PTFE Gasket Stress Relaxation Test: Standards, Methods and Evaluation

PTFE gasket stress relaxation testing measures how effectively polytetrafluoroethylene gaskets retain compressive sealing stress under constant deformation over time, particularly at elevated temperatures. This test is essential for predicting bolt load loss, leakage risk, and long-term sealing reliability in bolted flanged joints used across the chemical, pharmaceutical, petrochemical, and food processing industries. Accredited third-party laboratories conduct these evaluations in accordance with recognized standards such as ASTM F38, DIN 28090-2, and EN 13555 to support material qualification, supplier approval, and flange joint design.

Technical Overview

PTFE (polytetrafluoroethylene) gaskets are widely used in bolted flanged joints because of their outstanding chemical inertness, broad service temperature range, low friction, and excellent non-stick properties. However, PTFE is a viscoelastic material with a relatively high tendency to creep, which means it deforms continuously under sustained load. When a gasket is compressed between flanges and held at a fixed deformation, the internal sealing stress gradually decreases over time. This phenomenon is known as stress relaxation.

In practical terms, stress relaxation translates directly into bolt load loss. As the gasket material flows and relaxes, the clamping force provided by the bolts diminishes, reducing the residual gasket stress available to maintain a tight seal. If the residual stress falls below the minimum required sealing stress, leakage or even complete joint failure can occur, with consequences ranging from product loss to safety and environmental hazards. Stress relaxation testing therefore provides quantitative data on how much of the initial gasket stress is retained after a defined period, and it is one of the most important performance indicators for PTFE gasket materials, especially in applications involving elevated temperature and thermal cycling.

Testing Samples

Stress relaxation testing can be performed on a wide range of PTFE-based gasket products. Typical sample categories submitted to accredited third-party laboratories include:

Virgin PTFE Gaskets

Gaskets manufactured from unfilled, sintered PTFE resin. These materials offer maximum chemical resistance but exhibit the highest creep and relaxation tendency among PTFE products, making relaxation testing particularly critical for qualification.

Filled and Modified PTFE Gaskets

PTFE compounds reinforced with fillers such as glass fiber, carbon, graphite, bronze, mica, barium sulfate, or silica. Fillers significantly reduce creep and improve compression characteristics, and testing verifies the effectiveness of the formulation.

Expanded PTFE (ePTFE) Gaskets

Uniaxially stretched PTFE sheet or joint sealant tape with a highly conformable, fibrous microstructure. These materials conform well to irregular flange surfaces but must be evaluated for stress retention under sustained compression.

PTFE Envelope Gaskets

Composite gaskets consisting of an insert material (such as corrugated metal, solid metal, or non-asbestos sheet) fully or partially encapsulated in a PTFE envelope. Testing addresses the relaxation behavior of the complete composite structure.

Specimens are usually die-cut or machined from production sheet material or finished gaskets. Common specimen forms include ring or disc-shaped samples, typically with a standard width and thickness (for example, 10 mm wide strips or 50 mm diameter discs, depending on the selected test standard), conditioned at standard laboratory atmosphere prior to testing.

Testing Items

The core parameters evaluated during PTFE gasket stress relaxation testing include:

• Initial gasket stress: the compressive stress applied to the specimen at the start of the test, selected to represent typical flange assembly conditions.
• Residual (retained) gasket stress: the compressive stress remaining in the specimen after a specified test duration.
• Stress relaxation rate (creep relaxation value): the percentage loss of initial stress, calculated from the difference between initial and residual stress.
• Stress-versus-time relaxation curve: the continuous decay profile used to characterize the relaxation kinetics of the material.
• Temperature-dependent relaxation behavior: relaxation performance at ambient, elevated (for example 100 °C, 150 °C, or 200 °C), or application-specific temperatures.
• Thickness reduction (creep deformation): the change in specimen thickness recorded during the test, which complements the stress data.
• Relaxation factor: for tests performed under EN 13555, the ratio of residual to initial stress is reported as a design input for flange calculation methods such as the EN 1591 calculation procedure.

Testing Methods

Several internationally recognized standards govern the stress relaxation testing of gasket materials. The most commonly applied methods for PTFE gaskets are described below.

ASTM F38 – Creep Relaxation of a Gasket Material

ASTM F38 is one of the most widely used methods for flexible gasket materials. A ring-shaped specimen is placed between hardened platens, and the assembly is loaded by means of a calibrated bolt and load ring equipped with strain gauges. After applying the specified initial compressive stress, the nut is fixed so that the total strain remains essentially constant. The load ring output is monitored over time (commonly 24 hours, with intermediate readings at 30 minutes and 1 hour), either at room temperature using method A or at elevated temperature using a heated platen arrangement (method B). The creep relaxation is reported as the percentage loss of the initial stress.

DIN 28090-2 – Creep Relaxation Test for Flange Gaskets

This German standard specifies a test rig in which the gasket specimen is compressed between heated platens inside a loading frame. The specimen is loaded to the initial gasket stress, the deformation is then held constant, and the force decay is measured while the assembly is held at the specified temperature (for example, for PTFE materials commonly tested at 150 °C or higher). The standard defines the characteristic creep relaxation value as a function of time and temperature, providing design-relevant data for flange gasket selection.

EN 13555 – Gasket Parameters and Test Procedures

EN 13555 defines the test methods used to determine the gasket parameters required for the design of bolted flanged connections according to EN 1591. Within this framework, the creep relaxation test measures the decrease in gasket stress under constant total deformation at the test temperature. The resulting creep relaxation factor and cold creep behavior are essential inputs for calculating long-term tightness of the joint. This method is frequently requested for gaskets intended for pressure equipment applications in the European market.

General Test Procedure

Regardless of the specific standard applied, the typical test sequence is as follows:

• Specimen preparation: samples are conditioned at standard temperature and humidity, and their dimensions (inner diameter, outer diameter, thickness) are measured and recorded.
• Installation: the specimen is centered between the loading platens of the relaxation test rig, which may include an environmental chamber or heated platens for elevated temperature testing.
• Loading: compressive stress is applied at a controlled rate until the specified initial stress level is reached, for example 30 MPa for typical PTFE sheet materials.
• Hold phase: the deformation is locked and held constant while the compressive force is continuously or periodically recorded over the test duration, commonly 24 hours or longer for extended studies.
• Data evaluation: the residual stress and relaxation rate are calculated, and the stress-time curve is generated and compared against acceptance criteria or specification limits.
• Reporting: the test report documents the material identification, specimen dimensions, initial stress, test temperature, residual stress, relaxation percentage, and complete relaxation curve.

Acceptance Criteria

Acceptance limits depend on the material specification and application requirements. Many industrial specifications for PTFE sheet gaskets require a creep relaxation value of no more than a defined maximum percentage (for example, values below approximately 20 to 35 percent are commonly specified for quality filled PTFE materials, while virgin PTFE typically exhibits higher relaxation). For EN 13555 data, the creep relaxation factor is evaluated as part of the overall flange joint tightness assessment rather than as a simple pass/fail criterion.

Applications

Stress relaxation test data for PTFE gaskets are used across many industries and engineering activities:

• Chemical and petrochemical plants: qualification of gaskets for aggressive media piping, reactors, heat exchangers, and flanged joints subject to thermal cycling.
• Pharmaceutical and food processing: verification of sealing reliability for sanitary fittings, sterile process lines, and clean-in-place systems where PTFE is preferred for purity and non-contamination.
• Semiconductor and ultrapure applications: assessment of expanded PTFE seals used in high-purity fluid handling systems.
• Flange joint design and calculation: provision of relaxation factors and residual stress data as inputs to EN 1591-based calculations and other bolted joint design methods.
• Material development and supplier qualification: comparison of formulations, fillers, and manufacturing processes to improve stress retention.
• Quality control and failure analysis: routine batch verification and root cause investigation of leaking joints attributed to bolt load loss.
• Maintenance and re-torque planning: use of relaxation data to establish appropriate re-tightening intervals for critical flanged connections.

FAQ

What is the difference between creep and stress relaxation?

Creep refers to the continuous increase in deformation of a material under a constant stress, whereas stress relaxation refers to the decrease in stress when deformation is held constant. In a bolted flange, the two phenomena are coupled: creep of the gasket reduces its thickness, which lowers the bolt elongation and therefore the gasket stress. This combined behavior is often called creep relaxation, and it is what ASTM F38 and DIN 28090-2 measure.

Why is PTFE particularly sensitive to stress relaxation?

PTFE is a linear, non-crosslinked fluoropolymer with a relatively low elastic modulus and a melting point around 327 °C. Its molecular chains can slide past one another under sustained compressive load, especially above room temperature, resulting in cold flow and significant stress decay. Fillers, expanded structures, and modified polymer grades are used specifically to mitigate this behavior.

What initial stress and temperature are typically used?

The initial gasket stress is selected to reflect realistic installation conditions and typically ranges from about 20 to 50 MPa for PTFE sheet materials. Test temperatures are chosen according to the intended service, with 23 °C, 100 °C, 150 °C, and 200 °C being common set points. Elevated temperature tests are strongly recommended because PTFE relaxation increases markedly with temperature.

How long does the test take?

A standard relaxation test typically runs for 24 hours after reaching the initial stress, with intermediate readings at defined intervals. Extended-duration tests of 100 hours, 1000 hours, or longer can be performed to characterize long-term behavior for critical applications. Overall turnaround including conditioning and reporting is usually several working days for standard tests.

How are the results calculated?

The stress relaxation rate is calculated as the initial stress minus the residual stress, divided by the initial stress, multiplied by 100 percent. The residual stress is the force measured at the end of the hold period divided by the effective gasket contact area. Under EN 13555, the ratio of residual to initial stress is additionally reported as the creep relaxation factor for design purposes.

Can testing be performed on finished gaskets rather than sheet material?

Yes. Specimens can be die-cut from finished gaskets or from production sheet material. Testing on the actual finished product is recommended when the manufacturing process, envelope construction, or facing treatment may influence relaxation behavior.