Compressive creep testing evaluates the time-dependent deformation of sealing materials — including elastomeric O-rings, gasket sheets, molded seals and compression packings — under sustained compressive load, and it is one of the most critical indicators of long-term sealing performance and leakage prevention. This technical guide explains the underlying principles, sample requirements, measurable test items, internationally recognized standards (ASTM, ISO, GB), testing procedures, calculation methods and typical industrial applications of compressive creep and compression set testing for sealing materials.
Technical Overview
Creep is the progressive, time-dependent deformation of a solid material when it is subjected to a sustained mechanical stress. For sealing materials such as elastomers, expanded graphite, PTFE-based composites and rubber-bonded fibers, compressive creep manifests as the gradual thinning of a gasket, O-ring or seal under a constant clamping load, bolt preload or system pressure. Because polymeric sealing materials are viscoelastic in nature, their deformation does not stop once the load is applied; it continues to accumulate over hours, days and even years of service.
In a bolted flange joint or a clamped enclosure, the compressive creep of the seal reduces the residual contact stress between the sealing surfaces. When the contact stress falls below the minimum sealing pressure required to block the medium, leakage pathways open and the joint fails. Compressive creep is therefore directly linked to seal reliability, emission control, maintenance intervals and product safety.
In laboratory practice, three related but distinct phenomena are evaluated together to fully characterize a sealing material:
- Compressive creep — the increase in compressive deformation measured while the specimen remains under a constant load or constant deflection.
- Compression set — the permanent deformation retained after the load is removed and a defined recovery period has elapsed.
- Compressive stress relaxation — the decay of sealing force over time when the specimen is held at a constant deflection.
By combining these measurements across temperature, pressure and time, engineers can extrapolate the service life of a seal, select appropriate materials for specific duty conditions and demonstrate compliance with industry material specifications.
Testing Samples
Elastomeric O-Rings and Molded Seals
Torque-relevant molded components made from NBR, FKM (fluoroelastomer), EPDM, silicone (VMQ), fluorosilicone (FVMQ) or perfluoroelastomer (FFM) are tested either as complete O-rings mounted in standardized fixtures or as cylindrical buttons cut or molded from the same compound. O-rings are typically tested at 20% to 25% cross-section deflection to simulate real installation squeeze.
Flat Gaskets and Sheet Sealing Materials
Compressed fiber sheet, expanded graphite sheet, flexible graphite laminates, PTFE sheet, rubber-coated metal and beaded or Kammprofile composite gaskets are tested as discs or rectangular strips. Test deflection and stress levels are selected to reproduce the bolt load distribution of the intended flange design, often with a compressive stress in the range of 20 MPa to 60 MPa for high-load flange gaskets.
Standard Test Specimens
For material qualification, standardized geometries are preferred so that results are comparable between laboratories. The most widely used specimen types, defined in ASTM D395 and ISO 815-1, are:
- Type 1 cylindrical button — 29.0 mm diameter by 12.5 mm thickness.
- Type 2 cylindrical button — 13.0 mm diameter by 6.3 mm thickness.
- O-ring segments or complete O-rings — tested in rigid fixtures per ISO 3384 or ASTM D6147.
All specimens must be free of voids, flash, surface defects and contamination. Materials are conditioned before testing, typically at 23 °C and 50% relative humidity for a minimum of 16 hours, unless the governing standard specifies otherwise.
Testing Items
Depending on the standard and the engineering objective, the following parameters can be reported from a compressive creep testing program:
- Compressive creep deformation (mm) and creep strain (%) as a function of time under constant load.
- Creep rate — the slope of the deformation–time curve in the secondary (steady-state) creep region.
- Compression set (%) under constant deflection (Method A) or constant force (Method B).
- Residual sealing force or force retention (%) for compressive stress relaxation testing.
- Recovery and rebound behavior after unloading.
- Temperature-dependent creep behavior at elevated or sub-ambient temperatures.
- Creep behavior under cyclic temperature or cyclic load conditions, for thermal cycling service simulation.
- Time-to-failure (creep rupture) for structural polymeric sealing components tested per ASTM D2990.
Testing Methods
Test Apparatus
A typical compressive creep testing setup consists of a rigid compression platens and fixture system, a loading mechanism (dead weights, calibrated springs or servo-controlled actuators), precision deflection measurement (LVDT transducers, dial gauges or optical/laser displacement sensors), an environmental chamber or air-circulating oven for temperature control, and a data acquisition system for continuous recording of deformation or force. Compression set fixtures use fixed steel spacers (bars) of precisely machined thickness to maintain the target deflection throughout the aging period. All equipment used in accredited third-party laboratories is calibrated and traceable to national metrology standards.
Compression Set Under Constant Deflection — ASTM D395 Method A, ISO 815-1, GB/T 7759.1
The specimen is compressed between parallel plates to a defined deflection, normally 25% of the original thickness, and locked in position by spacers. The assembled fixture is placed in an oven at the specified test temperature for the specified duration (commonly 22 h, 24 h, 70 h, 168 h or 1000 h). After exposure, the specimen is removed, allowed to recover at standard laboratory temperature for 30 minutes, and its final thickness is measured. This method best simulates seals installed in rigid joints where the gap cannot close further.
Compression Set Under Constant Force — ASTM D395 Method B
A constant compressive force, typically 1.8 kN applied through a calibrated spring, is maintained on the specimen during the aging period. This approach better represents seals operating under a fixed dead load, such as packing elements and cushioning seals, where deflection is allowed to increase as the material creeps.
Compressive Creep of Plastic and Composite Seals — ASTM D2990
For PTFE, engineering polymers and composite sealing components, ASTM D2990 defines compressive creep testing under constant applied stress. The specimen is loaded to a defined stress level and the axial strain is recorded continuously or at logarithmic time intervals, generating a creep curve from which instantaneous elastic strain, transient creep and steady-state creep rate are derived. Tests are frequently run at multiple stress levels and temperatures to enable time–temperature superposition and long-term extrapolation.
Compressive Stress Relaxation — ISO 3384, ASTM D6147, GB/T 1685
Instead of measuring deformation, these methods measure the decay of counterforce when the seal is held at constant compression. Continuous measurement (Method A) records the force decay curve throughout the test, while intermittent measurement (Method B) periodically releases and re-applies compression to obtain point values. Force retention values are essential for flange gasket design, bolt load calculations and fire-safe or emission-critical sealing qualifications.
Gasket Creep Relaxation — ASTM F38
Specifically developed for gasket materials, ASTM F38 measures the percentage of compressive load retained by a gasket specimen after a sustained period, reflecting the combined effect of creep and relaxation under bolted-joint conditions.
Calculation of Results
The principal reported quantities are calculated as follows:
- Compression set under constant deflection: CB = (t0 − t1) / (t0 − ts) × 100, where t0 is the original thickness, t1 is the thickness after recovery and ts is the spacer thickness.
- Compression set under constant force: CA = (t0 − t1) / t0 × 100.
- Compressive creep strain: ε(t) = Δh(t) / h0 × 100, where Δh(t) is the change in thickness at time t and h0 is the initial thickness.
- Force retention: R = F(t) / F0 × 100, where F(t) is the residual counterforce and F0 is the initial compressive force.
Typical Test Conditions
Common test temperatures follow standard rubber material schedules — for example 70 °C, 100 °C, 125 °C, 150 °C, 175 °C, 200 °C or 250 °C depending on the polymer class — while low-temperature compression set per ISO 815-2 and GB/T 7759.2 is performed at temperatures from −10 °C down to −100 °C, with recovery measured at the test temperature. Test durations range from short 22 h screening tests to 1000 h and longer for qualification and life-assessment programs.
Applications
Compressive creep and compression set data directly support material selection, design validation and quality control across a wide range of industries:
- Automotive — cylinder head gaskets, valve cover seals, exhaust system gaskets, brake system cups and fuel injector O-rings evaluated for sustained compression at elevated engine temperatures.
- Aerospace — hydraulic, pneumatic and fuel system seals requiring long-term force retention under wide temperature ranges and vibration.
- Oil and gas — flange gaskets, valve seats, blowout preventer sealing elements and downhole packers qualified for sustained bolt load, high pressure and high temperature.
- Electronics and electrical enclosures — IP-rated enclosure gaskets, thermal interface pads and connector seals assessed for permanent set that would compromise ingress protection.
- Chemical and petrochemical processing — PTFE and graphite-based gaskets for aggressive media pipelines, reactors and heat exchangers.
- Construction — curtain wall, window and door sealing profiles subjected to permanent compression and weathering.
- Medical, food and pharmaceutical — silicone seals for sterilizable equipment, validated for compression set after repeated autoclave cycles.
Manufacturers routinely submit sealing materials to accredited third-party laboratories for compressive creep testing to support supplier qualification, incoming material inspection, failure analysis and certification against international and national standards.
FAQ
What is the difference between compressive creep and compression set?
Compressive creep is the increasing deformation measured while the specimen is still under load, whereas compression set is the permanent deformation that remains after the load is removed and a recovery period has elapsed. A material can show moderate creep but high set, or vice versa, so both parameters are usually specified in material datasheets.
Which standard should be selected for O-ring seals?
For O-rings and molded elastomeric seals, compression set is most commonly tested to ASTM D395 Method A, ISO 815-1 or GB/T 7759.1 at 25% deflection, while force retention is measured to ISO 3384 or ASTM D6147. The choice depends on whether the failure mode of concern is loss of seal height or loss of sealing force.
What deflection and stress levels are used?
Elastomeric compression set tests standardly use 25% deflection; harder compounds and many O-ring geometries use 15% to 20%. Constant-force and gasket tests are stress-controlled, typically 20 MPa to 60 MPa for flange gaskets and about 7 MPa for sheet material qualification, unless the product standard specifies otherwise.
How long does a typical test take?
Screening tests of 22 h or 24 h provide rapid comparative data, 70 h and 168 h tests are standard for material specification, and 1000 h or longer tests are used for qualification and long-term performance extrapolation. Low-temperature tests generally run for 24 h to 72 h at the specified cold temperature.
Can testing simulate real service conditions?
Yes. Tests can be combined with elevated or low temperatures, immersion in fluids and chemicals, thermal cycling and cyclic mechanical loading to reproduce actual duty conditions. Continuous deflection or force monitoring allows full creep and relaxation curves to be recorded for finite element analysis input and service life prediction.
What sample quantity and size are required?
Standard cylindrical buttons of 29 mm × 12.5 mm or 13 mm × 6.3 mm are preferred, and at least three specimens per condition are tested for statistical validity. For O-rings and finished gaskets, complete parts can be tested in dedicated fixtures provided the geometry is representative and documented in the test report.