Testing the Effect of Tensile Rate on Specimen Deformation: Strain Rate Sensitivity Evaluation

The speed at which a tensile test is conducted has a direct and quantifiable influence on how a specimen deforms, affecting measured values of strength, elongation, and modulus. This technical article examines the effect of tensile rate on specimen deformation, the test methods used to characterize strain rate sensitivity, and the international standards—such as ISO 6892-1, ASTM E8, and ISO 527—that govern rate selection. Understanding these effects is essential for laboratories and engineers who need reproducible, comparable mechanical property data.

Technical Overview

Tensile rate, expressed either as crosshead speed (mm/min) or as engineering strain rate (per second), is one of the most influential test parameters in mechanical characterization. The measured deformation response of a specimen is not purely a material constant; it is a combined result of intrinsic material behavior and the loading conditions applied by the testing machine. For metallic materials, higher strain rates generally raise yield and tensile strength because dislocation motion is a thermally activated process with limited time to proceed at faster loading. For polymeric materials, viscoelasticity dominates: increasing the tensile rate shifts the material response toward a stiffer, more glass-like behavior, increasing modulus and strength while reducing ductility and elongation at break.

Because of these effects, international test standards prescribe specific rate ranges to ensure that results from different laboratories are comparable. ISO 6892-1 defines distinct strain rates for determining yield and tensile strength of metals, while ISO 527 and ASTM D638 specify speed classes for plastics ranging from 1 mm/min to 500 mm/min. Deviating from the specified rate can shift measured strength values by several percent or more, which is why tensile rate effect testing is a critical component of method validation, dispute resolution, and material qualification.

Testing Samples

Metallic Materials

Metallic specimens are typically machined as proportional flat or round test pieces in accordance with ISO 6892-1 or ASTM E8/E8M. Common product forms include sheet, strip, plate, bar, rod, wire, and castings. Metallic samples display rate-dependent behavior through changes in yield-point phenomena, Lüders band propagation, work-hardening response, and post-necking elongation, with certain alloys such as low-carbon steels and aluminum-magnesium alloys showing pronounced strain rate sensitivity.

Plastic Materials

Plastic specimens are injection-molded or machined dumbbells conforming to ISO 527-2 (specimen types 1A/1B) or ASTM D638 (Types I through V). Amorphous, semi-crystalline, and reinforced thermoplastics all exhibit significant rate dependence due to viscoelastic and viscoplastic deformation mechanisms, making rate control especially critical for thermoplastics.

Rubber and Elastomeric Materials

Vulcanized rubbers and thermoplastic elastomers are tested using dumbbell or ring specimens per ISO 37 or ASTM D412. Elastomers are extremely rate sensitive; tensile strength, elongation at break, and stress at a given strain all change markedly when the crosshead speed is varied, and adiabatic heating at high rates can further alter deformation behavior.

Composites, Films, and Foils

Fiber-reinforced polymer composites are evaluated with straight-sided or tabbed specimens per ASTM D3039 or ISO 527-4/527-5, where rate influences matrix-dominated properties. Thin polymeric films and metal foils are tested per ASTM D882, using narrow strips and low-force load cells, where rate strongly affects measured modulus and break strain.

Testing Items

A tensile rate effect study quantifies how key deformation and strength parameters vary with loading speed. Typical measured items include:

• Yield strength and upper/lower yield-point behavior at each rate
• Ultimate tensile strength as a function of strain rate
• Young's modulus and chord modulus measured at standardized rates
• Uniform elongation, elongation at break, and total extension
• Reduction of area and post-necking deformation response for metals
• Strain rate sensitivity exponent (m-value) derived from flow stress at different rates
• Complete engineering and true stress-strain curves for comparison across rates
• Rate-dependent failure mode characterization (ductile versus brittle fracture appearance)

Testing Methods

Test Equipment and Control Modes

Testing is performed on a calibrated universal testing machine equipped with force cells appropriate to the specimen, and deformation is measured using contact extensometers, video extensometers, or bonded strain gauges. Three control modes are used to define the tensile rate: crosshead speed control (open-loop, simplest but least accurate), stress rate control, and closed-loop strain rate control, in which the extensometer signal feeds back to the machine controller. Modern standards such as ISO 6892-1 Method A favor closed-loop strain rate control because it guarantees identical deformation rates in the specimen gauge section regardless of machine compliance or specimen stiffness.

Rate Levels Specified by Standards

• ISO 6892-1 (metals): standard strain rates of 0.00007/s, 0.00025/s, and 0.0025/s, with different rates designated for modulus, yield, and tensile strength determination
• ASTM E8/E8M (metals): specified rates of stressing for yield determination and limits on crosshead speed for tensile strength, expressed in MPa/s and mm/mm/min
• ISO 527-1 and ASTM D638 (plastics): preferred speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min depending on specimen type and material rigidity
• ISO 37 and ASTM D412 (rubber): normal test speed of 500 mm/min for standard dumbbells
• ASTM D882 (thin films): speeds selected as a percentage of initial gauge length per minute

Typical Test Procedure

Specimens are first conditioned and dimensionally measured, then gripped with jaws suited to the material to prevent slip or premature failure. A series of replicate tests, typically three to five specimens per rate, is performed across a minimum of three rate levels spanning at least one order of magnitude, for example 0.00025/s, 0.0025/s, and 0.025/s for metals. Force-extension data are converted into engineering stress-strain curves, deformation stages (elastic, uniform plastic, necking) are identified at each rate, and results are compared statistically. For strain rate sensitivity evaluation, flow stress values at fixed plastic strain are plotted logarithmically against strain rate, and the slope yields the m-value, which quantifies the rate dependence of deformation resistance.

High Strain Rate Testing

When deformation behavior at crash, impact, or forming speeds is required, servo-hydraulic high-speed machines, drop towers, or split Hopkinson bar apparatus are used to reach strain rates from 1/s up to several thousand per second. Such tests require special attention to wave propagation effects, inertia compensation, dynamic force calibration, and adiabatic heating, since deformation becomes thermally non-isothermal at high rates.

Applications

Tensile rate effect testing supports a wide range of engineering and quality functions across industries:

• Automotive crashworthiness analysis, where material models require flow curves measured at dynamic strain rates
• Sheet metal forming simulation, including stamping, deep drawing, and rolling, which involve strain rates above quasi-static laboratory values
• Packaging film development, where sealing, tearing, and drop performance depend on rate-dependent tensile behavior
• Aerospace structural design, where rate-sensitive aluminum alloys and composites must be characterized across loading regimes
• Weld and joint qualification, ensuring tensile properties are measured under the rate conditions mandated by application standards
• Material datasheet generation and lot-release quality control, where rate compliance guarantees comparability of reported values
• Failure analysis and inter-laboratory dispute resolution, where unrecognized rate deviations are a common source of disagreement

FAQ

Why does tensile rate change the measured strength of a specimen? In metals, plastic deformation relies on thermally activated dislocation movement; at higher rates there is less time for dislocations to bypass obstacles, raising flow stress. In polymers, molecular chain relaxation cannot keep pace with the applied deformation, producing a stiffer and stronger but less ductile response.

What is the difference between crosshead speed and strain rate? Crosshead speed is the velocity of the testing machine actuator in mm/min, while strain rate is deformation speed normalized by gauge length in units per second. Machine compliance and specimen grip movement cause the actual gauge-section strain rate to differ from the nominal value, which is why closed-loop strain rate control is preferred by modern standards.

What is the strain rate sensitivity exponent m? The m-value is the slope of the log-log plot of flow stress versus strain rate at a fixed plastic strain. A higher m indicates stronger rate dependence; m also governs superplastic behavior, where values near or above 0.3 enable extreme uniform elongation.

Which tensile rate should be used for reporting results? The rate specified in the governing product or test standard must be used, such as 0.00025/s for yield strength of metals under ISO 6892-1 or 50 mm/min for rigid plastics under ISO 527. If no standard applies, the chosen rate should be stated explicitly in the test report because values at different rates are not interchangeable.

Are polymers or metals more sensitive to tensile rate? Polymers generally show much stronger rate dependence because viscoelastic relaxation times are comparable to test durations, while rate effects in metals at room temperature are more moderate but still significant, particularly in low-carbon steels, austenitic stainless steels, and aluminum-magnesium alloys.

Can tensile rate affect the failure mode of a specimen? Yes. Many ductile polymers and some metals transition toward more brittle, lower-elongation fractures as rate increases, and adiabatic heating at high rates can locally soften the material and alter necking behavior, so fractographic evaluation should always consider the rate at which the test was performed.