Tensile Set in Elastomers: Definition, Testing Method & Material Significance
What Is Tensile Set in Elastomers?
Tensile set is a measure of the permanent deformation retained by an elastomeric material after it has been stretched to a defined elongation and then allowed to recover. It quantifies the degree to which an elastomer fails to return to its original dimensions after deformation — the residual strain that remains after elastic recovery is complete.
Tensile set is distinct from compression set (which measures permanent deformation after compressive loading) and is particularly relevant for elastomeric seals, gaskets, diaphragms, belts, and flexible connectors that experience cyclic tensile or flexural straining in service.
Why Tensile Set Matters for Elastomeric Products
In an ideal elastic material, tensile set would be zero — the material would fully recover its original length after any elongation within the elastic range. Real elastomers exhibit some permanent set due to:
- Viscous flow: Movement of polymer chain segments that cannot be fully recovered elastically
- Crosslink rupture: Breaking of chemical crosslinks under high elongation
- Chain slippage: Relative movement of entangled polymer chains during deformation
- Stress relaxation during elongation: Particularly pronounced in thermoplastic elastomers (TPEs)
High tensile set indicates poor elastic recovery — a problem in sealing applications where dimensional recovery is needed after installation deformation, or in dynamic applications where accumulated permanent set reduces preload and sealing force over time.
Test Methods for Tensile Set of Elastomers
ASTM D412 Method B (Tensile Set)
ASTM D412 — the primary standard for rubber tensile properties — includes a tensile set measurement. The dumbbell or ring specimen is elongated to a defined percentage of its original length (typically 100%, 200%, or a specified elongation) using a testing machine. After holding at this elongation for a defined period (typically 10 minutes), the specimen is released and allowed to recover for 10 minutes. Gauge marks on the specimen are then measured to determine the permanent residual elongation.
Tensile set is expressed as a percentage: Tensile Set (%) = (Final gauge length − Original gauge length) / Original gauge length × 100
ISO 2285 – Tensile Stress Relaxation
ISO 2285 measures tensile stress relaxation and permanent set under sustained elongation as a function of time and temperature. This is particularly relevant for elastomers used in sustained-load sealing applications.
Factors Affecting Tensile Set
Elastomer base polymer type (natural rubber, EPDM, silicone, nitrile, fluoroelastomer), crosslink density, vulcanisation system, temperature, elongation level, and hold time all influence tensile set. Silicone elastomers generally exhibit low tensile set due to their flexible Si-O backbone and stable crosslink network. Natural rubber compounds designed for dynamic applications are optimised to minimise set through efficient crosslinking systems.
Industrial Applications
In automotive diaphragms (fuel pressure regulators, pneumatic actuators), a low tensile set ensures consistent actuating force and seal integrity throughout the component’s service life. In medical tubing and peristaltic pump tubing, a low tensile set ensures a consistent internal diameter after installation deformation. In industrial drive belts and conveyor belts, accumulated tensile set causes slack development and belt elongation that must be managed by tensioning systems.
Conclusion
Tensile set is a key performance indicator for elastomeric materials, reflecting their ability to recover after tensile deformation. A low tensile set is essential for maintaining dimensional stability, sealing performance, and functional reliability in applications subjected to repeated or sustained stretching. By evaluating tensile set, engineers can select and optimise elastomer formulations that deliver long-term durability and consistent performance under real service conditions.
Why Choose Infinita Lab for Elastomer Tensile Set Testing?
Infinita Lab provides elastomer tensile set testing per ASTM D412 and ISO 2285 through our nationwide accredited laboratory network, supporting rubber compound development, material qualification, and quality control programmes.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What does tensile set measure? It measures the permanent elongation remaining in an elastomer after it has been stretched and allowed to recover.
How is tensile set different from compression set? Tensile set evaluates deformation after stretching, while compression set measures deformation after compressive loading.
What is considered a good tensile set value? Lower values are better — ideally close to 0%, indicating near-complete recovery.
What factors influence tensile set? Polymer type, crosslink density, temperature, elongation level, and duration of applied strain.
Why is tensile set important in sealing applications? High tensile set reduces the material’s ability to return to its original shape, leading to loss of sealing force and potential leakage.
