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Stress Relaxation Testing of Materials

Significance and Purpose

Stress Relaxation Testing evaluates how a material’s internal stress decreases over time under constant strain. This is particularly important for elastomers used in seals, gaskets, and other applications where long-term force retention is crucial. Unlike creep testing (where strain increases under constant stress), stress relaxation testing measures the loss of stress at a fixed deformation. The results help in predicting the long-term sealing performance, viscoelastic behavior, and material stability under mechanical loads.

Relevant ASTM and DatapointLabs (DPL) Standards

Short-Term Stress Relaxation

  • Tensile Stress Relaxation:
    • DPL M-624: Stepwise tensile stress relaxation for elastomers (internal DatapointLabs standard)
    • ASTM D6048: Standard test for tensile stress relaxation over short durations (≤4 hours)
  • Compressive Stress Relaxation:
    • DPL M-625: Stepwise compressive stress relaxation for elastomers (internal DatapointLabs standard)

Long-Term Stress Relaxation (Extended Duration, Instrumented Testing)

  • Compressive Stress Relaxation (Long-Term, Instrumented Test):
    • ASTM D6147: Standard test method for evaluating long-term stress relaxation of elastomeric materials using instrumented setups

DatapointLabs Tests for Stress Relaxation Testing

Tests in the DatapointLabs test catalog that reference stress relaxation testing are as follows:

Stress Relaxation Testing Specific to Elastomers

Test ID Test Description Standards
M-624 Stepwise Tensile Stress Relaxation DPL M-624*
M-625 Stepwise Compressive Stress Relaxation DPL M-625*
M-626 Tensile Stress Relaxation (up to 4 hours hold time) ASTM D6048
M-266 Long Term Instrumented Stress Relaxation ASTM D6147

* Internal DatapointLabs Standard

Principle of Operation

Stress relaxation testing involves:

  1. Applying a constant or stepwise strain: Either tensile or compressive, to a test specimen.
  2. Measuring the decrease in stress over time: As molecular rearrangements within the material lead to force decay.
  3. Recording data for a set duration: Whether short-term or long-term, depending on the test standard.

In elastomers, stress relaxation occurs due to viscoelastic behavior, molecular disentanglement, and potential environmental effects (e.g., temperature, humidity).

Typical Procedure

  1. Sample Preparation
    • Test specimens are machined or molded according to standard dimensions.
    • Conditioning is done per test requirements (e.g., temperature stabilization).
  2. Test Setup
    • The specimen is placed in a tensile or compression fixture.
    • Strain is applied to a predefined level (e.g., 10%, 20%, or 50% elongation in tensile tests).
  3. Relaxation Phase
    • The applied strain is held constant or in a stepwise manner.
    • The decrease in force (stress) is recorded over time using load cells or other sensors.
  4. Data Recording and Analysis
    • Short-term tests (ASTM D6048, DPL M-624/M-625) may last minutes to hours.
    • Long-term tests (ASTM D6147) can last for up to 1000 hours under controlled conditions.

Specimen Types

Specimens used by DatapointLabs in various types of stress relaxation testing are as follows:

Specimen Type DatapointLabs Test IDs
Tensile Bars [Details] M-626
Prisms [Details] M-266
Contact Us [Details] M-624, M-625

Extensometry Techniques

Extensometry techniques typically employed by DatapointLabs in stress relaxation testing are as follows:

Extensometry Technique DatapointLabs Test IDs
Crosshead Displacement (Axial) M-624, M-625, M-626, M-266

Characterization Measurements

Stress Relaxation Curve

  • A stress relaxation curve plots stress vs. time at a constant strain.
  • The curve typically exhibits a rapid initial stress drop, followed by a gradual decline as the material stabilizes.
  • Key parameters that may be extracted from the curve include:
    • Initial stress (σ0) at the start of relaxation.
    • Residual stress (σt) after a defined time.
    • Stress retention percentage (σt/σ0×100%) after a given duration.
    • Relaxation rate (slope of stress decay over time).

Typical Data Reported (see test descriptions for exact details)

  • Stress Relaxation Curve: Plot of stress vs. time at constant strain.
  • Percentage of Stress Relaxation Over a Given Duration: Derived from stress relaxation curve.
  • Stress Retention at Various Time Points: Derived from stress relaxation curve.
  • Effect of Temperature on Stress Relaxation: Probing temperature dependency of stress vs. time at non-ambient temperatures.

 Suitable Material Types

  • Elastomers (Rubber Materials): Natural rubber, silicone rubber, EPDM, nitrile rubber, polyurethane elastomers.
  • Thermoplastic Elastomers (TPEs): TPU, TPO, SEBS-based elastomers.

Suitable Applications

  • Seals & Gaskets: Predicting long-term sealing effectiveness in automotive, aerospace, and industrial applications.
  • O-Rings & Rubber Components: Evaluating stress decay in dynamic or static sealing environments.
  • Medical Devices: Assessing force retention in soft elastomeric components such as tubing and stoppers.
  • Vibration Dampers & Shock Absorbers: Understanding viscoelastic stress relaxation properties for performance over time.
  • Electrical Insulation Components: Evaluating force decay in elastomeric parts used in cable and electronic housings.

Conclusion

Stress relaxation testing is essential for assessing the time-dependent mechanical stability of elastomers and other viscoelastic materials. Standards such as ASTM D6048, ASTM D6147, and DPL M-624/M-625 provide structured methodologies for evaluating stress decay under tensile and compressive conditions. The results guide material selection, performance prediction, and long-term design optimization in sealing, vibration damping, and soft polymer applications.

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