Analysis of the Effect of Strain Rate on Tensile Properties

Strain rate is one of the most decisive parameters in tensile testing, directly influencing the measured yield strength, tensile strength, and ductility of metallic and polymeric materials. This article presents a systematic technical analysis of how strain rate affects tensile properties, covering deformation mechanisms, applicable standards such as ASTM E8/E8M, ISO 6892-1, and ASTM D638, specimen preparation, testing methodologies, and industrial applications. Understanding these effects is essential for laboratories, design engineers, and manufacturers who require reliable and comparable material data.

Technical Overview

Strain rate is defined as the rate at which plastic or elastic deformation develops during a tensile test, expressed as the change in strain per unit of time (1/s). It is one of the fundamental controllable variables in mechanical testing, alongside temperature and specimen geometry. Because most engineering materials are strain-rate sensitive, identical specimens tested at different strain rates can produce significantly different stress-strain curves, which in turn affects material qualification, design allowables, and numerical simulation inputs.

At the microstructural level, strain rate sensitivity arises from the thermally activated nature of dislocation motion in metals and the viscoelastic response of polymers. At low strain rates, dislocations have sufficient time to bypass obstacles, resulting in lower flow stress. As strain rate increases, dislocations must overcome barriers within shorter time frames, raising the stress required for continued deformation. At very high rates, additional mechanisms such as phonon drag, inertial effects, and adiabatic heating (local temperature rise caused by insufficient time for heat dissipation) further modify the tensile response.

• In most metals, yield strength increases noticeably with strain rate, while the effect on ultimate tensile strength is comparatively smaller.
• Body-centered cubic (BCC) metals, such as carbon steels, generally exhibit stronger rate sensitivity than face-centered cubic (FCC) metals, such as aluminum alloys and austenitic stainless steels.
• Total elongation frequently decreases at high strain rates, although adiabatic heating can sometimes maintain or even improve ductility in certain alloys and polymers.
• In polymers, higher strain rates shift the material response toward glassy, more brittle behavior, increasing modulus and strength while reducing elongation at break.
• Strain rate sensitivity is commonly quantified by the exponent m, derived from the slope of flow stress versus strain rate on a logarithmic scale.

Because of these effects, international standards strictly define the permissible strain rate or stress rate windows for tensile testing. Test results are only comparable when the loading rate is controlled and reported, which is the central premise of any strain rate effect analysis program.

Testing Samples

Metallic Flat Sheet Specimens

Flat dog-bone specimens with proportional or non-proportional gauge lengths are machined from sheet and plate materials in accordance with ASTM E8/E8M, ISO 6892-1, EN ISO 6892-1, GB/T 228.1, or JIS Z 2241. Typical gauge widths range from 12.5 mm to 20 mm with thicknesses reflecting the supplied product. Specimens are cut along and perpendicular to the rolling direction to evaluate anisotropy under different strain rates.

Metallic Round Specimens

Cylindrical specimens with gauge diameters of 5 mm to 12.5 mm are prepared from bars, forgings, castings, and thick sections. Round specimens enable accurate reduction-of-area measurement, which is particularly valuable when assessing the loss of ductility at elevated strain rates.

Polymeric Specimens

Rigid and semi-rigid plastics are tested using Type I specimens per ASTM D638 or Type 1A/1B specimens per ISO 527-2 and GB/T 1040. Because polymers are strongly viscoelastic, specimen conditioning at 23 degrees Celsius and 50 percent relative humidity is mandatory before testing.

Special-Purpose Specimens

Welded joint cross-sections, additively manufactured coupons, fastener blanks, and thin foils can also be evaluated. For these products, specimen geometry is adapted to the governing product standard, and higher replicate counts are recommended because local microstructure amplifies rate effects.

For all specimen types, preparation must ensure smooth, notch-free machined edges, tight dimensional tolerances, and clearly marked original gauge lengths, since stress concentrations artificially depress measured ductility and distort rate sensitivity comparisons.

Testing Items

A comprehensive strain rate effect analysis evaluates the full set of tensile properties at each rate level and then compares them statistically. Core measured and derived items include:

• Upper and lower yield strengths (ReH, ReL) and 0.2 percent proof strength (Rp0.2)
• Ultimate tensile strength (Rm)
• Young's modulus (E) in the elastic region
• Uniform elongation (Ag), percentage elongation after fracture (A, A50mm), and total elongation at maximum force (Agt)
• Percentage reduction of area (Z) for round specimens
• Strain-hardening exponent (n-value) and plastic strain ratio (r-value) for sheet metals
• Strain rate sensitivity exponent (m-value) calculated across the tested rate range
• Complete engineering and true stress-strain curves, fracture mode characterization, and adiabatic shear observations at high rates

Each rate level is normally tested with at least three valid replicates to support meaningful statistical comparison of means and scatter.

Testing Methods

Quasi-Static Testing and Loading Rate Control

Quasi-static tensile testing covers strain rates from approximately 0.00001/s to 0.01/s using electromechanical or servo-hydraulic universal testing machines. ISO 6892-1 and GB/T 228.1 define four standard rates: 0.00007/s, 0.00025/s, 0.0025/s, and 0.0067/s, with lower rates applied to the elastic region and higher rates to the plastic region. ASTM E8/E8M specifies stress rate limits of 1.15 to 11.5 MPa/s for yield property determination and a strain rate not exceeding 0.015/s for elongation determination. For plastics, ASTM D638 and ISO 527 prescribe crosshead speeds such as 1, 5, 50, or 500 mm/min depending on specimen type and material rigidity.

Control Modes

Three control modes are used in rate-effect studies. Crosshead speed control is the simplest but introduces machine compliance errors, since the actual strain rate in the gauge section differs from the commanded value. Stress rate control regulates loading in the elastic range. Closed-loop strain rate control, designated Method A in ISO 6892-1, uses extensometer or virtual gauge length feedback to hold the true strain rate constant throughout the test, providing the highest accuracy and interlaboratory reproducibility.

Strain Measurement

Deformation is measured with clip-on extensometers, non-contact video extensometers, or digital image correlation (DIC) systems. DIC is particularly advantageous at intermediate and high strain rates because it captures non-uniform strain localization and adiabatic shear band formation that contact devices miss.

Intermediate and High Strain Rate Testing

For rates from about 1/s to 1000/s, high-speed servo-hydraulic machines with slack-type acceleration apparatus are used, following guidance in ISO 26203-1 for metallic materials. At very high rates, split Hopkinson (Kolsky) tension bar systems extend the range to several thousand per second. Force equilibrium verification and one-dimensional wave analysis are essential quality checks at these rates.

Data Evaluation

For every rate, stress-strain curves are digitized, key properties are extracted, and rate sensitivity is quantified by plotting flow stress at fixed plastic strain against strain rate on logarithmic axes; the slope yields the m exponent. Results feed constitutive models such as the Cowper-Symonds and Johnson-Cook formulations used in crash and impact simulation.

Applications

Strain rate effect analysis of tensile properties directly supports product safety, process design, and simulation accuracy across many industries:

• Automotive: generating dynamic flow curves for crashworthiness models, body-in-white steel, and energy-absorbing components.
• Aerospace: qualifying structural alloys for bird strike, foreign object damage, and dynamic load events.
• Sheet metal forming: calibrating rate-dependent material cards for stamping, rolling, and high-speed press operations.
• Civil and seismic engineering: verifying the elevated yield strength of structural steels under earthquake loading rates and assessing weld performance under dynamic conditions.
• Oil, gas, and pressure equipment: evaluating pipeline linepipe steels and pressure vessel materials for impact and rapid depressurization scenarios.
• Additive manufacturing: qualifying printed alloys whose rate sensitivity may differ from wrought equivalents.
• Plastics and packaging: predicting drop impact performance, high-speed filling, and brittle-ductile transitions in service.

By testing at rates that reproduce actual service or processing conditions, engineers avoid both overly conservative designs and dangerous overestimations of material capability.

FAQ

Why does yield strength increase with strain rate?

Plastic deformation in metals is controlled by thermally activated dislocation motion. At higher strain rates, dislocations have less time to overcome obstacles with the aid of thermal energy, so a higher applied stress is required, which raises the measured yield and flow stresses. In polymers, molecular chain relaxation cannot keep pace with the deformation, producing a stiffer, stronger response.

What strain rates do the main standards specify?

ISO 6892-1 and GB/T 228.1 specify strain rates of 0.00007/s, 0.00025/s, 0.0025/s, and 0.0067/s depending on the property being measured and the material class. ASTM E8/E8M defines stress rate limits of 1.15 to 11.5 MPa/s for yield determination, and ASTM D638 and ISO 527 define crosshead speeds for plastics ranging from 1 to 500 mm/min.

What is the difference between crosshead speed control and closed-loop strain rate control?

Crosshead speed control sets only the machine actuator velocity; machine compliance and specimen geometry then determine the actual gauge strain rate, which varies during the test. Closed-loop strain rate control continuously adjusts the actuator using extensometer feedback to maintain a constant strain rate in the gauge section, which is required for the most accurate and comparable results.

How is the strain rate sensitivity exponent determined?

The flow stress at a fixed plastic strain is measured at several strain rates, and the logarithm of stress is plotted against the logarithm of strain rate. The slope of the best-fit line is the m exponent, which quantifies how strongly the material strengthens with rate and is a key input for dynamic constitutive models.

Does a higher strain rate always reduce ductility?

No. Although elongation often decreases at high rates because deformation localizes rapidly, adiabatic heating at very high rates can thermally soften some steels, aluminum alloys, and polymers enough to sustain or even improve ductility. This is why empirical testing across the full rate range of interest remains necessary.

Where should strain rate effect testing be performed?

Testing should be carried out in ISO/IEC 17025-accredited third-party laboratories or qualified in-house facilities equipped with calibrated universal testing machines, validated strain rate control, and appropriate extensometry. Test reports must state the control mode, applied rates, standards used, and statistical evaluation to ensure the data are traceable and comparable.