PTFE specimen stress relaxation testing measures the gradual decay of internal stress in polytetrafluoroethylene samples held under constant strain, providing essential data for predicting long-term sealing force and dimensional stability. This technical guide covers test specimen preparation, applicable standards such as ASTM E328 and ASTM D2991, testing procedures, data interpretation, and typical industrial applications for virgin and filled PTFE materials.
Technical Overview
Stress relaxation is the time-dependent decrease in stress exhibited by a material when it is held under a fixed deformation. For polytetrafluoroethylene (PTFE), this behavior is particularly significant because the polymer is semicrystalline and strongly viscoelastic at normal service temperatures. When a PTFE test spline or specimen is stretched or compressed to a constant strain and held in that position, the internal resisting force decays continuously as molecular chains rearrange, crystallites reorient, and the material undergoes physical aging.
Unlike instantaneous elastic response, relaxation in PTFE progresses over logarithmic time scales, meaning that a seal or gasket compressed at installation can lose a substantial fraction of its initial clamping stress within hours, days, or years. This characteristic, often referred to as cold flow in practical engineering contexts, makes stress relaxation testing a fundamental requirement for any component that must maintain sustained contact pressure, such as flange gaskets, valve seats, and backup rings.
During a typical test, the specimen is deformed rapidly to a predetermined strain, after which the applied strain is held constant while the resulting force is continuously monitored. The recorded stress-versus-time curve, together with derived quantities such as the relaxation modulus, forms the basis for material qualification, finite element viscoelastic modeling, and long-term performance prediction.
Testing Samples
Tensile Relaxation Specimens
Tensile stress relaxation tests use dumbbell-shaped splines machined or die-cut according to ASTM D638, with Type I geometry being the most common for rigid and semi-rigid fluoropolymer stock. The preferred specimen thickness is approximately 3.2 mm, and specimens must be free of voids, nicks, and molding defects that could introduce premature failure or non-uniform stress distribution.
Compression Relaxation Specimens
Compression testing, which most closely simulates gasket and seal service conditions, employs cylindrical buttons or discs. Typical dimensions are 12.7 mm diameter by 6.35 mm height, or 13 mm by 6.3 mm in accordance with ISO 3384 practice. Discs may also be die-cut directly from PTFE or expanded PTFE sheet stock to evaluate commercial gasket materials in their as-supplied form.
Component-Level Specimens
Finished parts such as ring gaskets, valve seats, envelope gaskets, and custom-molded components can be tested directly using fixtures that replicate the actual confinement geometry. Testing at the component level captures the influence of part shape, surface finish, and installation constraints that are not reflected in molded button specimens.
- Specimens must be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours prior to testing.
- Filled grades, including glass-filled, carbon-filled, and bronze-filled PTFE, should be sampled in the flow and cross-flow directions when anisotropy is expected.
- Specimen ends must be parallel within tight tolerances for compression tests to avoid angular loading artifacts.
Testing Items
A comprehensive PTFE stress relaxation characterization program typically includes the following measured and derived quantities:
- Stress-versus-time relaxation curves at one or more fixed strain levels.
- Relaxation modulus E(t) in tension or compression, calculated as instantaneous stress divided by constant applied strain.
- Percentage stress decay at benchmark times such as 1 hour, 24 hours, 168 hours, and 1000 hours.
- Residual sealing stress, expressed as the ratio of stress at a given time to initial stress.
- Temperature dependence of relaxation behavior, typically evaluated at multiple temperatures from ambient to 200 °C or higher.
- Relaxation spectra and master curves constructed through time-temperature superposition for long-term prediction.
- Comparative rankings of virgin PTFE versus filled or expanded PTFE grades.
Testing Methods
Applicable Standards
- ASTM E328, covering stress relaxation tests for materials and structures in both tension and compression.
- ASTM D2991, the standard practice for stress relaxation testing of plastics, widely referenced for specimen splines.
- ASTM F38, specifically addressing creep relaxation of gasket materials under compressive loading.
- ISO 3384, which defines stress relaxation in compression procedures originally developed for elastomeric seals and adapted for fluoropolymers.
Equipment and Instrumentation
Testing is performed on a universal testing machine or a dedicated relaxation fixture equipped with a high-stiffness load frame, a precision load cell with excellent long-term zero stability, and a data acquisition system capable of logarithmic time sampling. An environmental chamber is required for tests at elevated or sub-ambient temperatures, and grip or platen compliance must be characterized and compensated so that the true strain on the specimen remains constant throughout the hold period.
Test Procedure
- Measure and record specimen dimensions at multiple locations before loading.
- Apply the target strain as rapidly as practical, typically within 5 to 30 seconds, defining the zero time at the start of the hold period.
- Select strain levels appropriate to the application, commonly 2% to 10% in tension and 10% to 25% compression for gasket simulation.
- Maintain the strain constant while recording load at logarithmic intervals such as 1, 2, 5, 10, 30, and 100 minutes, continuing to extended durations as required.
- Correct the measured force for frame compliance, thermal drift, and load cell zero shift at the conclusion of the hold.
Data Reduction and Long-Term Prediction
Raw force data are converted to engineering stress using the original cross-sectional area and plotted against logarithmic time. The relaxation modulus is reported alongside percentage stress decay values. For design purposes, tests conducted at several elevated temperatures can be shifted using time-temperature superposition principles, with WLF or Arrhenius shift factors, to construct master curves extending to service lifetimes of ten years or more without requiring decade-long experiments.
Applications
Stress relaxation data for PTFE specimens directly support engineering decisions in industries where sustained sealing pressure and leak integrity are critical:
- Bolted flange joints using virgin, filled, or expanded PTFE sheet gaskets, where relaxation results determine recommended bolt torque and retorque schedules.
- Valve seats and stem seals in chemical processing, where loss of contact stress leads to leakage past the sealing interface.
- Static seals, O-ring backup rings, and compression fittings in semiconductor wet-process fluid handling.
- Bearing pads, slide plates, and bridge bearing elements subject to long-term sustained compressive loads.
- Wire and cable insulation and pharmaceutical and food-grade seal components requiring documented long-term mechanical stability.
- Input parameters for viscoelastic finite element models used in gasket joint analysis and seal design verification.
FAQ
What is the difference between creep and stress relaxation?
Creep is the increase in deformation of a material held under constant stress, while stress relaxation is the decrease in stress of a material held under constant strain. The two phenomena share the same underlying molecular mechanisms but are measured under opposite boundary conditions, and both reflect the viscoelastic nature of PTFE.
How long does a typical stress relaxation test last?
Screening tests may run for 1 to 24 hours, while qualification tests commonly extend to 168 hours or 1000 hours. Where longer service lives must be predicted, accelerated testing at elevated temperatures combined with time-temperature superposition is used instead of extended real-time holds.
Which standard should be selected for PTFE testing?
ASTM E328 or ASTM D2991 is generally appropriate for molded specimen splines, whereas ASTM F38 is preferred when the objective is to evaluate gasket sheet materials under compressive loading representative of flanged joint service.
Does temperature strongly affect PTFE relaxation?
Yes. Relaxation rates in PTFE increase significantly with temperature, particularly as operating conditions approach the crystalline transition regions of the polymer. This is why multi-temperature test programs are essential for components used in elevated-temperature chemical or thermal service.
Can filled and expanded PTFE grades be tested with the same methods?
Yes. Glass-filled, carbon-filled, bronze-filled, and expanded PTFE materials can all be evaluated using the same specimen geometries and procedures, allowing direct comparative ranking of stress retention performance among different material grades.