Effect of Tensile Test Speed on Plastics: Tensile Rate Impact Testing to ISO 527 and ASTM D638

Tensile rate impact testing evaluates how crosshead speed and strain rate influence the tensile properties of plastic materials, which is essential because polymers are viscoelastic and their measured strength, modulus, and elongation change systematically with loading rate. Performed in accordance with ISO 527-1, ISO 527-2, and ASTM D638 (or their GB/T 1040 and JIS K 7161 equivalents), this testing provides critical data for material datasheets, quality control, and strain-rate-dependent simulation. This article explains the underlying mechanisms, standard test speeds, specimen requirements, test procedures, and typical applications of tensile rate effect studies for plastics.

Technical Overview

Plastics are viscoelastic materials whose mechanical response depends strongly on the timescale over which the load is applied. Unlike metals, which show only modest rate sensitivity at room temperature, polymers combine the elastic response of a molecular network with time-dependent mechanisms such as chain segment rotation, disentanglement, crazing, and viscous flow. As a direct consequence, the tensile strength, yield stress, modulus, and elongation at break measured on a universal testing machine can shift by 10 % to 50 % when the crosshead speed is varied across the range permitted by the relevant standards.

Tensile rate effect testing is a systematic study in which identical specimens are pulled to failure at a series of controlled speeds in order to quantify this speed dependence. The governing standards include ISO 527-1 and ISO 527-2 for plastics, ASTM D638 for tensile properties of plastics, GB/T 1040.1 and GB/T 1040.2 as the Chinese adoption of ISO 527, and JIS K 7161 and JIS K 7162 for the Japanese equivalents. ISO 527-1 defines a set of preferred crosshead speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min, while ASTM D638 lists speeds from approximately 0.3 mm/min up to several hundred mm/min, allowing a wide rate window to be explored.

The typical experimental trend is well established: as the test speed increases, the apparent modulus, yield stress, and tensile strength generally increase, while elongation at break decreases and ductile materials may shift toward brittle fracture. Classical rate-process (Eyring) theory predicts that the yield stress rises approximately linearly with the logarithm of the strain rate, and time–temperature superposition principles allow high-rate and low-temperature behavior to be correlated. Understanding and documenting these effects is essential, because a tensile result reported without its test speed is not reproducible and cannot be compared with data generated at other rates.

Testing Samples

Thermoplastic Moulding and Extrusion Compounds

Commodity and engineering thermoplastics such as PE, PP, PS, HIPS, ABS, SAN, rigid and flexible PVC, PMMA, PC, PA6, PA66, POM, PET, PBT, PPE blends, and TPU are the most frequently tested materials. Dog-bone specimens of type 1A or 1B per ISO 527-2, or Type I per ASTM D638, are injection moulded in accordance with ISO 294-1 or compression moulded per ISO 293, with a preferred thickness of 4 mm. These amorphous and semi-crystalline resins typically display pronounced rate sensitivity, making them the primary subject of tensile rate studies.

Thermosetting Materials

Epoxy, phenolic, unsaturated polyester, vinyl ester, and polyurethane thermosets are usually prepared as cast or compression-moulded plates and machined into type 1B or Type I specimens. Because these highly cross-linked networks offer fewer molecular relaxation pathways, their rate sensitivity is often lower than that of thermoplastics, but it remains relevant for adhesives, potting compounds, and structural castings subjected to rapid loading.

Filled and Fibre-Reinforced Compounds

Short-glass-fibre-reinforced grades (for example PA6-GF30 or PP-GF30), mineral-filled compounds, and talc-filled polypropylenes can be tested using the same specimen geometry. For continuous-fibre-reinforced composites, ISO 527-4 and ISO 527-5 apply. Reinforcement generally moderates the rate dependence of strength, yet the matrix-dominated modulus and failure strain still vary with speed, so rate screening is still recommended during compound development.

Plastic Films and Sheets

Thin films are tested per ISO 527-3 or ASTM D882 using strip or dumbbell specimens and correspondingly higher separation rates. Because film molecules are often highly oriented, tensile properties can be extremely rate sensitive, and both machine-direction and transverse-direction specimens should be evaluated when characterizing packaging, lamination, or capacitor film grades.

Testing Items

A comprehensive tensile rate study quantifies each of the following properties at every selected test speed, so that the full speed dependence of the material can be mapped:

• Tensile strength at break and tensile stress at yield, measured at each crosshead speed
• Tensile strain at yield and tensile strain (elongation) at break
• Tensile modulus (Young's modulus), determined as the chord modulus between 0.05 % and 0.25 % strain in accordance with ISO 527-1
• Secant modulus at specified strain levels, where low-strain linearity is absent
• Nominal stress and nominal strain curves, calculated with respect to grip separation for materials that neck extensively
• Rate-sensitivity parameters, such as the slope of yield stress versus the logarithm of strain rate, and Eyring-type activation parameters
• The ductile-to-brittle transition speed, identified where elongation at break collapses and fracture surfaces change character
• Complete engineering stress–strain data sets at each rate, formatted for use in finite element simulation

Each result is reported together with its associated test speed, since ISO 527 and ASTM D638 both require the speed to be stated explicitly in any valid test report.

Testing Methods

Specimen Preparation and Conditioning

Specimens are produced under documented moulding conditions, with injection-moulded panels gated so that flow direction is known, since orientation can confound rate effects. Before testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618. Hygroscopic materials such as polyamides require conditioning to a defined moisture state, because water content interacts with rate effects. ISO 527-1 calls for at least five valid results per condition, and ASTM D638 similarly recommends five specimens; higher replicate counts are advisable in rate studies because fracture data scatter widens at high speeds.

Apparatus and Calibration

Testing is performed on a screw-driven or servohydraulic universal testing machine whose force measurement is verified to ISO 7500-1 (class 0.5 or class 1) or ASTM E4. Strain is measured with a clip-on or video extensometer meeting ISO 9513 class 1 or ASTM E83 class B-1, since crosshead displacement alone cannot deliver accurate modulus values. Wedge or pneumatic grips with serrated or profiled faces prevent slippage at high loads. For speeds approaching or exceeding 1 m/s, conventional load-cell measurements become distorted by inertia and ringing; in that regime, servohydraulic high-speed machines, drop towers, and high-speed cameras with digital image correlation are used to obtain valid strain data, and greater specimen counts are tested to manage scatter.

Test Speed Selection and Test Procedure

The modulus of rigid plastics is determined at 1 mm/min using an extensometer, with the chord taken between strains of 0.0005 and 0.0025 as prescribed by ISO 527-1; ASTM D638 practice commonly uses 5 mm/min with extensometry for the same purpose. Strength and elongation for type 1A specimens are typically determined at 50 mm/min. For a dedicated rate study, speeds are selected at roughly decade spacing, for example 1, 5, 10, 50, 100, and 500 mm/min, spanning the nominal strain-rate window of roughly 10⁻⁴ to 10⁻¹ s⁻¹ that quasi-static machines can deliver. Each specimen is dimensioned, gripped with a small preload, and pulled to fracture at the chosen speed while force and strain are recorded continuously; the sequence is repeated for all speeds, ideally in randomized order to eliminate systematic drift.

Data Analysis and Reporting

For every speed, the mean and standard deviation of each property are calculated, and master plots of tensile strength, yield stress, modulus, and elongation at break are drawn against the logarithm of strain rate. A linear fit of yield stress versus log strain rate yields a practical rate-sensitivity index consistent with Eyring rate-process theory. When the data support simulation work, strain-rate-dependent constitutive coefficients, such as Cowper–Symonds or Johnson–Cook parameters, are fitted to the family of stress–strain curves for use in crash, impact, and drop-test models. Fracture surfaces are examined to confirm the ductile-to-brittle transition, and every reported value is accompanied by its test speed, specimen type, and conditioning history.

Applications

Tensile rate effect testing delivers value across the entire plastics product lifecycle, from material development to failure diagnosis:

• Generation and verification of material datasheets, where properties such as yield stress must be reported at the standardized speed of 50 mm/min to be comparable between suppliers
• Construction of strain-rate-dependent material cards for finite element analysis of impact, crash, drop, and fast-assembly loading conditions
• Incoming inspection and quality control, where deviations in rate sensitivity reveal changes in molecular weight, crystallinity, or additive content
• Material selection for snap-fits, clips, cable ties, fasteners, and packaging components that experience rapid loading in service
• Failure analysis, where comparing retained specimens at multiple speeds helps distinguish material brittlement from loading-rate effects
• Regulatory compliance and certification programs conducted by accredited third-party laboratories under ISO 527, ASTM D638, GB/T 1040, or JIS K 7161 frameworks
• Research applications, including time–temperature superposition studies, activation-volume determination, and validation of constitutive models for polymer processing

FAQ

The following questions address the most common practical concerns raised by designers, quality engineers, and laboratory staff regarding the influence of tensile test speed on plastic properties.

Why does test speed change tensile results for plastics so much more than for metals?

Plastics are viscoelastic: polymer chains need time to rotate, disentangle, and flow in order to relieve stress. At low crosshead speeds these relaxation mechanisms keep pace with the deformation, producing lower strength and higher elongation. At high speeds the molecular network cannot relax, so the material responds in a stiffer, stronger, and more brittle manner. Metals, by contrast, deform primarily through crystallographic slip, which is far less time-dependent at room temperature.

Which test speed must be used for the tensile modulus?

ISO 527-1 specifies 1 mm/min for modulus determination on type 1A and 1B specimens, with the chord modulus calculated between 0.05 % and 0.25 % strain using an extensometer. ASTM D638 practice typically employs 5 mm/min with equivalent extensometric strain measurement. Using the standardized low speed ensures that modulus values from different laboratories are genuinely comparable.

How many specimens should be tested at each speed?

Both ISO 527-1 and ASTM D638 require at least five valid replicate results per condition. For rate-sensitivity studies, particularly at high speeds where fracture scatter increases, eight to ten replicates per speed are recommended so that trends in strength and elongation can be distinguished from statistical noise.

Can tensile results measured at different speeds be compared directly?

No. Because yield stress, strength, modulus, and elongation all vary with loading rate, values obtained at different speeds are not interchangeable. Any comparison, specification limit, or acceptance criterion must reference the same test speed, and the speed must always be stated in the test report as required by ISO 527 and ASTM D638.

How is a practical rate-sensitivity test program designed?

A typical program selects four to six speeds spaced at roughly one decade apart, for example 1, 10, 50, 100, and 500 mm/min, chosen to bracket the strain rates expected in service. All specimens come from a single moulding lot, are conditioned identically, and are tested in randomized order. The resulting data set is plotted as property versus log strain rate, from which the rate-sensitivity index, the ductile-to-brittle transition, and simulation-ready material parameters are extracted.