Rigidly fixedTJ-146
Compression Anvils
Flat, parallel, hardened anvils. Their parallelism is part of the method — an anvil out of true compresses one side of the specimen first and both percentages carry the error.
SpecificationsTesting standard
ASTM F36
Standard Test Method for Compressibility and Recovery of Gasket Materials
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM F36 measures how much a gasket material compresses under load and how much of that compression it gets back. A specimen is loaded to a preload, then to a major load, then back to the preload, with thickness recorded at each step. Compressibility and recovery come out as percentages, and a gasket needs both — one to conform to the flange, the other to follow it.
At a glance
- Test type
- Compression — the specimen is squeezed
- Published by
- ASTM
- Material
- Rubber, elastomers & foams
- Runs on
- Series 7200 and Series 9000
What the test does
A gasket specimen is placed between two hardened anvils and a small preload is applied to establish a datum thickness. The load is then raised to a specified major load, held, and the thickness recorded again. The load is returned to the preload and, after a short recovery period, thickness is measured once more. Nothing is pulled and nothing fails — the whole test happens within the elastic and plastic response of the material.
What it measures, and why it matters
Two figures come out: compressibility, the proportion of the original thickness lost under the major load, and recovery, the proportion of that loss regained when the load is removed. A gasket must compress enough to conform to flange irregularities and recover enough to follow the joint as bolts relax and the flange breathes with temperature. A material that compresses well and recovers poorly seals on assembly and leaks later, which is the failure this test exists to predict.
The two figures pull against each other, and that tension is the whole reason both are reported. A soft, highly compressible sheet conforms to a warped flange easily but has little left to give back; a dense, low-compressibility sheet recovers well but may never seat against the irregularity in the first place. A gasket specification therefore sets a window on each rather than a minimum on either, and a material can fail by being too compressible as readily as by being too little.
Specimen and datum
- Specimen
- Disc or square, fully covered by the anvils
- Datum thickness
- Measured AT THE PRELOADNot with a micrometer. A compressible material reads differently under any two gauges, so the preload is what makes the datum comparable between laboratories.
- Anvils
- Flat, parallel, hardened
- Conditioning
- Standard laboratory atmosphereMany gasket materials are hygroscopic, and moisture moves both results.
- Measure deflection across the anvils
- Not from the crossheadDakOn a thin gasket the frame's own flex can exceed the specimen's compression, which reads as high compressibility and low recovery.
Loads and dwell
- Preload
- Small, establishing the datum
- Major load
- Specified for the material class
- Recovery period
- Defined, after returning to preloadCutting it short reports less recovery than the material actually has.
- Resolution
- A few micrometres on thicknessBoth results are ratios of small differences, so resolution matters far more than capacity.
Calculations
CompressibilityC
C = ((t₁ − t₂) / t₁) × 100
- t₁
- thickness at preload, mm
- t₂
- thickness at major load, mm
RecoveryR
R = ((t₃ − t₂) / (t₁ − t₂)) × 100
- t₃
- thickness after returning to preload and recovering, mm
A percentage of what was LOST, not of the original thickness. A material that compresses 40 % and recovers 50 % has come back 20 % of its original thickness — the two figures have to be read together.
How the test runs
- 01Cut the specimen cleanly, sized so the anvils cover it fully.
- 02Condition it in the standard laboratory atmosphere.
- 03Set up thickness measurement across the anvils rather than from crosshead travel.
- 04Apply the preload and record the datum thickness.
- 05Raise to the major load, hold for the specified dwell, and record thickness.
- 06Return to the preload.
- 07Wait the specified recovery period.
- 08Record thickness again.
- 09Compute compressibility and recovery and report both.
Watch the test
Grips and fixtures for this method
5 to 400 kNTJ-125
Direct Compression Fixture
Direct compression platens where the specimen is larger than the standard anvils cover.
SpecificationsWhat the report has to contain
- Reference to ASTM F36
- Gasket material identification and nominal thickness
- Preload and major load used
- Dwell and recovery periods
- How thickness was measured
- Compressibility as a percentage
- Recovery as a percentage
- Conditioning atmosphere
- Number of specimens, mean and standard deviation
What the machine must be capable of
Force demand is modest and the resolution requirement is severe: the preload must be applied repeatably and the thickness measured to a few micrometres, because both results are ratios of small differences. Anvils must be flat, parallel and hardened, and the deflection measurement must exclude the frame's own compliance — on a thin gasket the machine's flex can exceed the specimen's compression. A load cell of a few kilonewtons with clean low-end resolution is more useful here than a large one.
Because both results are ratios of small differences, the measurement chain matters more than the frame. A thickness resolution of a few micrometres is needed on a specimen that may only be a millimetre or two thick, and the deflection has to be read across the anvils rather than inferred from crosshead position — the frame's own flex under the major load is comparable with the compression being measured. The preload has to be repeatable to the same precision, since it sets the datum every later figure is referred to.
What goes wrong in practice
Machine compliance is the classic error: crosshead travel includes the frame's stretch, so compressibility reads high and recovery low unless the deflection is measured across the anvils themselves or the compliance is subtracted. Uneven anvils compress one side first. Rushing the recovery period reports less recovery than the material has. And using a micrometer thickness instead of the preload thickness makes the datum incomparable with anybody else's.
Two further faults recur beyond the obvious ones. Anvils that no longer sit parallel compress one edge of the disc first, so the recorded thickness change is an average across a wedge rather than a uniform compression, and the effect is invisible in the numbers. And specimens punched with a blunt die carry a compressed rim that behaves quite differently from the body of the material, which shows up as scatter between nominally identical discs rather than as an obviously wrong result.
F36 and F38 together
Two gasket properties that only mean something as a pair.
| ASTM F36 | ASTM F38 | |
|---|---|---|
| Measures | Compressibility and recovery | Creep relaxation |
| Timescale | Minutes | Hours, often at temperature |
| Answers | Will it conform, and spring back? | Will it stay loaded? |
| Failure it predicts | Poor initial seal | A leak after a thermal cycle |
A material with excellent compressibility and poor relaxation resistance seals beautifully on assembly and leaks a month later. Qualifying a gasket on F36 alone is how that gets missed.
Questions we are asked about this test
What is ASTM F36?
It is the ASTM test method for compressibility and recovery of gasket materials. A specimen is loaded to a preload to establish a datum thickness, then to a specified major load, then back to the preload, with thickness recorded at each stage. The results are compressibility and recovery, both as percentages.
Why is the datum thickness taken at a preload rather than with a micrometer?
Because a compressible material reads differently under any two gauges — a micrometer at whatever pressure the operator applies gives a thickness nobody else can reproduce. Taking the datum under a defined preload makes it a repeatable measurement, and it is why the preload is specified rather than nominal.
What do compressibility and recovery tell me together?
Whether the gasket will seal and stay sealed. Compressibility says whether it will conform to flange irregularities; recovery says whether it will follow the joint as bolts relax and the flange moves with temperature. A material that compresses well and recovers poorly seals on assembly and leaks later.
Recovery is a percentage of what?
Of the thickness LOST under the major load, not of the original thickness. A gasket that compresses 40 % and recovers 50 % has regained 20 % of its original thickness. Reading recovery as a fraction of the original overstates it substantially, and it is a common misreading.
Why does machine compliance matter so much here?
Because on a thin gasket the frame's own stretch can exceed the specimen's compression. If deflection is taken from crosshead travel, the machine's flex is counted as gasket compression — so compressibility reads high and recovery reads low, and both errors point the same way. Measuring across the anvils removes it.
How thick a gasket can ASTM F36 test?
There is no thickness limit in principle, but thin sheet is where the method gets difficult. Both results are ratios of small differences, so on a one-millimetre gasket the compression may be tens of micrometres — comparable with the frame's own flex under load. That is why the deflection is read across the anvils and why the preload must be repeatable to the same precision as the measurement.
Does ASTM F36 predict how a gasket will seal?
Only partly, and that is the point of reading it with ASTM F38. Compressibility says the gasket will conform to the flange and recovery says it will follow the joint as it moves, but neither says how much load it will still be carrying next month. A material can pass both handsomely and relax badly, which is the field failure the pair together is meant to catch.
Running ASTM F36 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 — the specified major load on a small disc, commonly well under 10 kN. Resolution on thickness matters far more than 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, hardened compression anvils covering the specimen fully. | Our compression anvils, built to the specimen |
| Environment | Ambient: standard laboratory atmosphere. Many gasket materials are hygroscopic. | 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.