Self-identifying
Load Cells
A load cell able to hold a constant force for weeks or months without drifting, which is a different requirement from resolving one accurately for a few minutes.
SpecificationsTesting standard
ISO 899-1
Plastics — Determination of creep behaviour — Part 1: Tensile creep
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 899-1 determines the tensile creep of plastics under specified conditions of pre-treatment, temperature and humidity. Unlike most plastics methods it is intended to provide data for engineering design as well as research, which is why its conditioning requirements are so strict.
At a glance
- Test type
- Creep & relaxation
- Published by
- ISO
- Edition
- ISO 899-1:2017
- Material
- Plastics, polymers & films
- Runs on
- Series 7200 and Series 9000
What the test does
Dumb-bell specimens, moulded directly or machined from sheet or articles, are pre-treated and conditioned to a specified temperature and humidity, and their thermal history recorded. A specified load is applied smoothly and then held constant while strain is measured on the specimen and logged along with temperature and humidity for the whole duration — weeks or months rather than minutes. Creep strain is reported at specified times, and creep modulus is reported against the time at which it was evaluated.
What it measures, and why it matters
How much a plastic keeps deforming under a load it has already survived. This is where polymers differ most consequentially from metals: a plastic creeps appreciably at room temperature under quite ordinary stresses, so a part that passes every short-duration test can still fail in service by slowly changing shape — the clip that loosens, the shelf that sags, the housing that stops sealing. Unusually among plastics methods, this one states that it is intended to provide data for engineering design as well as research, which is why its conditioning requirements are as strict as they are.
A test measured in weeks
Everything that is negligible over a five-minute tensile test — grip relaxation, temperature drift, gauge slip — becomes the measurement over a month.
- Loading
- Constant load, applied and heldNot a rate. The variable is time, and the load is the condition.
- Specimens
- Dumb-bells, moulded directly or machined from sheet or articles
- Materials
- Rigid and semi-rigid, non-reinforced, filled and fibre-reinforced
- Purpose
- Engineering design as well as research and developmentUnusual. Most plastics test methods disclaim design use; this one is written for it.
- Thermal history
- Can have profound effects on creep behaviourStated in the standard. Two specimens of the same grade with different moulding or annealing histories creep differently.
- Log the conditions continuously, not just at the start
- DakA laboratory that drifts two degrees overnight has changed the test, and only a continuous record will show it.
Creep is where a plastic's difference from a metal is most consequential. A polymer under constant load keeps deforming at room temperature, so a part that passed a tensile test can still fail in service by slowly changing shape.
Test speed
- Loading
- Applied smoothly, then held constant
- Reported
- Creep strain against time, and creep modulus where required
- Conditions
- Pre-treatment, temperature and humidity all specified
- Record the specimen's moulding and annealing history
- DakThe standard says thermal history has profound effects; a report without it cannot be reproduced.
Calculations
Creep strainε(t)
ε(t) = ΔL(t) / L₀
- ΔL(t)
- extension at time t
- L₀
- original gauge length
Measured on the specimen throughout the test, not inferred from crosshead position.
Creep modulusEc(t)
Ec(t) = σ / ε(t)
- σ
- the constant applied stress
- ε(t)
- creep strain at time t
A modulus that falls with time. Quoting one without the time it belongs to is meaningless, which is the single commonest misuse of creep data.
Why time is the variable—
Stress is held; strain is observed
The inverse of a tensile test, where strain rate is held and stress is observed. Reading creep data as though it were tensile data is a category error.
How the test runs
- 01Prepare dumb-bell specimens, moulded directly or machined, and record how.
- 02Record the thermal history — moulding conditions and any annealing.
- 03Pre-treat and condition to the specified temperature and humidity.
- 04Measure the cross-section.
- 05Fit extensometry that can remain attached for the duration.
- 06Apply the specified load smoothly and start timing.
- 07Hold the load constant for the whole test.
- 08Log strain, temperature and humidity continuously.
- 09Record the creep strain at the specified time intervals.
- 10Continue to the required duration, or to rupture where creep rupture is wanted.
- 11Report creep modulus against the time it was evaluated at, never on its own.
Grips and fixtures for this method
Axial & transverse
Clip-On Extensometers
Extensometry that can stay attached for the whole duration; a gauge that creeps itself, or slips over a month, writes its own behaviour into the result.
SpecificationsWhat the report has to contain
- Reference to ISO 899-1 and the edition
- Material, grade and any filler or reinforcement
- How the specimens were produced and their thermal history
- Pre-treatment, temperature and humidity, with the record through the test
- Applied stress and how it was held
- Gauge length and how strain was measured
- Creep strain at each specified time
- Creep modulus with its time
- Total duration, and whether rupture occurred
- Any interruption to load or conditions
What the machine must be capable of
Holding still, accurately, for a very long time. The load must stay constant for weeks or months and the extensometer must remain attached and stable for the same period — which is a quite different requirement from resolving force or strain well for five minutes. A cell that drifts over a month and a gauge that slips imperceptibly both write their own behaviour into what is recorded as material creep, and nothing in the data separates them afterwards. The conditioned atmosphere has to hold for the duration too.
What goes wrong in practice
Quoting a creep modulus without the time it belongs to, which is the commonest misuse of creep data and always flatters the material, since the modulus falls continuously. Omitting the thermal history. Logging conditions only at the start, so an overnight drift of a couple of degrees goes unrecorded. Reading a creep curve as though it were a tensile curve, when the variables are swapped. And using flexural creep data where tensile design values were needed — flexural rigs are cheaper and more numerous, which is why so much published data is flexural and why a designer working in tension is so often handed the wrong curve.
ISO 899-1 or ISO 899-2
| Part 1 — tensile creep | Part 2 — flexural creep | |
|---|---|---|
| Loading | Constant tensile load | Constant flexural load |
| Specimen | Dumb-bell | Bar in bending |
| Practicality | Needs a dead-weight or servo frame per specimen | Simple rigs, many specimens at once |
| Data | Directly usable for tensile design | Convenient, but bending mixes tension and compression |
Flexural creep rigs are cheaper and easier to run in numbers, which is why much published creep data is flexural. Tensile creep is harder to obtain and is the cleaner input for a design in tension.
Questions we are asked about this test
What is ISO 899-1?
It is the ISO method for the tensile creep of plastics — the slow deformation of a polymer held under a constant load. Specimens are dumb-bells, moulded directly or machined from sheet or articles, and it suits rigid and semi-rigid materials whether unfilled, filled or fibre-reinforced. The current edition is ISO 899-1:2017, the third.
How is a creep test different from a tensile test?
The variables are swapped. A tensile test holds a strain rate and observes the stress; a creep test holds the stress and observes the strain as time passes. That makes time the independent variable, and it is why the outputs are a strain-versus-time curve and a modulus that falls as the test runs. Reading creep data as though it were tensile data is a category error rather than an approximation.
Why does creep matter so much more for plastics than for metals?
Because a polymer creeps appreciably at room temperature under quite ordinary loads, while a metal generally does not until it is hot. A plastic bracket that passed its tensile test can still fail in service by slowly changing shape under a load it comfortably survives — the clip that loosens, the shelf that sags, the housing that no longer seals. That failure mode is invisible to every short-duration test.
Why must a creep modulus be quoted with a time?
Because it falls continuously as the test runs. A creep modulus is the constant applied stress divided by the strain reached at a particular moment, so a hundred-hour value and a thousand-hour value for the same material are different numbers and both are correct. Quoting one without its time is the commonest misuse of creep data, and it always flatters the material.
Why does thermal history matter?
The standard states that it can have profound effects on creep behaviour. Moulding conditions and any subsequent annealing set the polymer's internal stress state and crystallinity, and creep is exquisitely sensitive to both. Two specimens of nominally the same grade, moulded differently, will creep differently — so how the specimen was produced belongs in the report as firmly as the material name does.
What makes the equipment requirement unusual?
Duration. The load has to stay constant for weeks or months, and the extensometer has to stay attached and stable for the same period. A load cell that resolves force beautifully for five minutes may drift over a month; a gauge that grips well initially may slip imperceptibly. Both write their own behaviour into what is recorded as material creep, and nothing in the data separates them afterwards.
Why is this method written for design use?
Because creep data is what a plastic part is actually designed against, and there is no shortcut to it. Most plastics test methods disclaim design use and offer comparison instead; this one states that it is intended to provide data for engineering design as well as research. That is also why its conditioning and reporting requirements are stricter than a comparative method would need.
Running ISO 899-1 on the Series 7200 and Series 9000
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 | Low and, crucially, constant — held for weeks or months rather than minutes | 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 7500-1 Class 1, with stability over the whole duration as the real requirement | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Dead-weight or servo-held loading with grips that will not relax, in a controlled atmosphere | Wedge, vice-action, pneumatic and hydraulic grips, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 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.