Rigidly fixedTJ-146
Compression Anvils
Hardened parallel anvils holding a fixed separation for hours. Stability over time is the requirement here rather than capacity.
SpecificationsTesting standard
ASTM F38
Standard Test Methods for Creep Relaxation of a Gasket Material
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM F38 measures creep relaxation of a gasket material — how much of the bolt load a gasket loses over time. The specimen is compressed to a specified stress, the platens held at fixed separation, and the falling force watched. The result is the percentage of the original load still there after a stated period, often at temperature.
At a glance
- Test type
- Creep & relaxation
- Published by
- ASTM
- Material
- Rubber, elastomers & foams
- Runs on
- Series 7200 and Series 9000
What the test does
A gasket specimen is compressed between platens to a specified stress and held there while the load is monitored, either at room temperature or at an elevated one. Nothing moves after the initial compression: the platens are held at fixed separation and the force the gasket exerts is watched as it decays. The test reports how much of that force is left after a stated time.
What it measures, and why it matters
The result is creep relaxation — the percentage of the original bolt load a gasket loses over the test period. It is the single most useful gasket property for a bolted flange, because a joint does not fail when the gasket is crushed; it fails when the residual load falls below what the internal pressure needs. A material with excellent compressibility and poor relaxation resistance will seal on assembly and leak after a thermal cycle, which is exactly the field failure this test predicts.
The figure is read against the joint rather than against the material. A flange bolted to a given preload has a residual load below which the internal pressure will lift the seal, and creep relaxation says how quickly the gasket walks the joint toward that point. Two materials with identical compressibility can behave entirely differently here, because relaxation is governed by the binder and the reinforcement rather than by how readily the sheet compresses.
Specimen and soak
- Specimen
- Disc or ring, sized to the platens
- Elevated temperature
- Heated and SOAKED before measurement beginsA material relaxes far faster while its temperature is still changing, so measuring during the warm-up records the heating rather than the creep.
- Initial stress
- Specified, representing bolt load
- Hold
- Fixed platen separationDisplacement is held; force is what moves.
- Verify the frame does not creep
- Before trusting the specimenDakA frame that relaxes under load unloads the gasket, and the recorded decay belongs to the machine.
Time, not speed
- Compression
- Applied once, to the specified stress
- Then
- Displacement held constant
- Measurement period
- Hours, from the standard or the specification
- Temperature
- Ambient or elevated, held throughoutAnd reported — a room-temperature figure quoted against an elevated-temperature requirement is a substitution that looks reasonable and is not.
Calculations
Creep relaxationCR
CR = ((F₀ − F_t) / F₀) × 100
- F₀
- force immediately after compression, N
- F_t
- force remaining after the stated period, N
The percentage LOST. A low number is the good result — it means the gasket is still doing the job the bolts set it up to do.
How the test runs
- 01Cut the specimen to size and condition it.
- 02Verify the frame's own relaxation under load is negligible over the test period.
- 03Compress to the specified stress and record the initial force.
- 04For elevated-temperature work, heat and soak until the assembly is stable before starting the clock.
- 05Hold the platen separation fixed.
- 06Log force continuously, or record it at the specified intervals.
- 07Continue for the stated period.
- 08Compute the percentage of load lost and report it with the temperature and duration.
Grips and fixtures for this method
Liquid CO₂ option
Environmental Chamber 3009-006
A controlled enclosure for elevated-temperature relaxation, held closely enough that the temperature is not itself the variable being measured.
SpecificationsWhat the report has to contain
- Reference to ASTM F38
- Gasket material identification and nominal thickness
- Initial compressive stress applied
- TEMPERATURE, and the soak time before measurement began
- Duration of the measurement period
- Force at the start and at the end
- Creep relaxation as a percentage
- Evidence that frame relaxation was accounted for
- Number of specimens
What the machine must be capable of
The binding requirement is holding a fixed displacement while measuring a falling force — a stiff frame, a load cell stable over hours, and no drift in the displacement control. Elevated-temperature work needs a heated platen arrangement or a chamber, held closely enough that the temperature is not itself the variable being measured. Force capacity is modest; stability over time is everything.
The demand is stability rather than capacity. The load cell must hold its calibration over the whole measurement period, the displacement control must not drift, and the frame itself must not creep — because every one of those failures unloads the specimen and is recorded as gasket relaxation. Running a rigid dummy through the same schedule is the practical way to establish how much of the observed decay belongs to the machine before any material result is trusted.
What goes wrong in practice
Frame relaxation is mistaken for gasket relaxation more often than anything else: a frame that creeps under load unloads the specimen, and the recorded decay belongs to the machine. Insufficient soak time before the measurement period starts inflates the apparent relaxation. And reporting a room-temperature figure against a specification written for elevated temperature is a substitution that looks reasonable and is not.
Elevated-temperature work adds its own traps. Thermal expansion of the load train changes the effective compression as the assembly heats, so a test started before the temperature is stable measures expansion as well as relaxation. And a specimen that has dried out or absorbed moisture during a long soak is no longer the material that was specified, which is why the conditioning atmosphere is held rather than merely established at the start.
F38 and F36 together
| ASTM F38 | ASTM F36 | |
|---|---|---|
| Measures | Load lost over time | Compressibility and recovery |
| Timescale | Hours, often hot | Minutes |
| Answers | Will it stay loaded? | Will it conform and spring back? |
| Failure it predicts | A leak after a thermal cycle | A poor initial seal |
A bolted flange does not fail because the gasket was crushed. It fails when the residual load falls below what the internal pressure needs — which is what this test measures and F36 does not.
Questions we are asked about this test
What is ASTM F38?
It is the ASTM test method for creep relaxation of a gasket material. The specimen is compressed to a specified stress, the platens are held at a fixed separation, and the force the gasket exerts is monitored as it decays. The result is the percentage of the original load lost over a stated period.
Why is creep relaxation the most useful gasket property?
Because a bolted joint does not fail when the gasket is crushed — it fails when the residual load falls below what the internal pressure requires. Relaxation is the mechanism that gets it there, so a material that holds its load is the one that keeps sealing through thermal cycles and time.
Why must the specimen soak before measurement starts?
Because a gasket material relaxes far faster while its temperature is still changing. Starting the clock during warm-up records the heating alongside the creep, and the reported relaxation comes out high for a reason that has nothing to do with the material's long-term behaviour.
How do I know I am measuring the gasket and not the machine?
By checking the frame first. A frame that creeps under sustained load unloads the specimen, and that decay is recorded as gasket relaxation. Running the test with a rigid dummy in place of the gasket, over the same period, shows how much of the fall belongs to the machine.
Can I quote a room-temperature figure against an elevated-temperature specification?
No. Relaxation is strongly temperature-dependent, and a material that loses a few percent at ambient can lose a great deal more at service temperature. Substituting the easier test is a common shortcut and it produces a figure that will not hold up when the joint is hot.
How long should a creep relaxation test run?
As long as the specification requires, and the period is part of the result — a twenty-two-hour figure and a hundred-hour figure describe different amounts of decay in the same material. Longer runs discriminate better between materials because relaxation slows with time, so most of the difference between compounds has emerged by the end of a long test and not by the end of a short one.
Can I run F38 at room temperature if the joint runs hot?
It will give a number and the number will flatter the gasket. Relaxation is strongly temperature-dependent, so a material that loses a few per cent at ambient can lose several times that at service temperature. Where the specification names a temperature, testing below it is not a conservative simplification — it is the wrong test.
Running ASTM F38 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 | Modest force, but held for hours. Stability of the load cell over time matters more than its capacity. | 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 | Flat parallel platens holding a fixed separation, heated where elevated-temperature relaxation is required. | Our compression anvils, built to the specimen |
| Environment | Ambient or elevated. Temperature is held throughout and reported with the result. | 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.