Analysis of the Influence of Tensile Testing Speed on Rubber Properties

Tensile testing speed is one of the most critical yet frequently underestimated parameters in rubber mechanical characterization. Because rubber is a viscoelastic material, its measured tensile strength, elongation at break, and modulus can vary significantly with crosshead speed, making standardized test conditions essential for comparable and reliable results. This article analyzes how tensile speed affects rubber test outcomes, reviews the requirements of ISO 37, ASTM D412, and GB/T 528, and provides practical guidance for engineers, quality managers, and accredited third-party laboratories.

Technical Overview

Rubber is a classic viscoelastic material, meaning its mechanical response combines elastic energy storage with viscous energy dissipation. When a rubber specimen is stretched, polymer chains uncoil, disentangle, and slip past one another, and this molecular rearrangement takes time. The speed at which the specimen is extended—typically expressed as crosshead speed in mm/min—therefore has a direct and measurable effect on the stress–strain curve recorded during a tensile test.

In general terms, increasing the tensile speed produces the following tendencies for most rubber compounds:

• Tensile strength increases, because molecular chains have less time to relax and redistribute stress, so a higher stress is required to initiate and propagate fracture.
• Modulus values at specified elongations (such as 100% and 300% tensile stress) increase for the same reason.
• Elongation at break tends to decrease, since the material cannot undergo sufficient viscous flow and chain alignment before failure occurs.
• The stress–strain curve becomes steeper overall, reflecting a more glassy, less relaxed material response.

Several mechanisms underlie these effects. First, stress relaxation time scales matter: when the deformation rate approaches or exceeds the relaxation rate of the polymer network, the material behaves more stiffly. Second, filler–rubber interactions amplify rate sensitivity; highly filled compounds with carbon black or silica show stronger speed dependence than unfilled gum stocks. Third, strain-induced crystallization, which is prominent in natural rubber (NR), interacts with test speed because crystallite formation requires both strain and time. Fourth, rapid extension generates internal heat (hysteresis), which can locally soften the material in thick specimens and complicate the interpretation of results at very high speeds.

The practical consequence is that tensile results obtained at different speeds are not directly comparable. A tensile strength measured at 50 mm/min may be 10–30% lower than the same compound measured at 500 mm/min, and elongation values can differ by a similar relative margin. This is precisely why international standards specify fixed test speeds and why test reports must always state the speed used.

Testing Samples

Natural Rubber (NR) Compounds

Natural rubber exhibits pronounced strain-induced crystallization, which gives it high tensile strength and tear resistance. Because crystallization kinetics depend on both strain level and time, NR compounds are among the most speed-sensitive materials in tensile testing. Faster crosshead speeds can delay full crystallization development, altering measured strength and elongation values.

General-Purpose Synthetic Rubbers (SBR, BR, NBR, CR, EPDM)

Styrene-butadiene rubber, butadiene rubber, nitrile rubber, chloroprene rubber, and ethylene-propylene-diene rubber are non-crystallizing or only weakly crystallizing under strain. Their rate dependence is governed mainly by chain relaxation and filler network breakdown. Carbon black filled grades of these polymers show clearly measurable speed effects, which is important for quality control lot acceptance.

Specialty Elastomers (FKM, VMQ, EPDM Peroxide Cures)

Fluorocarbon (FKM) and silicone (VMQ) compounds are widely used in seals and high-temperature service. Their tensile data are strongly dependent on post-cure state and test speed, so accredited third-party laboratories routinely verify that specification limits correspond to the standard speed stated in the relevant material specification.

Specimen Geometry

Speed effects interact with specimen geometry. Standardized specimens include:

• ISO 37 dumbbell specimens, types 1, 2, 3, and 4, cut with standard dies from sheets of 2 mm nominal thickness.
• ASTM D412 dumbbells, types A through F, and ring specimens, for which the effective strain rate distribution differs from dumbbells.
• GB/T 528 specimens, harmonized with ISO 37, used widely in Chinese industry and certification.

Smaller specimens are stretched at lower specified speeds so that the strain rate in the narrow central section remains comparable across specimen types.

Testing Items

The speed dependence of rubber affects all of the following commonly reported tensile parameters:

• Tensile strength (TS): the maximum stress recorded before rupture; consistently increases with test speed for most compounds.
• Elongation at break (Eb): the strain at rupture; generally decreases with increasing speed, though some crystallizing compounds show mixed behavior.
• Tensile stress at 100% elongation (M100) and at 300% elongation (M300): the so-called 100% and 300% moduli; both increase with speed.
• Tensile stress at yield, where applicable, particularly for harder or highly filled compounds.
• The complete stress–strain curve shape, which is used for material modeling, finite element analysis input, and failure investigation.
• Comparative rate-sensitivity studies, in which one compound is tested at multiple speeds to characterize its viscoelastic behavior for dynamic service predictions.

When a test report lists any of these values without the associated crosshead speed, the data lose traceability and cannot be validly compared against specification limits.

Testing Methods

ISO 37 Method

ISO 37, together with specimen preparation and conditioning requirements in ISO 23529, specifies a series of allowable test speeds, including 1, 2, 10, 25, 50, 100, 200, 250, and 500 mm/min. For the commonly used type 1 and type 2 dumbbells, 500 mm/min is the standard speed for most routine testing, while slower speeds are designated for smaller specimen types and for special investigations. Testing machines must comply with the force and speed accuracy classes of ISO 5893. Elongation is preferably measured with a contact or non-contact extensometer on marks within the parallel central section rather than inferred from crosshead travel alone.

ASTM D412 Method

ASTM D412 designates 500 mm/min (approximately 20 in./min) as the customary speed of testing for vulcanized thermoset rubbers, with slower speeds such as 50 mm/min (2 in./min) and 20 mm/min permitted for thermoplastic elastomers, stiff compounds, or specification-defined cases. The standard requires reporting the speed used, the specimen type, and the die identification, since all of these influence the results. Dumbbell and ring specimen procedures are described separately because ring specimens experience non-uniform strain rates around their circumference.

GB/T 528 Method

GB/T 528, the Chinese national standard for tensile testing of vulcanized and thermoplastic rubbers, is technically aligned with ISO 37. It adopts the same dumbbell specimen types and the same speed series, with 500 mm/min as the conventional speed for routine testing. Products certified against GB standards must therefore use the speed stated in the applicable product standard to demonstrate conformity.

Practical Control Points for Reliable Results

• Condition specimens at 23 ± 2 °C and 50 ± 5% relative humidity for the minimum period specified in ISO 23529 or ASTM D1349 before testing.
• Verify die sharpness and specimen thickness measurement, as nicks and thickness errors amplify scatter independently of speed.
• Calibrate crosshead speed and force measurement at the exact speed used, since speed accuracy tolerances are speed-dependent.
• Use an extensometer for elongation and modulus values whenever specification limits are tight.
• Always record the test speed, standard edition, specimen type, and die reference in the test report.
• When comparing historical data or interlaboratory results, confirm that identical speeds were used before drawing conclusions about material quality.

Where dynamic service conditions matter, engineers may complement standard-speed tensile testing with rate-sweep studies and time–temperature superposition analysis to build master curves that describe stiffness and strength across a wide range of effective strain rates.

Applications

Understanding and controlling tensile speed effects supports a wide range of industrial applications:

• Automotive component qualification: seals, bushings, CV joint boots, and hoses are accepted against tensile limits that assume the standard speed; using the wrong speed can cause false pass or false fail decisions.
• Tire compound development: tread and sidewall compounds operate under rapidly varying strain rates, so rate-sensitivity characterization informs performance predictions for rolling, cornering, and impact.
• Industrial products: conveyor belts, V-belts, diaphragms, and rubber springs benefit from modulus and strength data measured under standardized, reproducible conditions.
• Medical and food-contact elastomers: tubing, closures, and gloves are specified with tensile minima tied to fixed speeds in pharmacopeial and industry standards.
• Quality control and supplier verification: incoming compound inspection relies on speed-controlled tensile testing to detect under-cure, contamination, or improper formulation.
• Failure analysis and litigation support: comparing failed parts against reference data requires matching the original test speed to isolate genuine material degradation from mere rate differences.
• Certification testing in accredited third-party laboratories: consistent application of ISO 37, ASTM D412, and GB/T 528 speeds ensures that results are accepted across markets and regulatory regimes.

FAQ

Why does tensile strength increase with higher testing speed?

At higher strain rates, polymer chains and filler networks have insufficient time to relax and redistribute stress through viscous mechanisms. The material therefore resists deformation more stiffly, and a greater stress is needed to reach fracture, which raises the measured tensile strength and modulus values.

Why does elongation at break usually decrease at higher speeds?

Slow extension allows chains to uncoil, slip, and align gradually, letting the specimen reach very high strains before separating. Rapid extension reaches the fracture stress of the network at a lower overall strain, so the specimen breaks sooner and the recorded elongation at break is reduced.

Which tensile speed should be used for rubber testing?

Use the speed specified by the governing product standard or material specification. In routine practice, ISO 37, ASTM D412, and GB/T 528 all commonly use 500 mm/min for standard dumbbell specimens, while slower speeds are prescribed for smaller specimens, thermoplastic elastomers, and special investigations.

Can results measured at different speeds be compared?

Not directly. Because of viscoelasticity, tensile strength and modulus at a higher speed will generally exceed values at a lower speed by a margin that can reach tens of percent. Any comparison must either use identical speeds or explicitly account for the rate difference through material characterization.

How large is the speed effect in practice?

For typical filled rubber compounds, changing the speed by one decade—such as from 50 mm/min to 500 mm/min—can shift tensile strength by roughly 10–30% and elongation at break by 5–20%, depending on the polymer, filler level, and cure state. Unfilled gum rubbers tend to show somewhat smaller effects, while highly filled and strain-crystallizing compounds show larger ones.

Does specimen type change the effective strain rate?

Yes. For a given crosshead speed, the strain rate in the gauge section depends on the specimen's gauge length, and ring specimens additionally experience varying rates around their circumference. This is why standards pair each specimen type with a designated speed.

What should a compliant test report include regarding speed?

A compliant report states the test standard and edition, specimen type and die, conditioning environment, crosshead speed, tensile strength, elongation at break, and any modulus values with their reference elongations. Without the speed and specimen details, the reported values cannot be reproduced or validly compared with specifications.