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.
