by Sara McCaslin Sara McCaslin No Comments

From Datasheet to Service Life: A Better Framework for Seal Qualification

In our previous article, we discussed the problems that can arise in high-performance seals when engineers focus too much on compressive stress when choosing a sealing solution. That leads to the question: How does one achieve a better characterization of seal behavior? Consider the following methods.

Compression Stress Relaxation (CSR) testing (ASTM D6147 / ISO 3384)

This approach measures retained sealing force over time under constant strain. CSR testing directly models what happens in a real gland rather than just measuring recovery after the fact. ISO 3384-1:2024 covers constant-strain conditions; ISO 3384-2:2019 extends this to alternating compression cycles, useful for simulating thermal cycling effects on effective gland compression. 

CSR is better aligned with force retention, but standard test results alone cannot predict installed life. A standard specimen compressed between parallel plates does not reproduce:

  • O-ring curvature and nonuniform strain
  • Actual gland squeeze and volume fill
  • Surface texture and groove corner effects
  • Internal pressure loading and pressure direction
  • Clearances, backup rings, and extrusion gaps
  • Media exposure, permeation, or fluid aging
  • Thermal gradients, dwell periods, and transient cycles
  • Dynamic motion or system vibration

CSR data can be an important input to a calibrated service-life assessment, but it is not a standalone life prediction. Correlation requires representative temperature, media, deformation, hardware compliance, pressure, and validation testing. 

Percent Retained Sealing Force (%RSF) 

This particular approach serves as a more directly actionable output than a compression set percentage. It answers the question engineers actually care about: how much sealing force is left after time and temperature have done their work. 

However, be aware that a normalized percentage can hide an inadequate initial sealing load. For example:

  • Material A retains 80% of an initially inadequate force.
  • Material B retains 65% of a substantially higher initial force.

Material B may retain more absolute force and provide the better seal. The design-relevant question is whether the absolute residual contact pressure/force exceeds the minimum required sealing threshold by a margin.

When using %RSF, report force decay or force retention together with the initial counterforce and a defined minimum acceptable residual force or contact-pressure threshold. A percentage alone can be misleading if the starting force differs between materials or geometries.

Dynamic Mechanical Analysis (DMA)

Dynamic mechanical analysis (DMA) evaluates an elastomer’s viscoelastic behavior by applying a small oscillating deformation while changing temperature, frequency, strain amplitude, or time. Its primary outputs are storage modulus, loss modulus, and tan δ\deltaδ. Here is what they tell us:

  • Storage modulus indicates the material’s elastic stiffness
  • Loss modulus reflects energy dissipation through internal molecular motion
  • tan δ\deltaδ, the ratio of loss modulus to storage modulus, indicates damping behavior

For sealing applications, DMA is especially useful for identifying temperature ranges where stiffness changes rapidly. A decrease in storage modulus at elevated temperature can reduce the contact force generated at a given gland squeeze and may reduce extrusion resistance under pressure. At low temperature, approaching a glass-transition or secondary-transition region can make the material less conformable, reducing its ability to accommodate surface roughness, dimensional variation, and thermal contraction. DMA can therefore help compare compounds with similar room-temperature hardness but that behave differently across the operating temperature range.

Keep in mind that DMA is a material-screening and diagnostic tool, not a direct seal-performance or service-life test. Test reports should state the test mode, frequency, strain amplitude, temperature ramp, specimen geometry, and the convention used to identify transitions, since reported transition temperatures can vary with method. Use DMA alongside compression stress relaxation, media aging, extrusion resistance, pressure cycling, and functional leakage testing on representative gland hardware.

Media-Representative Testing

Media-representative testing evaluates the seal while exposed to the actual service fluid or gas (or a technically justified simulant) rather than just relying solely on dry compression testing. The exposure should reproduce the relevant fluid composition, concentration, contaminants, temperature, pressure, dwell time, and fluid-aging or replenishment conditions. This is actually key because a material rated broadly as “compatible” may behave differently when it comes to real mixtures containing additives, water, cleaning agents, dissolved gases, oxidation products, or contaminants.`

Fluid exposure can cause issues such as shrinkage, swelling, plasticization and softening, extraction of compounding ingredients, hardening, cracking, embrittlement, and loss of mechanical properties. Such changes may alter both the installed squeeze and the force the seal retains over time. 

Standard liquid-exposure methods such as ASTM D471 evaluate changes in volume, mass, hardness, tensile strength, and elongation after immersion. This provides highly useful compatibility-screening data but does not, by itself, demonstrate leak-tight service performance.

For demanding applications such as those under discussion, it is wise to evaluate media-aged specimens using the properties that connect exposure to function:

  • Compression stress relaxation or retained counterforce
  • Dimensional change
  • Visible damage
  • Leakage performance in representative gland hardware. 

Where pressure, thermal cycling, or dynamic motion is relevant, it is also prudent to conduct exposure and functional testing under those combined conditions rather than aging the material separately in a static immersion vessel. The objective is to establish that the seal retains adequate contact pressure, dimensional stability, and structural integrity for the required service life—not merely that it survives a short, dry compatibility test.

Gland-Representative Geometry 

Material properties do not directly translate into installed seal performance unless the test configuration reflects the actual gland. Gland geometry controls the installed squeeze, stress distribution, contact width, available void volume, and deformation under pressure.

 Relevant factors include seal cross-section, groove depth and width, corner radii, installed stretch, squeeze, gland fill, clearance gap, pressure direction, and any backup-ring configuration. 

Excessive stretch can reduce effective cross-section and squeeze, while excessive gland fill can leave insufficient room for thermal expansion, media-induced swell, and pressure-driven deformation.

Representative testing should include nominal dimensions and tolerance extremes for the seal and hardware, including 

  • Cross-section variation
  • Groove dimensions
  • Concentricity
  • Runout
  • Thermal expansion
  • Pressure-induced housing deflection. 

A nominal-only test can miss low-squeeze conditions that reduce contact-force margin or high-fill conditions that increase assembly damage and extrusion risk.

Surface condition and pressure loading should also reflect the real assembly. Surface roughness, waviness, machining lay, scratches, edge breaks, pressure direction, and worst-case clearance gap can all affect leakage and extrusion. For high-pressure service, test the actual anti-extrusion configuration and verify functional leakage performance in representative hardware—not only material behavior in a parallel-plate fixture.

Focus on the Application

High-performance seals require more attention to the actual operating conditions than the compressive set. The methods we covered in this article can assist engineers in choosing the right sealing solution for high-performance applications.

by Daniel Mays Daniel Mays No Comments

Creep and Stress Relaxation in High-Performance Polymer Seals

Creep and stress relaxation are types of time-dependent deformation that matter in sealing as too many engineers in the field typically see “assembled dry, passed leak test, then seeps later.” Sealing force is not a fixed number: it decays over time. And polymer seals can be affected by factors such as viscoelasticity, temperature sensitivity, and constraint effects. 

This article explores core definitions and concepts related to creep and stress relaxation, then covers how different polymer sealing materials behave and tips for the design and installation of seals to minimize these issues.

Definitions and Concepts for Creep and Stress Relaxation

Creep is defined as the increase in strain under constant applied stress. The constant stress can be, for example,  contact stress from interference, bolt load transferred through a gasket, or differential pressure loading. The results of creep are dimensional change, extrusion growth, reduced interference, and/or a contact pattern shift.

Cold flow refers to creep at moderate or ambient temperature and is controlled by a combination of stress and constraints. As a type of creep, cold flow is dominated by a combination of viscoelastic and viscoplastic deformation under a sustained compressive load.

Stress relaxation is decreasing stress under constant strain as the result of fixed gland volume, captured seal, or fixed squeeze. This can be a problem for static seals, where the gland maintains constant displacement, not constant stress. The results of stress relaxation include clamp-load loss, loss of sealing force, and an increased possibility of leakage.

Polymers can still look like they kept their shape, but they may not be pushing as hard against the metal anymore. In elastomers, “compression set” is mainly about the rubber not springing back. In polymers, the bigger issue is that the internal stress slowly bleeds off over time, so sealing force drops even if the part does not look significantly deformed.

When a polymer is compressed, part of the squeezed portion will spring back right away, but part of it returns slowly, and another part never returns because the material has permanently shifted shape. The longer a seal is under compression, the more the polymer begins to relax and flow, so even after the load is removed, it may not be able to rebound to restore the original sealing force. 

And if you compress a polymer seal and then release it, the force on the way back will usually be lower than on the way in because some energy is lost inside the material. That’s why repeated squeeze-and-release cycles will not bring the seal back to the original force level.

Material Behavior in High-Performance Seal Polymers

PTFE (unfilled): PTFE has extremely low friction and is very chemically resistant, but it gives up the sealing load over time. Virgin PTFE tends to creep and relax under sustained compression, therefore requiring a strong gland support, tight extrusion-gap control, or spring energization.

PTFE (filled): Filled PTFE holds up better because fillers increase stiffness and reduce cold-flow behavior. Filled PTFE can usually retain its sealing force longer than virgin PTFE, but the filler used can also increase friction and may affect counterface wear.

PEEK: PEEK is typically chosen when long-term load retention matters greatly. PEEK’s higher stiffness means better resistance to creep and stress relaxation, though solid gland design and surface control still matter. PEEK is also available in filled variants that can impact its properties.

UHMW-PE: UHMW-PE is excellent for abrasion and low friction, but it can still relax under long compressive dwell, especially if stresses are high or support is limited. It performs best when the design itself minimizes sustained stress and prevents extrusion.

PAI (Torlon): PAI offers the strongest resistance to time-dependent deformation in this group. It retains shape and sealing load well, making it a strong fit for high loads and elevated temperatures where other polymers may drift.

MaterialCreep ResistanceStress Relaxation ResistanceRebound After Long DwellExtrusion Risk (if poorly supported)
PTFE (unfilled)LowLowLowHigh
PTFE (filled)ModerateModerateLow–ModerateModerate
PEEKHighHighModerate–HighLow–Moderate
UHMW-PELow–ModerateLow–ModerateModerateModerate–High
PAI (Torlon)Very HighHigh–Very HighHighLow

Design Variables That Control Creep and Relaxation

Gland constraint is the first major factor. A fully confined gland gives the seal fewer places to move, which cuts down creep flow and helps prevent extrusion. If the gland is only partially confined, any clearance becomes an escape route for the seal, and support has to be both radial and axial. Radial support keeps the polymer from pushing into the extrusion gap under pressure. Axial support helps prevent shifting and uneven edge loading. The small geometry details count as well; add corner radii and lead-in chamfers, and avoid sharp edges that create stress concentrations. Also, remember tolerance stack-up: as the seal relaxes, the “effective” clearance and contact conditions can change even if the metal parts do not.

More squeeze is not going to automatically be safer. Higher initial stress can accelerate creep and stress relaxation, especially with heat. The goal is to start with enough contact stress to seal, then still have enough after the material settles. That means designing around the minimum required contact stress at end-of-life, not just at assembly.

Extrusion gap control is about finding where pressure can escape and blocking it. The gap changes with temperature, pressure-driven hardware deflection, and assembly variation. Backup rings help by mechanically closing off that path. Their details matter, though.

Surface finish can make or break long-term performance. Roughness peaks concentrate stress and encourage localized flow, and surface lay can create leak paths. With filled polymers, counterface hardness matters because wear risk can increase with the wrong pairing. Aim for a finish that reduces stress peaks without creating new friction or lubrication issues.

Hardware stiffness also impacts load retention. Flexible joints can magnify clamp-load loss as polymers relax, so stiffer flanges, spacers, and bolt patterns will significantly assist with stability. For demanding duty cycles, spring-energized seals are an excellent option as they add an additional force to compensate for potential issues, such as relaxation, wear, thermal cycling, and small misalignment. 

Installation Tips for Mitigating Creep and Stress Relaxation

Many issues with creep start at installation, where a small nick, a cut, or a twisted seal can leak early, then get blamed on cold flow. Over-compressing the seal during assembly also makes it worse by driving high stress that speeds up relaxation and can leave permanent deformation. A simple fix is better handling and proper lubrication during installation to reduce the potential for surface damage and help the seal seat without problems due to uneven stress.

Load management matters just as much after assembly. Polymer gaskets and seats often benefit from controlled retorque protocols (when the application allows it) because the initial load can drop quickly during the first dwell. A common approach is initial torque, a short wait, then a retorque and verification check. Keep in mind that if over-torque pushes stress too high, it can accelerate creep and shorten the sealing life.

Finally, storage can quietly pre-load your failure. If a seal sits compressed on the shelf, it may relax before it sees service, starting life with serious issues related to sealing force. Temperature history matters as well, especially if parts are stored near heat sources or in hot warehouses. When possible, ship and store seals uncompressed, and for critical applications, controlled conditioning and careful packaging can protect long-term load retention.

Conclusion

Creep, cold flow, and stress relaxation are not mysterious defects, but rather predictable behaviors that appear whenever polymers sit under load for long periods. For this reason, treat them as design inputs and build a sealing system around them by choosing the right material, controlling deformation with proper gland constraint, relying on stiff hardware to maintain load, and validating the design with tests that match real pressure, temperature, and dwell-time conditions.

Advanced EMC is here to help with all your sealing needs, and our engineers are happy to help you navigate your way through potential creep and stress relaxation issues. Contact us today!