Compression Set — ISO 815 / ASTM D395 · Rubber Testing

What Is Compression Set?

Permanent compressive deformation test is a test in which the sample is compressed at a defined rate, kept at a certain temperature and time, and then released; It is an experiment in which the percentage of deformation that is not recovered is determined.

Purpose of Compression Set

The aim is to evaluate whether the gasket can maintain its sealing ability under long-term compression.

Which Products Is Compression Set Applied To?

O-ring and static seals
Flange and cover gaskets
Vibration isolation elements
Sealing profiles

In Which Sectors Is Compression Set Used?

RubberAutomotiveMachinery

Standards Related to Compression Set

ISO 815-1
ISO 815-2 (low temperature)
ASTM D395 Method B

Compression Set Conditions

Clamping: The ratio defined in the standard; typically 25%
Temperature: Defined in the specification; from room temperature to elevated temperature
Time: Defined in the standard; typically 22 or 72 hours
Geri kazanma: After release, the time defined in the standard is waited and measured.

Equipment for Compression Set

Compression set plates and spacers
Calibrated thickness measuring instrument
temperature controlled oven
Conditioning room

Sample Requirements for Compression Set

Sample sizes must be defined in the standard
Spacer thickness determines compression ratio and must be verified
Sample surfaces must be flat and parallel
The number of specimens is defined in the standard

Duration of Compression Set

3–7 days depending on duration.

Compression Set Procedure

1Specimen thickness is measured.
2The sample is compressed to the defined ratio between the plates.
3It is kept at a defined temperature and time.
4The sample is released and rested for a defined period of time.
5The thickness is measured again.
6Percentage of permanent deformation is calculated.

Compression Set Acceptance Criteria

The acceptance criterion is the maximum permanent deformation percentage given in the specification.

Evaluation of Results

A compression set value given without specifying temperature and time cannot be compared. Failure to comply with the recovery period makes it look worse than it actually is; The elastomer recovers for a while after being released.

Compression Set Report

Standard and method applied
Compression ratio
Temperature and duration
recovery time
Permanent deformation values

Accreditation

EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.

Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.

Scope and authorisations

Related Regulations

Gasket and sealing specifications

Rubber Testing — What Does This Testing Field Cover?

The section below describes the field that the Compression Set test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.

Rubber testing determines the mechanical behaviour of an elastomer and its resistance to environmental effects. Elastomers are assessed with a different logic from metals and plastics: the real question is not strength but ELASTIC RECOVERY and ageing behaviour.

The function of an elastomer is usually to deform and return: a gasket is compressed and provides a seal, a mount absorbs vibration, a hose expands under pressure. What is measured in the field is therefore not "how many newtons does it withstand" but "how far does it recover after deformation, and how long does it keep doing so".

COMPRESSION SET is therefore the most decisive measurement in the field. If a gasket cannot recover after being held compressed for a long period, it has lost its sealing function even though it has not torn at all. This measurement is made together with temperature, because permanent set increases rapidly with temperature.

Hardness is the most common measurement in the field, using the Shore A and IRHD scales. It is fast and practical but does not define the material on its own: two different compounds of the same hardness can show completely different ageing and fluid resistance. Hardness is valuable in monitoring compound consistency.

On the environmental resistance side, hot air ageing, swelling in fluids, ozone cracking and low temperature behaviour are examined. FLUID RESISTANCE is particularly critical: the same elastomer behaves differently in fuel, oil, coolant and brake fluid — swelling in one, shrinking in another, hardening in a third. The material is therefore tested with the actual fluid it will contact and at the actual temperature.

Ozone cracking is a silent cause of failure in parts used outdoors. Low concentrations of ozone in the atmosphere create surface cracks in unsaturated elastomers under strain; the part cracks while appearing to carry no load at all. An ozone chamber accelerates and reveals this behaviour.

Low temperature behaviour is the second silent risk. As an elastomer approaches its glass transition temperature it loses elasticity and behaves like a rigid plastic; a gasket starts to leak in the cold. Low temperature recovery and brittleness tests establish this limit.

A change of compound is not a silent change in this family. When filler content, plasticiser or the vulcanisation system changes, mechanical properties and fluid resistance shift together; verification is repeated.

Method groups in this field

Mechanical
Tensile stress-strain properties, tear strength, abrasion (rotating drum and Pico), fatigue and tear propagation.
Hardness
Shore A and IRHD indentation hardness; a rapid indicator of compound consistency.
Permanent deformation
Compression set and stress relaxation; decisive for sealing components.
Fluid resistance
Swelling in fuel, oil, coolant and chemicals, change in mass and volume, loss of hardness and strength.
Ageing
Hot air ageing, ozone cracking, the effects of UV and humidity.
Low temperature
Low temperature recovery, brittleness temperature and flexibility measurements.

Commonly confused in this field

Hardness does not define the material; two compounds of the same hardness can show different ageing and fluid resistance.
Fluid resistance must be tested with the actual fluid and the actual temperature; a general claim of oil resistance is insufficient.
Without measuring compression set, no decision can be made about sealing life.
Ozone cracking occurs under strain; a test with an unstrained specimen does not show the real risk.
A change of compound requires verification to be repeated; a supplier may change its compound silently.
A gasket whose low temperature behaviour is not stated produces unexpected leaks in cold climates.

Which decisions these results support

Material selection for gaskets and sealing components
Automotive and industrial component approval
Verification of a compound change
Root cause analysis of field leaks and cracking
Declaration of the operating temperature range and fluid compatibility

Related Tests

Frequently Asked Questions About Compression Set

Is a gasket with a lower compression set value always better?

A low value is positive in terms of tightness, but it is not the only criterion. Temperature resistance, chemical resistance and compression stress relaxation should be evaluated together.