ISO 899-2

What the test does

A rectangular bar is laid on two supports, loaded at mid-span, and left there. The load does not change and nothing moves quickly. What is recorded is deflection against time, often over 1000 hours, and the curve that comes out is the material's creep behaviour in bending.

The fixture is the three-point arrangement described in ISO 178, so a laboratory running short-term flexure already has the geometry. The specimen is 10 × 4 × 80 mm, taken from the mid-section of the multi-purpose specimen — the same bar that feeds tensile, flexural and impact testing, which is what makes results across those methods comparable.

Deflection is taken either from the die movement, corrected for the self-deformation of the test arrangement, or optically with a video measuring system. The correction is not optional bookkeeping: over a thousand hours a fixture that has settled by a fraction of a millimetre will otherwise report that settlement as creep in the plastic.

Temperature and humidity are recorded throughout, because both change the answer and neither holds itself steady for six weeks without being watched.

What it measures, and why it matters

Short-term flexural testing tells you what a plastic does in a minute. Creep tells you what it does in a year, and for a loaded plastic part those are different materials. A bracket that passes a flexural test at its design load will still sag out of tolerance if the load stays on it, and no short-term number predicts by how much.

The method covers rigid and semi-rigid plastics: unreinforced, filled and fibre-reinforced. That breadth matters, because the filled and reinforced grades are the ones specified for load-bearing parts on a short-term modulus, and the ones where the filler changes creep in ways that figure does not reveal.

Results feed design, quality control and development. In design they become the creep modulus used to size a part for its service life; in quality control they catch a compound change a tensile test misses — a regrind fraction that has crept up, a stabiliser reformulated.

Part 1 of the same standard covers tensile creep. The two are not interchangeable, and the difference is not cosmetic: a bar in bending has its top surface in compression and its bottom in tension, and a plastic that creeps differently in the two states behaves differently again in flexure. A part loaded in bending should be characterised in bending.

Specimen

The same bar that feeds tensile, flexural and impact testing, which is what makes results across those methods comparable for one material.

Size

10 × 4 × 80 mmTaken from the mid-section of the multi-purpose specimen, or moulded directly.

Also permitted

Machined from sheet or from moulded articlesWithin scope, and often the only option when the question is about a real part rather than a grade.

Materials

Rigid and semi-rigid: unreinforced, filled and fibre-reinforcedThe filled and reinforced grades are the ones most often specified on a short-term modulus, and the ones where the filler changes creep most.

Where it came from

Same position, same tool, every timeDakOrientation, residual stress and skin-core structure all influence creep and all vary across a moulding.

Test speed

There is no crosshead rate. The load arrives once and then stays.

Application

Smoothly, without jolts, then held constantAn impact at the start puts strain into the specimen that is not creep and cannot be separated from it afterwards.

Duration

Commonly 1000 hoursRoughly six weeks per specimen, which is why creep rigs are built in banks rather than tying up a load frame.

Deflection

Corrected die movement, or video measurementThe correction for self-deformation of the arrangement is not bookkeeping: a fixture that settles a fraction of a millimetre over six weeks reports that settlement as creep.

Environment

Temperature and humidity recorded throughoutBoth change the answer, and neither holds itself steady for six weeks unwatched.

How the test runs

1. 01Condition the specimen and record the atmosphere.
2. 02Set up the ISO 178 three-point fixture and characterise its self-deformation.
3. 03Apply the calculated load smoothly, without jolts.
4. 04Record deflection against time, typically to 1000 hours.
5. 05Subtract the rig's self-deformation from the measured movement.
6. 06Derive the creep modulus at the times the design requires.

What travels with an ISO 899-2 result

A creep figure without its conditions is not comparable with anything.

• The applied stress and the time the modulus was taken at.
• Temperature and humidity across the run, not just at the start.
• Specimen origin — moulded directly, or machined from what.
• Whether deflection came from corrected die movement or from video.
• That the rig's self-deformation was characterised and subtracted.

What the machine must be capable of

Not force, but patience and stability. The load is modest and constant; what the equipment has to do is hold it unchanged for weeks without drift, and measure a slowly growing deflection to a resolution that makes the early part of the curve meaningful.

That usually means dead-weight loading rather than a servo frame, and rigs built in banks so many specimens run at once. The rig must sit in a controlled environment with temperature and humidity logged rather than assumed, and be mechanically stable: settlement in the frame or supports is indistinguishable from creep unless it has been characterised and subtracted.

What goes wrong in practice

Not correcting for the self-deformation of the arrangement is the classic error, and it always adds apparent creep.

Applying the load with a jolt is the second. The load has to arrive smoothly and then stay put; an impact at the start puts strain into the specimen that is not creep and cannot be separated from it afterwards.

Letting the environment drift is the third. A test that runs through a season without conditioning has temperature and humidity as uncontrolled variables, and both move creep rates substantially.

Finally, comparing a flexural creep modulus with a tensile one. They are different numbers about different loading states, and a design that uses one where it needed the other will be wrong in a direction nobody checks.

Part 1 or Part 2

Same standard, same conditioning, different loading — and they are not interchangeable.

A plastic that creeps differently in tension and compression behaves differently again in flexure. Using a tensile creep modulus for a part loaded in bending is wrong in a direction nobody checks.

Questions we are asked about this test

Related and equivalent standards

ISO 899-1 is the tensile creep companion and shares the conditioning and reporting approach. ISO 178 supplies the three-point geometry this part uses. ASTM D2990 is the ASTM counterpart, covering tensile, compressive and flexural creep in one document rather than splitting them across parts.