider this test in the decision for a pipe supplier?

One of the biggest advantages of polyethylene (gravity) pipes is flexibility, but the pipes are sold according to stiffness classes ☹ To produce a stiff pipe some pipe producers are mixing cheap stiff material in the pipe (like filled by calcium carbonate up to 50%) – by doing this; the pipe will break under overload conditions or unexpected soil movements.

Deflection at 30%
Picture 2: Deflection at 30%

How do Krah Pipes handle these tests?
Krah pipes produced with the Krah production technology have no problem to comply to both tests, even higher deflections – up to 90% can be handled – but only if the raw material is good. Especially taking into account the cost pressure, many thermoplastic pipe producers try to reduce the costs of their pipe by using high filled thermoplastics, or they are running their lines in a high controlled speed, so that the pipe will have many frozen stresses in the pipe wall, or the waterway wall is detached from the structured wall.

Additional remark: it would be good also to also check the joints with the same test, to prove that not only the pipe works after/during a deflection, because in the end the designers / clients want to have a secure and sustainable pipe system and not only „a“ pipe.

Alexander Krah
CEO Krah Pipes