Determination of the Relationship Between Tensile Testing Speed and Strength

The measured tensile strength of a material is not a fixed property but depends strongly on the speed—or strain rate—at which the test is performed. Determining the relationship between tensile testing speed and strength is essential for material qualification, quality control, and generating reliable input data for simulation of real-world loading conditions. This article explains the underlying mechanisms, specimen types, standard test methods, data analysis procedures, and typical applications of rate-dependent tensile strength characterization.

Technical Overview

When a specimen is pulled in tension, the speed of deformation—expressed either as crosshead speed (mm/min) or true strain rate (s⁻¹)—directly influences the measured strength values. For most polymeric materials, higher test speeds produce higher tensile strength, higher yield stress, and lower elongation at break, because viscoelastic molecular chains have less time to relax and untangle during loading. For metallic materials, increased strain rate raises yield and ultimate tensile strength through rate-dependent dislocation motion and adiabatic heating effects, although the sensitivity is generally smaller than in plastics.

Establishing this speed–strength relationship requires testing replicate specimens at several controlled rates under identical conditioning, then correlating strength against strain rate, typically on a logarithmic scale. Accredited third-party laboratories routinely perform this characterization in accordance with international standards such as ISO 527-1, ASTM D638, ISO 6892-1, ASTM E8/E8M, ASTM D412, ISO 37, and the corresponding national standards GB/T 1040 and GB/T 228.1. The resulting data support material comparison, specification compliance, and the calibration of strain-rate-dependent constitutive models used in finite element analysis.

Testing Samples

Rigid and Semi-Rigid Thermoplastics

Injection-molded or machined dumbbell specimens, typically ISO 527-2 Type 1A (gauge length 50 mm) or ASTM D638 Type I, are used for materials such as PP, PE, ABS, PC, PA, and POM. Specimens must be free of flash, notches, and voids, since stress concentrators distort the rate sensitivity of measured strength.

Metallic Materials

Flat rectangular or cylindrical proportional specimens conforming to ISO 6892-1 or ASTM E8/E8M are prepared from sheet, plate, bar, tube, or wire. The gauge length is typically 5.65√S₀ (proportional) or a fixed 50 mm, and surface preparation must avoid heating or cold work that alters the rate response.

Elastomers and Flexible Polymers

Vulcanized rubber and thermoplastic elastomers are tested using dumbbell or ring specimens per ASTM D412 or ISO 37. These materials exhibit pronounced rate sensitivity, making multi-speed testing particularly important for sealing, damping, and tire applications.

Plastic Films and Thin Sheets

Strip specimens cut per ASTM D882 or ISO 527-3 are used for packaging films, where production line speeds create high-rate loading during filling, sealing, and transport.

Adhesives, Foams, and Composites

Bulk adhesive specimens, foam blocks, and composite coupons (for example per ASTM D3039 for continuous-fiber composites) can also be evaluated across a range of test speeds when rate-dependent design data are required.

Testing Items

A comprehensive speed–strength relationship study typically measures the following properties at each test rate, with a minimum of five valid replicates per condition:

• Ultimate tensile strength (σm) as a function of crosshead speed or strain rate
• Yield strength (σy) and stress at break (σb) at each rate
• Tensile modulus (Etm or Et), noting that modulus rises moderately with rate in polymers
• Elongation at yield and elongation at break, which generally decrease with increasing rate in ductile polymers
• Full engineering and true stress–strain curves at each rate for simulation input
• Strain rate sensitivity exponent (m-value), derived from the slope of log σ versus log ε̇
• Energy to break (toughness), computed from the area under the stress–strain curve

Testing Methods

Specimen Conditioning

Prior to testing, specimens are conditioned to a standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity per ISO 291 or ASTM D618 for a minimum of 24 hours (40 hours for polyamides and other moisture-sensitive materials). Testing is conducted in the same controlled environment, since temperature shifts the rate–strength curve through time–temperature superposition.

Speed Control Strategies

Two control philosophies are recognized by the standards. Constant crosshead speed testing, the traditional approach used in ISO 527 and ASTM D638, maintains a fixed separation rate of the grips; the strain rate in the gauge section is therefore only approximately constant and depends on specimen stiffness. Constant strain rate control, offered by Method B of ISO 6892-1, uses closed-loop feedback from an extensometer or video system to hold the gauge-section strain rate constant, which improves reproducibility when the specimen geometry changes during necking. For metals, ISO 6892-1 Method A specifies estimated stress rates in the elastic region, followed by a fixed strain rate for the remainder of the test.

Multi-Rate Test Matrix Design

To characterize the speed–strength relationship, specimens are tested at a geometrically spaced series of rates. A typical polymer matrix covers 1, 5, 10, 50, 100, 250, and 500 mm/min, corresponding approximately to quasi-static strain rates from 10⁻⁴ s⁻¹ to 10⁻¹ s⁻¹. Metallic specimens are often evaluated at strain rates from 0.00007 s⁻¹ to 0.8 s⁻¹ as defined in ISO 6892-1. When higher-rate behavior is required, servo-hydraulic high-speed systems extend coverage to intermediate rates of 1–100 s⁻¹, bridging the gap between screw-driven machines and split Hopkinson pressure bar setups.

Data Regression and Modeling

Strength values from each rate are averaged and plotted against strain rate on log–log axes. For many polymers and metals over moderate rate ranges, the relationship follows a power law of the form σ = C·ε̇ᵐ, where the exponent m quantifies strain rate sensitivity. Linear regression on the logarithmic data yields m with its confidence interval. For metals, results are frequently fitted to constitutive models such as Cowper–Symonds or Johnson–Cook to provide rate-dependent material cards for crash and impact simulation. Reportable outputs include the regression equation, coefficient of determination (R²), and the scatter of replicates at each rate.

Instrumentation and Measurement Accuracy

Testing is performed on calibrated universal testing machines with load cells of appropriate capacity (typically breaking load between 20 % and 80 % of full scale) and accuracy class 0.5 or better per ISO 7500-1. Strain is measured with clip-on extensometers (ISO 9513 class 1 or better) or non-contact video extensometers, which are preferred for materials that knife-edges would damage, such as films and soft elastomers. Grip pressure, alignment, and preload (typically below 5 % of yield force) are controlled to prevent slip and bending stresses that bias the measured strength.

Validation and Reporting

The final report should state the standard followed, specimen geometry and preparation method, conditioning, control mode (crosshead speed or strain rate), each rate level, individual and mean strength values, the fitted speed–strength relationship, and any observed failure mode changes (for example, ductile-to-brittle transitions at high rates). Testing performed by accredited third-party laboratories to ISO/IEC 17025 ensures metrological traceability and third-party acceptance of the data.

Applications

• Crash and impact simulation: rate-dependent stress–strain curves feed material models used in automotive, aerospace, and rail structural analysis
• Packaging engineering: verifying that films and seals retain strength at the high deformation rates of filling and palletizing lines
• Material selection and comparison: ranking candidate polymers or alloys by strength at the strain rate representative of the end use rather than at a single default speed
• Quality control: detecting process drift by monitoring strength at a defined reference speed in incoming and outgoing inspection
• Specification compliance: demonstrating conformance when a product standard, customer drawing, or regulatory requirement specifies strength at a particular test speed
• Design safety margins: quantifying the strength penalty of slow, long-term loading versus short-duration overload events
• Research and development: characterizing new compounds, blends, heat treatments, and additive formulations for rate sensitivity

FAQ

Why does tensile strength increase with test speed?

In polymers, molecular chains and entanglements need time to relax and reorganize; at high speeds this relaxation cannot occur, so the material responds with a higher, stiffer resistance. In metals, dislocations move and multiply at a finite rate, so faster deformation requires higher stress to sustain plastic flow. Both mechanisms cause measured strength to rise as test speed increases.

How much can test speed change the measured strength?

For glassy and semi-crystalline thermoplastics, increasing the speed from 1 mm/min to 500 mm/min can raise tensile strength by 20 % to 60 % while reducing elongation at break substantially. For ductile metals, the effect is typically a few percent over the same quasi-static range but becomes significant at dynamic rates.

How many test speeds are needed to establish the relationship?

A minimum of four to five rate levels spread over at least two decades, each with at least five valid replicates, is recommended to obtain a statistically meaningful regression of strength versus strain rate and a reliable sensitivity exponent.

Which standard test speeds apply by default?

ISO 527-1 lists preferred speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min, with 50 mm/min common for rigid plastics modulus and strength; ASTM D638 commonly uses 50 mm/min for Type I specimens. For metals, ISO 6892-1 Method A defines strain-rate ranges such as 0.00007 s⁻¹ (for yield determination) and 0.0025 s⁻¹ up to 0.008 s⁻¹ for higher-rate testing.

Can strength values measured at different speeds be compared directly?

No. Comparing tensile strength results obtained at different test speeds is a frequent source of disagreement between suppliers and customers. Data are only comparable when the test speed, strain measurement method, specimen geometry, and conditioning are identical, or when a validated speed–strength relationship is used to normalize the values.

Do temperature and test speed interact?

Yes. Rate effects and temperature effects are coupled through time–temperature superposition in polymers: testing fast at a low temperature can resemble testing slowly at an even lower temperature. Consequently, rate-sensitivity studies should always report and control the test temperature.

What equipment is required for this testing?

A calibrated universal testing machine with a wide, programmable speed range, precision load cell, and extensometry (contact or video) is sufficient for quasi-static rates up to approximately 500 mm/min. Intermediate and high strain rates require servo-hydraulic high-speed test systems, and very high rates above several hundred s⁻¹ require specialized dynamic rigs. Accredited third-party laboratories can supply the full rate range with ISO/IEC 17025 traceability.