Rubber Tension Set Testing: Evaluating Permanent Deformation After Elongation

Rubber tension set testing, also known as tensile permanent deformation testing, measures the residual elongation that remains in a rubber specimen after it has been stretched, held for a defined period, and released. The test, governed by standards such as ISO 2285 and ASTM D412, is a key indicator of elastic recovery and long-term performance for sealing compounds, elastic components, and thermoplastic elastomers. This article explains the test principle, specimen requirements, methods, calculations, typical test conditions, and industrial applications.

Technical Overview

Rubber products such as seals, gaskets, hoses, elastic bands, and flexible couplings are routinely subjected to tensile strain in service. In an ideal elastomer, the material returns completely to its original dimensions once the strain is removed. In practice, molecular chain slippage, incomplete crosslinking, filler–polymer interaction, and viscoelastic flow cause a portion of the deformation to be retained. This retained deformation after stretching is called tension set, or tensile permanent deformation.

Definition and Test Principle

Tension set testing quantifies the ability of a rubber material to recover after elongation. A marked specimen is extended to a specified elongation or held under a specified tensile load for a defined time at a defined temperature. The specimen is then released and allowed to recover under standard laboratory conditions. The distance between the benchmark marks is measured after recovery, and the residual extension is calculated as a percentage. A lower tension set value indicates better elastic recovery and a more dimensionally stable rubber network.

Why Permanent Set Matters

Excessive tension set directly degrades product performance. Seals and O-rings that take a permanent stretch no longer maintain proper interference fit, leading to leakage. Elastic fasteners, straps, and spring elements lose clamping force. Hoses and bellows may exhibit dimensional drift and reduced fatigue life. Tension set is therefore widely used in material qualification, compound development, quality control, and failure analysis.

Governing Standards

The primary international standard is ISO 2285, which covers tension set under constant elongation as well as tension set, elongation, and creep under constant tensile load. ASTM D412 includes tension set determination as part of tensile testing of vulcanized rubber and thermoplastic elastomers. Corresponding GB/T, EN, and JIS national standards adopt or closely align with ISO 2285. Supporting standards include ISO 37 and GB/T 528 for dumbbell specimen geometry, ISO 23529 for conditioning and dimensional measurement, ISO 188 for heat aging, and ISO 1817 for resistance to liquids.

Testing Samples

Standard Dumbbell Specimens

Dumbbell-shaped specimens are the most common sample type. They are die-cut from flat sheets with a typical thickness of 2.0 mm ± 0.2 mm using the standard die types defined in ISO 37, ASTM D412, and GB/T 528. Benchmark marks are applied within the narrow parallel section, typically 20 mm or 25 mm apart depending on the specimen type. At least three specimens are tested, and the median value is reported.

Ring Specimens

Small ring specimens can be used, particularly for constant-load testing where a dead weight is hung directly on the ring. Rings avoid grip slippage issues but require precise measurement of internal circumference before and after testing.

Strip and Product Specimens

Cut strips, sections taken from finished products such as tubing, cord, weatherstrip profiles, or elastic webbing, may also be tested. Results from non-standard product specimens are useful for comparative purposes but are not directly interchangeable with results from standard dumbbells.

Preparation and Conditioning

Specimens must be free of surface defects, and marks must be applied without damaging the surface. Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a minimum of 16 hours before testing. Specimens that break, tear at the grips, or show marks outside the readable range are rejected and retested.

Testing Items

Testing programs typically include the following measurement items:

• Tension set under constant elongation: the specimen is held at a fixed strain, such as 50 %, 100 %, 200 %, or 300 % extension, for a defined duration, and the residual extension is measured after release.
• Tension set under constant tensile load: the specimen is suspended under a fixed force, usually expressed as a applied stress based on cross-sectional area, and permanent set is measured after release.
• Elongation under constant load: the total extension measured while the specimen is held under load, recorded immediately after loading and at defined intervals.
• Creep under constant load: the increase in extension over time while the load is maintained, reflecting time-dependent viscoelastic behavior.
• Combined-conditioning tension set: tension set measured after additional exposure such as heat aging (ISO 188) or fluid immersion (ISO 1817) to evaluate long-term aging resistance.

Testing Methods

Equipment and Apparatus

Testing is performed with a calibrated universal tensile testing machine or dedicated tension set fixtures consisting of clamps, spools, or rigid jigs that maintain the required extension. Elevated temperature tests use a forced-air laboratory oven with temperature control compliant with the relevant standard. Additional apparatus includes a rotating cutting die, benchmark marking tool, thickness gauge with a suitable presser foot, dead weights for constant-load testing, and a calibrated length-measuring device.

Constant-Elongation Procedure

The standard constant-elongation method proceeds as follows:

• Step 1: Die-cut the dumbbell specimens, measure thickness at three points, and apply benchmark marks defining the initial gauge length L0.
• Step 2: Condition the specimens for at least 16 hours at 23 °C ± 2 °C and 50 % ± 5 % relative humidity.
• Step 3: Clamp the specimen in the set fixture and extend it to the target elongation, so the stretched mark distance L1 equals L0 multiplied by (1 + E/100), where E is the specified elongation percentage.
• Step 4: Hold the specimen at the specified temperature for the specified duration, for example 24 hours, 72 hours, or 168 hours at 23 °C, 70 °C, 100 °C, or 150 °C, placing the jig in the oven when elevated temperature is required.
• Step 5: Release the specimen without shock. For oven tests, allow the specimen to cool to laboratory temperature.
• Step 6: After a recovery period, typically 30 minutes at 23 °C unless otherwise specified, measure the final mark distance L2 to the precision required by the standard.
• Step 7: Calculate the tension set and report the median of at least three specimens together with the complete test conditions.

Constant-Load Procedure

In the constant-load method, the specimen cross-section is measured and a dead weight or static force is applied so that the nominal stress equals the specified value, commonly in the range of 0.5 MPa to 2.0 MPa or as defined by the product specification. Elongation is recorded immediately after loading and again at defined intervals to determine creep. After the holding period, the load is removed and the residual extension is measured after recovery in the same manner as the constant-elongation method.

Calculation and Reporting

Two reporting conventions are in common use, and the basis must always be stated with the result. Under the convention used in ASTM D412, tension set is the residual extension expressed as a percentage of the original gauge length: tension set equals (L2 minus L0) divided by L0, multiplied by 100. Under the convention described in ISO 2285 procedures, set may be expressed as a percentage of the applied extension: (L2 minus L0) divided by (L1 minus L0), multiplied by 100, which describes the fraction of imposed deformation that is retained. The test report should include the material identification, specimen type, elongation or load, temperature, duration, recovery time, individual and median results, and any deviations from the standard.

Typical Test Conditions

• Elongation levels: 50 %, 100 %, 200 %, or 300 %, with 100 % being the most common specification level for sealing compounds.
• Holding durations: 24 h, 72 h, or 168 h, with longer exposures used to simulate extended service.
• Test temperatures: 23 °C for standard room-temperature evaluation, and 70 °C, 100 °C, 125 °C, or 150 °C for heat resistance assessment.
• Recovery time: 30 minutes at 23 °C as the standard interval, with longer recovery periods such as 24 h specified in some product standards.

Applications

Tension set testing supports material selection, design validation, and quality control across a broad range of industries. Accredited third-party laboratories routinely perform the test for supplier qualification, incoming inspection, and certification dossiers.

• Sealing systems: O-rings, gaskets, diaphragms, and door and window seals where stretch installation or thermal cycling demands high recovery.
• Automotive components: weatherstrips, coolant and fuel hoses, engine mounts, bushings, bellows, and vibration isolators.
• Industrial rubber goods: conveyor belts, rollers, flexible couplings, rubber cords, and drive belts.
• Medical and pharmaceutical products: elastic tubing, tourniquets, strap components, and latex articles requiring dimensional stability.
• Consumer and apparel products: elastic bands, waistbands, straps, and sports equipment components.
• Materials development: comparing crosslink systems, filler loadings, polymer blends, and thermoplastic elastomer grades during compound formulation and optimization.

FAQ

What is the difference between tension set and compression set?

Tension set measures residual deformation after a specimen has been stretched, while compression set measures residual deformation after a specimen has been compressed. Both quantify permanent set, but they simulate different service modes. Seals installed with stretch are better evaluated by tension set, while seals loaded in compression are evaluated by compression set per ISO 815 or ASTM D395.

Which elongation level and holding time should be selected?

The choice depends on the intended application and product specification. A common screening condition is 100 % elongation for 24 hours at 23 °C or 70 °C. Components subjected to severe stretching or elevated temperature may be evaluated at 200 % to 300 % elongation for 72 hours to 168 hours at 100 °C to 150 °C.

How are results interpreted?

Lower tension set values indicate better elastic recovery. Typical acceptance limits for quality sealing compounds range from about 10 % to 25 % depending on the elastomer type and test severity. A significant increase in tension set after heat aging or fluid immersion signals degradation of the crosslink network.

Can the test be performed at elevated temperature?

Yes. The specimen, held at the specified elongation in a rigid jig, is placed in a forced-air oven at the target temperature for the specified duration. It is then removed, released, and measured after cooling and recovery at laboratory temperature.

Why is the recovery time before measurement fixed?

Recovery is time-dependent: the specimen retracts rapidly at first and then more slowly. Measuring at a standardized interval, typically 30 minutes, ensures reproducible and comparable results between laboratories. Different recovery times must never be compared directly.

What causes high tension set in rubber compounds?

Common causes include insufficient or non-uniform crosslinking, excessive filler loading, poor filler dispersion, plasticizer migration, chain scission from heat or oxidative aging, swelling from fluid contact, and the Mullins effect after the first elongation cycle.

How many specimens are required?

Standards such as ISO 2285 call for a minimum of three specimens, with the median value reported. Additional specimens are recommended when results are scattered or when product specifications demand tighter statistical confidence.