Testing standard
ASTM E139 Creep, Creep-Rupture and Stress-Rupture Testing of Metals
Standard Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM E139 covers creep, creep-rupture and stress-rupture testing of metals. A bar is held at temperature under a constant force — often for thousands of hours — while extension is tracked, or simply left until it breaks. It yields the creep curve, the minimum creep rate, times to specified strains and rupture life: the data behind every high-temperature design allowable.
At a glance
- Test type
- Creep & relaxation
- Published by
- ASTM
- Edition
- E139-24
- Material
- Metals, alloys & welds
- Runs on
- Series 7200 and Series 9000
From the test method to your testing system
Explore DAK equipment for ASTM E139, then review the specimen and setup requirements below.
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01Understand the method
What the test does
A machined metal bar is installed in a load train running vertically through a split furnace, brought to temperature and held until the whole gauge length is at heat. A constant force is then applied smoothly, without shock — usually through a lever arm multiplying a hanging dead weight — and left in place. Either the extension is tracked continuously, or the bar is left until it breaks and the elapsed time recorded. Runs of tens of hours to 10 000 h are routine; frames are built for durations approaching 100 000 h.
What it measures, and why it matters
A creep test yields extension against time at one stress and one temperature: the decaying primary stage, the steady secondary stage that gives the minimum creep rate, and the accelerating tertiary stage ending in fracture. Times to specified strains come off the same curve. A stress-rupture test drops strain measurement and reports rupture life alone, with elongation and reduction of area from the broken halves.
These numbers set allowable stresses for anything that runs hot under load — turbine blading, boiler and superheater tubing, pressure-vessel internals, hot fasteners. Minimum creep rate governs how far a component distorts before replacement, which for a rotating part is a clearance question, not a strength one. Rupture life across stresses and temperatures is what time-temperature parameter methods extrapolate from, so a design life is only as sound as these runs.
02Prepare the specimen and test settings
Specimen and the load train
The specimen is conventional. What is unusual is that it must survive being correct for thousands of hours, with nothing drifting.
- Specimen
- Machined bar, threaded or shouldered ends
- Furnace
- Split, multi-zone, closed around the gauge
- Thermocouples
- On the gauge length itselfAnd more than one, because a gradient along the gauge means the hottest section creeps fastest and the result belongs to that section.
- Loading
- Applied smoothly, without shockUsually through a lever arm multiplying a hanging dead weight — a genuinely constant force, which a servo frame has to work to imitate.
- Duration
- Tens of hours to 10 000 h routinelyFrames are built for durations approaching 100 000 h. That is over eleven years.
- Alignment
- Verified before the run startsDakThere is no correcting it later. A bending component discovered at 4 000 hours has cost four thousand hours.
There is no speed — only load and time
- Force
- Constant, held for the duration
- Creep test
- Extension tracked continuously
- Stress-rupture test
- Strain not measured; time to break recordedWith elongation and reduction of area from the broken halves afterwards.
- Temperature stability
- Held for the whole runThe binding requirement of the whole method. Creep rate is exponentially sensitive to temperature.
03Build the test setup on a DAK machine
What the machine must be capable of
The method prescribes an accuracy class, not a capacity: force is governed by Practices E4. Sizing follows the specimen. A 6 mm bar of about 28 mm² at creep stresses between 50 and 400 MPa needs roughly 1.4 to 11 kN, a 10 mm bar at 300 MPa about 24 kN. Lever creep frames for this work are commonly rated 20 to 50 kN, ratios of 20:1 or 50:1 keeping the dead weights manageable.
No crosshead speed or strain rate is programmed. The frame must hold a force absolutely steady for the whole run and keep the load train straight: bending superimposed on the axial stress shortens rupture life without warning, and alignment is verified separately under Practice E1012.
Strain measurement is performance-based rather than tied to a named class. Practice E83 is referenced for extensometer classification and the class used must be reported, but the requirement is that the system resolve creep strain finely enough for the intended use of the data, and that it sense the specimen rather than the parts attached to it. Extension pieces bring the sensing element out of the hot zone.
Temperature is the other half of the machine: a multi-zone furnace holding the set point for the entire run, thermocouples on the reduced section, permitted deviation banded by temperature in the standard's own text. No bounds are set — furnaces sold against the method typically cover roughly 200 °C to 1 200 °C, which is equipment capability, not a method limit. Humidity is not controlled.
Running ASTM E139 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 | The method prescribes an accuracy class, not a capacity, so sizing follows the specimen: a 6 mm diameter round bar of about 28 mm² at creep stresses of 50–400 MPa needs roughly 1.4–11 kN, and a 10 mm bar at 300 MPa about 24 kN. Commercial lever creep frames for this work are commonly rated 20–50 kN, with 20:1 or 50:1 lever ratios doing the multiplying. | 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 | Lever or dead-weight creep frame with threaded, shouldered or pinned pull rods inside a multi-zone split furnace | Our a fixture built for this method, built to the specimen |
| Environment | Constant elevated temperature held for the entire test, measured by thermocouples on the reduced section, with the permitted deviation banded by temperature; no humidity requirement | 3009 series chambers, −150 °C to +400 °C — temperature only |
04Run the test
How the test runs
- Machine the bar with threaded or shouldered ends and measure the gauge.
- Verify the alignment of the load train before anything is heated.
- Install the specimen and close the split furnace around it.
- Attach thermocouples to the gauge length, more than one along it.
- Heat and soak until the whole gauge is at temperature and uniform.
- Apply the force smoothly, without shock.
- Log extension continuously for a creep test, or leave it for a rupture test.
- Hold temperature and load for the duration — hours to years.
- Read the primary, secondary and tertiary stages from the curve.
- On fracture, measure elongation and reduction of area on the broken halves.
- Report every figure against the stress and the temperature it belongs to.
The commonest way to lose a creep test is a temperature excursion nobody noticed. Continuous logging of the furnace as well as the specimen is what lets a questionable result be defended or discarded on evidence rather than on argument.
05Calculate, report and interpret
Reading the creep curve
ε̇_min = slope of the secondary stage
The steady stage between the decaying primary and the accelerating tertiary. It is the single most used number from a creep test and the basis of most extrapolation to service life.
t_ε = elapsed time at a nominated creep strain
Read off the same curve. Design codes commonly specify 1 % creep strain in a stated number of hours.
t_r = elapsed time to fracture at one stress and temperature
With elongation and reduction of area measured on the broken halves, which distinguish a ductile creep failure from a brittle one.
What the report has to contain
- Reference to ASTM E139
- Material identification, heat, product form and heat treatment
- Specimen dimensions and gauge length
- Applied stress and how the force was produced
- TEST TEMPERATURE and the stability actually held
- Soak time before loading
- Creep curve, or rupture time for a stress-rupture test
- Minimum creep rate and times to specified strains
- Elongation and reduction of area after fracture
- Any temperature or load excursion during the run
What goes wrong in practice
Off-axis loading is the classic invisible error. A load train fractionally out of line puts bending on one side of the gauge section; the test looks normal, the rupture time comes out short, and nothing in the record explains it.
Thermocouple drift is the long-run equivalent. Over thousands of hours at temperature a thermocouple decalibrates, and a set point that has quietly slipped a few degrees moves rupture life far more than the same error would move a tensile result.
Oxidation of the gauge section in air masquerades as creep: a thin bar loses section to scale, true stress climbs during the run, and the specimen fails early — worst on small diameters and long durations. A fracture at or near a shoulder, from pull-out or thread relaxation, is no result at all whatever the elapsed time says.
06Compare methods and find answers
Creep, rupture and short-term strength
| ASTM E139 creep | Stress rupture | ISO 6892-2 hot tensile | |
|---|---|---|---|
| Duration | Hours to 100 000 h | Hours to thousands | Minutes |
| Load | Constant force | Constant force | Increasing to fracture |
| Output | Creep curve and rate | Time to break | Proof and tensile strength |
| Answers | How much will it stretch in service? | How long until it breaks? | How strong is it right now? |
A hot tensile test says nothing about creep. A material can meet its elevated-temperature proof strength comfortably and still deform unacceptably over a year at a fraction of that stress — which is precisely why creep programmes run for years rather than being inferred from a short test.
Questions we are asked about this test
What is ASTM E139?
It is the ASTM practice for conducting creep, creep-rupture and stress-rupture tests on metallic materials. A specimen is held at temperature under a constant force, and either its extension is tracked over time or it is left until it breaks and the elapsed time recorded.
What is the difference between a creep test and a stress-rupture test?
What is measured. A creep test tracks extension continuously and yields the creep curve, the minimum creep rate and times to specified strains. A stress-rupture test drops the strain measurement and reports only how long the specimen lasted, with elongation and reduction of area taken from the broken halves afterwards. The second is cheaper and answers a narrower question.
What are the three stages of a creep curve?
Primary, where the rate decays as the material work-hardens; secondary, a long steady stage that gives the minimum creep rate; and tertiary, where the rate accelerates as damage accumulates and necking begins, ending in fracture. The secondary stage is what most design extrapolation is built on.
How long does a creep test take?
Tens of hours to 10 000 hours routinely, and frames are built for durations approaching 100 000 hours — over eleven years. That is why creep data is expensive, why it is shared across industries, and why extrapolation from shorter tests is such a well-developed and carefully policed discipline.
Why is a dead weight used rather than a servo frame?
Because a hanging weight through a lever arm is genuinely constant, indefinitely, with no control loop to drift and no power supply to fail. A servo frame can hold a force very well, but over ten thousand hours the simplest mechanism is the one least likely to have quietly done something else.
Why does temperature control matter more than anything else?
Because creep rate is exponentially sensitive to temperature. A few degrees high for a few hundred hours can consume a substantial fraction of the specimen's life, and the curve afterwards looks entirely plausible. Continuous logging of both furnace and specimen is what allows a questionable run to be defended or discarded on evidence.
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Planning ASTM E139 testing?
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.