Tensile Strength Test Speed: Standard Rates, Specifications and Technical Requirements

Tensile strength test speed is the controlled rate at which a universal testing machine stretches a specimen, and it has a direct influence on the measured values of strength, modulus, and elongation. International standards such as ISO 527, ASTM D638, ISO 6892-1, and ASTM E8 prescribe specific crosshead speeds or strain rates so that results remain reproducible and comparable between laboratories. This technical guide explains the standard speed requirements for plastics, metals, rubbers, and films, and why selecting the correct testing rate is essential for generating valid material data.

Technical Overview

Tensile strength test speed refers to the controlled rate at which the movable crosshead of a universal testing machine separates during a tensile test. It is expressed either as a crosshead speed in millimeters per minute (mm/min) or, in more rigorous methods, as a strain rate in units of s⁻¹ or %/min. Because the measured mechanical response of a material is time-dependent, the test speed is a fundamental test parameter that must be specified, controlled, and reported alongside the results.

The influence of speed is particularly pronounced for polymeric materials. Plastics and rubbers are viscoelastic: at higher strain rates, polymer chains have less time to relax and reorient, so the specimen exhibits a higher apparent modulus and tensile strength together with a lower elongation at break. Increasing the test speed by an order of magnitude can raise the measured tensile strength of a thermoplastic by several percent or more, while metallic materials show a smaller but still measurable sensitivity near the yield point. For this reason, results obtained at different speeds are not directly comparable.

Modern tensile standards define three families of rate control, and accredited third-party laboratories apply the mode required by the applicable specification:

• Crosshead speed control: a constant rate of grip separation, used for most plastics, films, elastomers, and composites.
• Strain rate control: closed-loop control using an extensometer signal, mandated by newer metal testing methods such as ISO 6892-1 Method A.
• Stress rate control: limiting the loading rate in the elastic region, retained in legacy metal procedures such as ASTM E8 Method A1.

Testing Samples

Rigid and Semi-Rigid Plastics

Dumbbell-shaped specimens such as Type 1A and 1B per ISO 527-2 or Type I through Type VI per ASTM D638 are machined or injection molded. Test speeds typically range from 1 mm/min to 50 mm/min, with the modulus measured at a slower rate than strength and elongation. The corresponding national standards GB/T 1040 and JIS K 7161 adopt equivalent specimen geometries and speed series.

Thin Plastic Films and Sheets

Strip specimens with extended parallel sections are tested per ISO 527-3 and ASTM D882. Because films deform over very large strains and the gauge length is short, prescribed speeds are comparatively high, commonly in the range of 50 mm/min to 500 mm/min depending on the film type and grip separation.

Rubber and Elastomers

Dumbbell specimens of Types 1 to 4 per ISO 37 or ASTM D412 are die-cut from sheets. These materials are tested at a single high speed, classically 500 ± 50 mm/min, to complete the test within a practical time frame given elongations that can exceed several hundred percent.

Metallic Materials

Machined round or flat proportional and non-proportional specimens per ISO 6892-1, ASTM E8/E8M, GB/T 228.1, or JIS Z 2241 are tested at strain rates orders of magnitude lower than those used for polymers, typically 0.00007 s⁻¹ to 0.00025 s⁻¹ in the elastic and yield regions.

Fiber-Reinforced Composites and Adhesive Joints

Rectangular unidirectional or multidirectional coupons per ASTM D3039 and ISO 527-4/-5 are usually tested at a nominal crosshead rate of 2 mm/min. Lap-shear adhesive joints per ASTM D1002 are tested at a slow constant speed of approximately 1.3 mm/min to capture the shear response accurately.

Testing Items

A tensile test performed at the prescribed speed yields a complete force-extension curve from which the following properties are derived, each with a characteristic sensitivity to test rate:

• Tensile strength (σM or Rm): the maximum stress sustained; increases measurably with test speed in rate-sensitive polymers.
• Yield stress and proof strength (σY, Rp0.2, ReH): affected by the strain rate applied through the yield transition, which is why metal standards strictly define this rate.
• Tensile stress at break (σB): sensitive to speed-induced changes in failure mode.
• Elongation at break and elongation at yield: elongation at break generally decreases as speed increases in ductile polymers.
• Tensile modulus (Et): determined at a dedicated slow speed, 1 mm/min under ISO 527-1 and 5 mm/min under ASTM D638, measured between prescribed strain limits using an extensometer.
• Secant modulus and Poisson-related quantities, where applicable, also requiring modulus-level speeds.

Reporting requirements in ISO and ASTM test reports always include the test speed or strain rate used, confirming that this parameter is part of the result identity rather than incidental machine setup.

Testing Methods

Plastics: ISO 527, ASTM D638, GB/T 1040, JIS K 7161

ISO 527-1 specifies a preferred series of nine test speeds, namely 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min. Unless a material standard states otherwise, the tensile modulus is determined at 1 mm/min, while strength and elongation are measured at the rate assigned by the product specification, frequently 5 mm/min or 50 mm/min for rigid thermoplastics. ASTM D638 defines an equivalent ladder of standard speeds and directs the user to the material specification; in common practice, 5 mm/min is applied for modulus determination and 50 mm/min is frequently specified for tensile strength and elongation of Type I specimens.

Metallic Materials: ISO 6892-1, ASTM E8/E8M, GB/T 228.1, JIS Z 2241

ISO 6892-1 Method A applies closed-loop strain rate control at 0.00007 s⁻¹ up to and through yield for improved reproducibility, then permits an increased rate of 0.00025 s⁻¹ after yield for determining tensile strength. Method B uses a single strain rate of 0.00025 s⁻¹ throughout. ASTM E8/E8M limits the stress rate in the elastic region to no more than approximately 11.5 MPa/s and prescribes a nominal strain rate of 0.015 min⁻¹, equivalent to 0.00025 s⁻¹, through the yield region, aligning closely with ISO Method B.

Rubber: ISO 37, ASTM D412, JIS K 6251

Both ISO 37 and ASTM D412 specify a standard crosshead speed of 500 ± 50 mm/min for the tensile testing of vulcanized and thermoplastic rubbers. Any deviation must be agreed and reported, since rubber properties such as tensile strength and elongation at break are strongly rate-dependent.

Thin Films: ISO 527-3, ASTM D882

For film and thin sheeting, the test speed is selected from the prescribed series according to the initial grip separation and the ductility of the material, so that specimen rupture occurs within the time window defined by the standard. Typical production-film testing runs at 100 mm/min to 500 mm/min.

Equipment and Verification

Testing machines used by accredited third-party laboratories verify force accuracy per ISO 7500-1 or ASTM E4, extensometer classification per ISO 9513 or ASTM E83, and crosshead speed per ISO 5893 or ASTM E2658. Class A extensometers are generally required for modulus determination, and grip compliance must be considered when converting crosshead speed into true strain rate.

Applications

Correctly specified tensile test speeds underpin material data used across a wide range of industries and product categories:

• Plastics and compounding: incoming resin qualification, lot release testing, and datasheet generation at ISO or ASTM standard speeds.
• Automotive and aerospace: design allowable data for polymers, composites, and metals, where strain-rate consistency is mandatory for certification packages.
• Packaging: film strength and elongation control for converting lines, validated at film-specific high speeds.
• Wire and cable: insulation and sheath tensile testing of polymer compounds under the relevant cable material standards.
• Medical devices: catheter tubing, packaging films, and polymer components tested at speeds matched to end-use loading conditions and regulatory expectations.
• Quality assurance and failure analysis: comparing retained specimens against original baseline data, which is only meaningful when the original test speed is reproduced.

FAQ

Why does tensile strength change with test speed?

Most engineering materials, and polymers in particular, respond in a time-dependent manner. At a higher speed, molecular segments cannot relax as the load increases, so the material appears stiffer and stronger while failing at a lower strain. Metals show a smaller but systematic increase in yield and tensile strength with strain rate.

What is the standard test speed for plastic tensile testing?

There is no single universal value. ISO 527 and ASTM D638 provide a series from 1 to 500 mm/min; the modulus is determined at 1 mm/min (ISO) or 5 mm/min (ASTM), while strength and elongation are measured at the speed named in the applicable material specification, very often 5 or 50 mm/min for rigid plastics.

Can tensile results obtained at different speeds be compared?

No. Because strength, modulus, and elongation all vary with rate, results are only validly compared when tested at the same speed, temperature, humidity conditioning, and specimen geometry. The test speed must always be stated in the test report.

What is the difference between crosshead speed and strain rate?

Crosshead speed is the machine displacement rate in mm/min and is affected by machine and grip compliance. Strain rate, expressed in s⁻¹, is calculated from the measured deformation of the specimen gauge length, usually with an extensometer, and is the physically meaningful quantity. Modern metal standards therefore control strain rate directly.

Which speed should be used if no material specification exists?

The default speeds given in the base test method should be followed, and the chosen rate must be documented. When behavior under realistic service loading is required, supplementary tests at application-relevant rates can be performed, clearly distinguished from the standardized reference results reported by accredited third-party laboratories.