Influence of Tensile Test Speed on Plastic Tensile Testing Results: Standards, Methods, and Practica

Tensile test speed is one of the most critical parameters in plastic mechanical testing, because polymers are viscoelastic materials whose strength, modulus, and ductility vary significantly with strain rate. This article explains how test speed influences plastic tensile test results, how ISO 527, ASTM D638, and GB/T 1040 regulate speed selection, and how engineers and laboratories can design speed-influence studies to obtain reliable and comparable data.

Technical Overview

Plastics are viscoelastic materials, which means their mechanical response depends not only on the applied stress but also on the rate at which that stress is applied. During a tensile test, the crosshead speed of the testing machine determines the strain rate experienced by the specimen, and this strain rate directly affects the ability of polymer molecular chains to slip, rotate, orient, and relax under load.

As a general rule, increasing the tensile test speed produces the following trends in most thermoplastics:

• Higher measured tensile strength and tensile stress at break, because the molecular chains have less time to relax and the material behaves in a stiffer, more brittle manner.
• Higher apparent tensile modulus, since the material responds closer to its instantaneous (glassy) stiffness rather than its relaxed stiffness.
• Lower elongation at break and reduced ductility, and in many materials a transition from ductile failure with necking to brittle fracture.
• An upward shift of the entire stress-strain curve, while the yield point may become sharper and more pronounced.

Conversely, testing at very low speeds allows creep-like deformation and chain disentanglement, resulting in lower strength values and greater elongation. Because of this sensitivity, ISO 527, ASTM D638, and GB/T 1040 all specify standardized test speeds and require the selected speed to be clearly reported. Comparing tensile data obtained at different speeds, or from different test standards with different nominal strain rates, can lead to serious misinterpretation of material performance.

Testing Samples

ISO 527-2 Multipurpose Specimens (Type 1A and 1B)

The preferred specimens for rigid and semi-rigid thermoplastics are the ISO 527-2 Type 1A (gauge length 50 mm, suitable for directly injection-molded parts) and Type 1B (gauge length 50 mm, typically machined from sheets, compression-molded plates, or finished products). These dumbbell-shaped specimens provide consistent stress distribution within the gauge section and are the reference geometry for most published material data sheets.

ASTM D638 Specimens (Type I to Type V)

ASTM D638 defines five specimen types for different material forms and thicknesses. Type I is the standard specimen for rigid plastics with a thickness of 7 mm or less, Type IV is commonly used for materials that cannot be machined into larger specimens, and Type V is intended for small-scale testing where only limited material is available. GB/T 1040 adopts specimen geometry consistent with the ISO 527 system, so results from GB and ISO test pieces are more directly comparable with each other than with ASTM results.

Film and Sheet Specimens

For plastic films and thin sheets below 1 mm in thickness, ISO 527-3 and ASTM D882 specify strip or dumbbell specimens tested at dedicated low-force speeds. Because films are especially rate-sensitive due to their molecular orientation, speed control is even more critical for reproducible results.

Specimen Preparation and Conditioning

All specimens must be prepared under controlled conditions, since molding parameters, gate location, weld lines, and machining quality introduce additional sources of variation. Specimens are conditioned in a standard atmosphere of 23°C and 50% relative humidity in accordance with ISO 291 or ASTM D618 before testing, and moisture-sensitive materials such as polyamides may require additional drying or humidity conditioning per ISO 1110.

Testing Items

A tensile speed influence test program typically measures and compares the following properties at each selected speed:

• Tensile stress at yield and tensile strain at yield.
• Tensile strength and tensile stress at break.
• Tensile strain at break and nominal elongation at break.
• Tensile modulus (Young's modulus), including chord modulus determined between defined strain limits such as 0.05% and 0.25% per ISO 527-1.
• Full stress-strain curves at each speed, used to visualize the shift in yield behavior and the ductile-to-brittle transition.
• Percentage speed sensitivity, calculated as the change in a given property between the lowest and highest tested speeds.

For a complete speed-influence characterization, the same batch of specimens is tested at several graded speeds, for example 1, 5, 10, 20, 50, 100, 200, and 500 mm/min, with a statistically sufficient number of replicates at each speed.

Testing Methods

Testing is performed on a calibrated universal tensile testing machine equipped with suitable grips, an appropriately sized load cell meeting the accuracy requirements of ISO 7500-1 or ASTM E4, and a contact or non-contact extensometer meeting ISO 9513 or ASTM E83. The general procedure is as follows:

• Measure the width and thickness of each specimen at several points within the gauge length and calculate the cross-sectional area.
• Select the test speed according to the governing standard and the purpose of the test. ISO 527-1 designates 1 mm/min as the reference speed for modulus determination, while ISO 527-2 lists preferred speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min for strength determination; 50 mm/min is the most common speed for Type 1A specimens. ASTM D638 prescribes speeds from 0.05 to 500 mm/min depending on the specimen type, with 50 mm/min typical for Type I specimens.
• Mount the specimen in the grips, apply the extensometer, and run the test at constant crosshead speed until rupture.
• Record force and extension continuously, calculate stress and strain, and derive modulus, yield, and break properties from the stress-strain curve.
• Repeat the test at each planned speed using fresh specimens, keeping temperature, humidity, specimen geometry, and grip pressure constant so that speed is the only variable.
• Plot each property against test speed or strain rate and analyze the trend, including any change in failure mode from ductile necking to brittle fracture.

It is important to note that the true strain rate in the gauge section differs from the nominal strain rate calculated from crosshead speed, particularly after yield when necking concentrates deformation. For rigorous rate-sensitivity studies, extensometer-based strain measurement or video extensometry is strongly recommended. The test report must always state the crosshead speed, specimen type, standard used, and environmental conditions, because tensile results without speed information are not comparable.

Applications

Tensile speed influence testing serves a wide range of engineering and commercial purposes:

• Generation of consistent material data sheets, ensuring that all published properties are measured at standardized speeds and can be compared across materials.
• Quality control of incoming resins, compounds, and molded parts in injection molding and extrusion production, where detecting batch-to-batch variation requires tightly controlled test speeds.
• Research and development of new formulations, including evaluation of how fillers, glass fibers, impact modifiers, plasticizers, and recycled content change the rate sensitivity of a polymer.
• Input for computer-aided engineering and finite element simulation, where rate-dependent material models require test data at multiple strain rates to accurately predict real-world behavior.
• Automotive, aerospace, and electronics applications where plastics experience rapid loading during crash, drop, vibration, or assembly, making high-speed tensile characterization essential.
• Packaging and medical device development, where films and components are loaded at speeds very different from standard laboratory speeds and the deviation must be quantified.
• Failure analysis and dispute resolution, where differences in test speed are frequently identified as the root cause of conflicting test results between laboratories.

Accredited third-party laboratories routinely perform speed-comparison tensile testing programs to support material qualification, supplier audits, and product certification against ISO, ASTM, EN, GB, UL, and JIS requirements.

FAQ

Why does tensile test speed change the measured strength of plastics?

Plastics are viscoelastic, so their molecular chains need time to slip and rearrange under load. At high speeds, the chains cannot relax quickly enough, the material appears stiffer and stronger, and elongation decreases. At low speeds, chain relaxation and creep-like deformation dominate, producing lower strength values and higher elongation.

How large is the speed effect in practice?

The effect is material-dependent and can be substantial. For many thermoplastics, increasing the test speed from 5 mm/min to 500 mm/min can raise tensile strength by ten to forty percent and reduce elongation at break dramatically, sometimes shifting the failure mode from ductile to brittle. Rigid amorphous and highly crystalline polymers often show pronounced rate sensitivity, while some elastomer-modified compounds show complex non-linear behavior.

Can results from ISO 527 and ASTM D638 be compared directly?

Not directly. Even when the nominal crosshead speed is identical, the two standards use different specimen geometries, and the resulting strain rate in the gauge section differs. When comparing ISO and ASTM tensile data, differences in specimen dimensions, gauge length, and modulus calculation method must be taken into account, and dedicated comparative testing at matched strain rates is recommended.

Which test speed should be selected for routine testing?

The speed specified by the relevant product standard or material specification should always be used. In the absence of a specification, ISO 527-2 commonly uses 50 mm/min for strength measurement with Type 1A specimens and 1 mm/min for modulus determination, while ASTM D638 commonly uses 50 mm/min for Type I specimens. Consistency is essential: results are only valid for comparison when the same speed is used.

Should the modulus be measured at a different speed from the strength?

Yes. ISO 527-1 specifies 1 mm/min as the reference speed for modulus determination because modulus measurement occurs at very small strains and must be quasi-static to avoid dynamic effects. Many laboratories run a separate low-speed test for modulus and a higher-speed test for yield and break properties, using two specimens from the same batch.

How is behavior at very high strain rates evaluated?

Standard tensile machines are generally limited to 500 mm/min. For impact-level strain rates, dedicated high-speed tensile testing equipment, tensile impact tests per ISO 8256 or ASTM D1822, or dynamic material testing methods are used to characterize the material response under rapid loading conditions relevant to drop, crash, and impact events.