Tensile Test Speed Effect Testing: How Strain Rate Influences Tensile Properties

Tensile test speed effect testing, also known as strain rate sensitivity testing, evaluates how the rate of loading influences mechanical properties such as yield strength, tensile strength, elongation, and modulus. Because virtually all engineering materials exhibit some degree of rate-dependent behavior, controlling and documenting test speed is essential for obtaining reproducible, comparable, and design-valid tensile data in accordance with ISO 6892-1, ASTM E8/E8M, ISO 527, and ASTM D638. This article explains the technical basis of speed effects, applicable sample types, tested parameters, methodologies, industrial applications, and frequently asked questions.

Technical Overview

Tensile testing is the most widely used method for characterizing the mechanical response of materials under uniaxial loading. However, the measured properties are not absolute constants: they depend on the rate at which the specimen is deformed. Tensile test speed effect testing systematically quantifies this dependence by evaluating tensile properties across a controlled range of crosshead speeds, stress rates, or strain rates.

The underlying mechanism is rate-dependent (viscoelastic and viscoplastic) material behavior. In polymers and elastomers, molecular chains require time to disentangle, uncoil, and slide past one another; when deformation is rapid, the material responds with higher apparent stiffness and strength but reduced ductility. In metallic materials, dislocation motion and diffusion-assisted relaxation processes are time-dependent, so yield and tensile strengths typically increase with strain rate while uniform elongation may decrease. Mild steels show pronounced rate sensitivity, whereas aluminum alloys and many high-strength steels are less sensitive.

International standards address this issue explicitly. ISO 6892-1 and its equivalent national standard GB/T 228.1 define multiple control approaches, including Method A (closed-loop strain rate control), Method B (crosshead speed based on estimated strain rate), and Method C (stress rate control). ASTM E8/E8M provides comparable framework with defined loading rates for determining yield properties. For plastics, ISO 527 and ASTM D638 specify a series of standard test speeds ranging from 1 mm/min to 500 mm/min, while ISO 37 and ASTM D412 govern speed selection for rubbers and elastomers. Accurate control and reporting of the testing rate is therefore a precondition for laboratory-to-laboratory comparability and for valid use of tensile data in product design and simulation.

Testing Samples

Metallic Materials

Specimens are prepared from sheet, plate, strip, bar, rod, wire, forgings, castings, and tubes in accordance with ISO 6892-1 or ASTM E8/E8M. Typical geometries include machined rectangular flat specimens with proportional gauge lengths, round specimens with 5:1 or 4:1 gauge length-to-diameter ratios, and small-size specimens for limited material volumes. Machining must avoid overheating, cold working, and surface damage that could alter measured properties, particularly when comparing results across different strain rates.

Plastics and Polymers

Rigid and semi-rigid plastics are tested using 1A/1B dumbbell specimens per ISO 527-2 or Type I through Type V specimens per ASTM D638. Films are evaluated using ISO 527-3 specimen geometries. Specimens must be molded or cut free of voids, notches, and residual stresses, and conditioned at standard laboratory atmosphere of 23°C and 50% relative humidity prior to testing, since moisture content and temperature further interact with rate effects.

Rubbers and Elastomers

Vulcanized thermoset rubbers and thermoplastic elastomers are tested with dumbbell or ring specimens per ISO 37 or ASTM D412. These materials exhibit the strongest rate dependence among common engineering materials, so specimen thickness measurement, die-cut quality, and speed control are especially critical for meaningful comparative data.

Other Materials

Testing can also be extended to composites, adhesives, foils, textiles, weld joints, and additively manufactured components, provided specimen geometry, grip method, and rate range are adapted to the material characteristics and referenced to the appropriate test standard.

Testing Items

Tensile test speed effect testing measures the complete set of tensile properties at each controlled rate and evaluates how each property varies with strain rate. Typical measured and derived parameters include:

• Upper and lower yield strength (ReH, ReL) for materials exhibiting a yield plateau
• Proof strength, plastic extension (Rp0.2) and proof strength, total extension (Rt) for materials without a distinct yield point
• Tensile strength (Rm) and force at fracture
• Young's modulus (E) determined within the elastic range at the standard low rate
• Uniform plastic elongation at maximum force (Ag) and total percentage elongation after fracture (A)
• Percentage reduction of area (Z) for round metallic specimens
• Yield and tensile strength as a function of strain rate, plotted as rate-sensitivity curves
• Strain rate sensitivity exponent (m), calculated from the slope of flow stress versus strain rate on logarithmic axes
• Work-hardening behavior (n-value) evaluated in combination with rate variation for metallic sheet materials

Testing Methods

Control Modes and Rate Ranges

Three principal control modes are used. Stress rate control applies a constant loading rate to the specimen, historically used for yield determination in metallic materials. Crosshead speed control maintains a constant actuator velocity and is the default mode for polymeric materials, with standard speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min. Closed-loop strain rate control, the preferred Method A in ISO 6892-1, uses extensometer or crosshead displacement feedback to maintain a constant strain rate in the specimen gauge section, offering the highest reproducibility. For metallic materials, quasi-static strain rates typically span from approximately 0.00007 s-1 to 0.8 s-1, while specialized servo-hydraulic systems extend characterization to intermediate rates up to several hundred s-1 for crash and impact simulation.

Test Procedure

The test sequence begins with dimensional measurement of the specimen cross-section, followed by verification that the testing machine load cell and extensometer are calibrated in accordance with ISO 7500-1/ISO 9513 or ASTM E4/ASTM E83. The specimen is mounted in appropriate grips with careful alignment to eliminate bending stresses, and a small preload is applied. Each specimen is then deformed to fracture at one designated rate, with strain measured by a contact extensometer, video extensometer, or digital image correlation system as required by the material and gauge length.

Rate Comparison Program

A typical speed effect study evaluates at least three to five strain rates or crosshead speeds, with a minimum of three replicate specimens per rate to support statistical validity. Modulus is conventionally determined at the lowest specified rate, while yield and fracture properties are recorded at each rate. Results are compiled into property-versus-rate tables and curves, and the strain rate sensitivity exponent is derived where flow stress data permit. All reports must state the control mode, target rate, actual achieved rate, temperature, and equipment class, because these factors directly influence the reported values.

Equipment and Environment

Testing is performed on universal testing machines equipped with closed-loop electronic control, high-speed data acquisition, and grips suited to the material. Tests are conducted at room temperature within the tolerance defined by the governing standard, typically 23°C for plastics and 10°C to 35°C for metallic materials unless otherwise specified. For high-rate programs, accelerated test devices, drop towers, or servo-hydraulic high-speed frames may be employed with appropriate wave-analysis corrections.

Applications

Speed effect tensile data serve a broad range of engineering and industrial purposes:

• Automotive crashworthiness analysis, where constitutive models for steel and aluminum sheet require flow stress curves measured at dynamic strain rates representative of collision events
• Sheet metal forming process design, including stamping, deep drawing, and roll forming, where strain rates differ substantially from quasi-static laboratory conditions
• Finite element analysis input, including parameter identification for Johnson-Cook and Cowper-Symonds rate-dependent material models
• Plastic component design and processing optimization, where apparent modulus and strength must match real loading speeds encountered in service and assembly
• Packaging film evaluation for high-speed filling and sealing lines, where materials experience rapid tensile loading
• Quality control and supplier data verification, ensuring that results from different accredited third-party laboratories are compared at identical rates
• Standards compliance demonstration for metallic and polymeric products under ISO, ASTM, EN, GB, and JIS frameworks, including certification of structural and safety-critical components
• Academic and industrial research on deformation mechanisms, viscoelasticity, and dynamic material behavior

FAQ

Why does tensile test speed change the measured results?

Deformation mechanisms in materials are time-dependent. At higher rates, molecular chains in polymers and dislocations in metals have less time to rearrange, so the material exhibits higher flow stress and stiffness, while ductility and elongation often decrease. The magnitude of the effect varies with material class, microstructure, and temperature.

What speed should be used for a standard tensile test?

The speed must follow the governing product or test standard. For metallic materials, ISO 6892-1 recommends strain rates of approximately 0.00025 s-1 for yield determination and 0.0067 s-1 for post-yield deformation. For plastics, the most common speeds are 1 mm/min for modulus and 5 to 50 mm/min for strength, while rubbers are commonly tested at 500 mm/min. Any deviation must be reported.

What is the difference between crosshead speed, stress rate, and strain rate?

Crosshead speed is the displacement velocity of the testing machine actuator and includes machine and grip compliance. Stress rate is the rate of increase of stress in the elastic region. Strain rate is the rate of deformation within the specimen gauge length and is the most physically meaningful parameter; closed-loop control is used to keep it constant during the test.

Do all materials show significant rate sensitivity?

No. Elastomers and thermoplastics are highly sensitive, mild steels are moderately sensitive, high-strength aluminum alloys show limited sensitivity, and ceramics and glasses are essentially rate-insensitive at room temperature. A screening test at two rates can quickly establish whether a full rate-sensitivity study is warranted.

How many specimens are required for a speed effect study?

A minimum of three valid replicates per rate is recommended for statistical confidence, with five or more preferred when data scatter is high or when results will feed simulation models. Invalid fractures outside the gauge length must be discarded and retested.

Can results from different laboratories be compared directly?

Only when the control mode, rate, specimen geometry, temperature, and calculation methods are identical. This is precisely why ISO 6892-1 emphasizes closed-loop strain rate control and full reporting of test conditions, and why accredited third-party laboratories always document the applied rate alongside the reported properties.