Testing standard
Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D2990 covers tensile, compressive and flexural creep and creep-rupture of plastics. A specimen is loaded to a fixed force and left alone — a thousand hours being the usual minimum — while its deformation is read on a widening schedule. From a family of such curves come creep modulus, isochronous stress-strain curves and creep-rupture life.
From the test method to your testing system
Explore the DAK machines already listed for ASTM D2990, then review the grips, measurement and setup requirements below.
Universal Testing MachineSeries 7200Explore the machine →
Universal Testing MachineSeries 9000Explore the machine →01Understand the method
A specimen is loaded to a fixed force and then left alone while its shape slowly changes. Three loading modes sit in the one document: a dumbbell bar gripped at both ends under a constant dead load, a short specimen squeezed between flat platens, and a bar on two supports carrying a weight at mid-span through a stirrup. The load comes on smoothly, within about five seconds, then holds — a thousand hours being the usual minimum. Deformation is read on a widening schedule: minutes, then hours, then hundreds of hours, then at least monthly.
The primary output is a creep curve — strain, or mid-span deflection, against elapsed time at one stress and one temperature. From a family of such curves come creep modulus, isochronous stress-strain curves and, where the test runs to fracture, creep-rupture life.
A plastic under sustained load behaves nothing like the same plastic in a short tensile test. A snap-fit, a pressurised pipe wall, a bracket carrying its own equipment: each keeps deforming under a load it survived comfortably on day one. Creep modulus is what a designer substitutes for short-term modulus when the load is permanent, and it can be a small fraction of it. Creep-rupture data exposes stresses that look safe in a quick test and break the part after months; tension is the preferred mode where rupture life is the object.
02Prepare the specimen and test settings
There is no test speed. What is prescribed is when to look, and the schedule widens because the curve does.
03Build the test setup on a DAK machine
No capacity is prescribed. What the method demands is a load accurately known and genuinely constant for the whole run, which is why dead-weight and lever frames remain the standard tool: a hanging mass does not drift. Sizing follows the specimen. Flexural racks are commonly built for a few hundred newtons per station — of the order of 445 N on a 4:1 lever stand — and dead-load tensile stands for soft polymers work at tens of newtons, while a rigid Type I bar of roughly 40 mm² at 10 to 40 MPa needs about 0.4 to 1.6 kN. A 1 to 5 kN lever or servo frame covers most rigid grades.
There is no crosshead speed and no strain rate to control; time is the variable. Resolution near the origin governs data quality, since the creep strains of design interest are often below one or two per cent, while a rupture run on a ductile polyolefin may end at tens of per cent. No force accuracy class, strain-measurement precision or extensometer class is quoted here; those requirements are set out in the standard itself and should be read there before a frame is specified.
Environment is part of the test: 23 °C and 50 % relative humidity must hold for the entire run, which over a thousand hours means a room or chamber that survives weekends and power cuts. Elevated-temperature ranges quoted commercially, up to about 120 to 150 °C, are equipment capability rather than a method requirement.
A controlled enclosure for the whole run, not just the loading. Over a thousand hours a hygroscopic polymer will reach whatever moisture state its surroundings dictate, and that changes the creep curve.
Specifications
For the compressive mode, where the specimen sits between flat platens under a constant load rather than being gripped.
SpecificationsDak 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 | The method fixes no capacity, only a constant and accurately known load. Dead-load flexural creep racks are built for a few hundred newtons per station — about 445 N on a 4:1 lever stand — and dead-load tensile stands for soft polymers and rubbers work at tens of newtons, while a rigid D638 Type I bar of roughly 40 mm² section at creep stresses of 10–40 MPa needs about 0.4–1.6 kN. A 1–5 kN lever or servo creep frame therefore covers most rigid grades. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | unknown | ISO 7500-1 Class 0.5 — the method sets no class of its own |
| Gripping | Dead-weight or lever creep frame: tensile grips, compression platens, or a multi-station three-point flexural rack with mid-span stirrup | Our self-tightening serrated wedge grips, with V-jaws for round specimens or compression anvils, built to the specimen |
| Environment | Standard laboratory atmosphere of 23 °C and 50 % RH after D618 conditioning, held for the whole run; elevated-temperature chambers (commercially to about 120–150 °C) and controlled humidity are used where the test plan calls for them | 3009 series chambers, −150 °C to +400 °C — temperature only |
04Run the test
05Calculate, report and interpret
E_c(t) = σ / ε(t)
A TIME-DEPENDENT modulus. Quoting a creep modulus without its time is meaningless — the same material has a different value at 1 hour and at 1 000 hours.
Stress plotted against strain at a FIXED elapsed time, across several creep curves
Built by slicing a family of creep curves vertically. It is what a designer actually uses, because it answers how much a part will have deformed after a given service period.
Grip creep is the quiet one. A wedge or vice grip that holds a plastic tab well enough for a five-minute test beds into it over weeks, and the extra displacement reads as material creep unless strain is taken on the gauge section.
Conditioning drift is invisible in the data. A room that wanders off 50 % relative humidity changes the moisture content of a polyamide, and the curve stays smooth and plausible while being wrong.
In compression, a slender specimen or platens out of parallel give buckling rather than creep; the deflection accelerates and is easily mistaken for tertiary creep. The reading schedule can also hide the end — once observations are monthly, a specimen runs away and fractures between them, leaving rupture time known only to within the interval.
06Compare methods and find answers
| ASTM D2990 | ASTM D638 / D695 / D790 | |
|---|---|---|
| Duration | 1 000 h minimum, often far longer | Minutes |
| Load | Constant | Increasing |
| Specimen | Borrowed from the short-term method | The same bars |
| Answers | How much will it deform over months? | How strong is it now? |
| Output | Time-dependent modulus | A single modulus |
The same bar, a different question. D638, D695 and D790 measure response over minutes at a controlled rate; this measures what happens over months at a fixed load. A short-term modulus used in a long-term deflection calculation will understate the deformation substantially, and that is one of the commonest errors in plastics part design.
It is the ASTM practice for tensile, compressive and flexural creep and creep-rupture of plastics. A specimen is loaded to a fixed force and left, usually for at least a thousand hours, while its deformation is read on a widening schedule. The output is a creep curve at one stress and one temperature.
Because it is a time-dependent quantity. Creep modulus is the applied stress divided by the strain reached at a given elapsed time, so the same material has one value at an hour and a substantially lower one at a thousand hours. A creep modulus without its time is not a number anybody can use.
A slice through a family of creep curves at a fixed elapsed time — stress plotted against the strain each specimen had reached at, say, 1 000 hours. It is what a designer actually works from, because it answers directly how much a part will have deformed after a given period in service.
Because it gives a single curve at one stress and one temperature. Design needs a matrix: several stresses, and often several temperatures, so that creep modulus and isochronous curves can be derived across the range the part will see. That is why creep racks hold many stations rather than one, and why the data is expensive.
No, and doing so is one of the commonest errors in plastics part design. A D638 or D790 modulus describes response over minutes at a controlled rate; a part under sustained load goes on deforming for years. Using the short-term figure understates the eventual deflection substantially — sometimes by a factor rather than a percentage.
Because the curve does. Most of the action is in the first minutes and hours, and creep is roughly linear against the logarithm of time over much of its range afterwards. Reading densely early and sparsely later captures the shape with a sensible amount of effort — and plotting on a log time axis from the start is what makes that shape visible.
Discuss your specimen, test requirements and reporting needs with DAK engineering.
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.
