Elongation at Break Testing of Sealing Strips: Standards, Methods and Technical Requirements

Elongation at break testing is one of the most important mechanical evaluations for sealing strips used in doors, windows, curtain walls, automotive bodies, and appliance gaskets. This test, typically performed in accordance with ISO 37, ASTM D412, GB/T 528, and JIS K 6251, measures the maximum deformation a sealing strip material can withstand before fracture, directly reflecting its flexibility, resilience, and long-term sealing reliability. This article provides a comprehensive technical overview of sample preparation, testing procedures, calculation methods, applicable standards, and practical considerations for elongation at break testing of rubber and plastic sealing strips.

Technical Overview

Sealing strips, also known as weatherstrips or gasket profiles, are flexible elastomeric components extruded from materials such as EPDM rubber, silicone rubber, thermoplastic vulcanizates (TPV), thermoplastic elastomers (TPE), chloroprene rubber (CR), and plasticized PVC compounds. They are widely installed in building doors and windows, curtain walls, vehicle doors and trunks, and household appliances to provide sealing against air, water, dust, noise, and thermal transfer.

Elongation at break, expressed as a percentage, is defined as the increase in gauge length of a test specimen at the moment of rupture relative to its original gauge length. It quantifies the ductility and deformability of the sealing material. During installation and service, sealing strips are repeatedly stretched, compressed, and flexed. A material with insufficient elongation capacity will crack, tear, or lose its sealing function prematurely. Therefore, elongation at break is a mandatory quality control index in virtually all national and international specifications for sealing strips.

The test is performed on a universal tensile testing machine by stretching a dumbbell-shaped or ring-shaped specimen at a constant crosshead speed until it fractures. The result is calculated as: Elongation at break (%) = (Lb − L0) / L0 × 100, where L0 is the initial gauge length and Lb is the gauge length at rupture.

Testing Samples

Standard Dumbbell Specimens

Dumbbell specimens are punched or die-cut from flat sheet material prepared from the same compound as the sealing strip, or from the flattened body of the finished strip. Common specimen types include Type 1, 2, 3, and 4 dumbbells as specified in ISO 37 and GB/T 528, or Die C specimens as specified in ASTM D412. Type 2 dumbbells with a 20 mm gauge length are frequently used for sealing strip materials due to their convenient preparation from extruded profiles.

Ring Specimens

Small ring specimens may be cut directly from tubular or hollow sealing profiles. Ring testing is useful when it is difficult to obtain flat dumbbell specimens from complex cross-sections, and the results reflect the performance of the actual finished product rather than a re-processed sheet.

Finished Profile Sections

For sponge and dense rubber automotive or architectural profiles, straight sections of the extruded strip may be tested as full cross-section samples with an agreed initial distance between grips. This approach is often used for comparative quality control, provided the specification permits it.

Material Scope

• Dense EPDM sealing strips for doors and windows
• Sponge or foam EPDM and CR sealing profiles
• Silicone rubber sealing strips for high-temperature and curtain wall applications
• TPV and TPE thermoplastic sealing profiles for automotive glazing and body sealing
• Plasticized PVC and PVC/NBR blend sealing strips for doors and windows

Testing Items

The core testing item is elongation at break. In practice, it is evaluated together with related tensile properties to fully characterize the material:

• Elongation at break (%) — the principal parameter of this test
• Tensile strength (MPa) — maximum tensile stress before rupture
• Tensile stress at 100% elongation (100% modulus, MPa) — indicator of stiffness
• Tensile stress at a given elongation as required by product specifications
• Percentage change in elongation at break after heat aging or compression set conditioning, when required

Typical acceptance criteria in common product specifications range from a minimum of 200% for PVC-based sealing strips to 300%–400% or higher for dense EPDM compounds. The exact limit depends on the applicable product standard, the material grade, and the end-use requirements agreed between manufacturer and purchaser.

Testing Methods

Standards and References

• ISO 37 / GB/T 528 — Determination of tensile stress-strain properties of vulcanized or thermoplastic rubber (dumbbell and ring specimens)
• ASTM D412 — Standard test methods for vulcanized rubber and thermoplastic elastomers in tension
• JIS K 6251 — Japanese standard for tensile properties of rubber
• ISO 5893 — Requirements for tensile testing machines with constant rate of traverse
• ISO 23529 / ASTM D1349 — Specimen preparation, conditioning, and standard test temperatures
• GB/T 12002 — Product standard for plasticized PVC sealing strips for doors and windows, which specifies elongation at break requirements

Test Procedure

• Specimen preparation: Cut at least three (preferably five) specimens from each sample lot. Measure the thickness and width of the narrow section at three points using a calibrated thickness gauge, and record the median values.
• Conditioning: Condition the specimens at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours, or as specified by the governing standard.
• Gauge length marking: Mark the gauge length (20 mm for Type 2 specimens, 25 mm for Type 1 specimens) on the parallel section of the dumbbell using a benchmark tool.
• Machine setup: Select a load cell appropriate to the expected force range so that the measured force falls within the calibrated range of the sensor. Fit pneumatic, wedge, or pneumatic side-action grips with smooth or serrated faces.
• Test speed: Apply a constant crosshead speed of 500 mm/min for Type 1 and Type 2 dumbbell specimens, or as specified (ASTM D412 commonly uses 500 mm/min for Die C).
• Measurement of elongation: Record the increase in gauge length using a contact extensometer, non-contact video extensometer, or grip separation measurement where permitted by the standard. Continue extension until the specimen ruptures.
• Validity of results: Discard specimens that break outside the gauge length. Repeat the test until the required number of valid results is obtained.

Calculation and Reporting

Calculate the elongation at break for each valid specimen and report the median value. The test report should include the sample identification, material and product type, specimen type and dimensions, test standard, conditioning atmosphere, test speed, individual and median results of elongation at break, and any related tensile values such as tensile strength.

Applications

• Quality control of building door and window sealing strips to verify conformity with product specifications before shipment
• Automotive sealing system development and incoming material inspection for door seals, trunk seals, glass run channels, and hood seals
• Curtain wall and structural glazing projects, where silicone and EPDM gaskets must demonstrate reliable elongation performance
• Appliance industry verification of refrigerator and oven door gaskets
• Material research and formulation comparison for new elastomer compounds and recycled-content materials
• Failure analysis of cracked or degraded sealing strips after weathering, heat aging, or long-term field service
• Type testing and certification support conducted by accredited third-party laboratories for product conformity declarations

FAQ

What is elongation at break and why is it important for sealing strips?

Elongation at break is the percentage increase in length a material achieves before fracturing. For sealing strips, it indicates flexibility and resistance to cracking during installation stretching, thermal movement, and repeated mechanical cycling, which are essential for maintaining a continuous seal.

Which standards govern this test?

The primary methods are ISO 37 and GB/T 528 for dumbbell and ring specimens, ASTM D412 for tension testing of rubbers and thermoplastic elastomers, and JIS K 6251. Product-level requirements for PVC sealing strips are specified in GB/T 12002 and related national and industry standards.

Can the test be performed on the finished strip instead of a pressed sheet?

Yes. Where the product standard permits, ring specimens or full cross-section sections cut from the finished profile may be tested. However, results from specimens of different types are not directly comparable and must be clearly identified in the report.

What is the typical test speed and temperature?

The standard test speed for common dumbbell specimens is 500 mm/min, and tests are conducted at 23 ± 2 °C and 50 ± 5% relative humidity after adequate conditioning.

How many specimens are required?

A minimum of three valid specimens is normally tested, and the median value is reported. Five specimens are recommended to improve statistical reliability.

What are typical acceptance values?

Common product specifications require elongation at break of at least 200% for plasticized PVC strips and 300% or more for dense EPDM compounds, with silicone and special grades often requiring higher values. The applicable limit must always be confirmed against the governing product standard or customer specification.

How long does the test take?

Excluding conditioning time, the mechanical test itself typically takes only a few minutes per specimen. Including specimen preparation, conditioning, and reporting, a complete evaluation is usually completed within two to three working days at an accredited third-party laboratory.