Plastic Standard Specimen Tensile Testing: Standards, Specimens, Procedures, and Applications

Plastic standard specimen tensile testing is the most widely used method for determining the tensile strength, tensile modulus, yield behavior, and elongation properties of plastic materials, performed on standardized dumbbell-shaped specimens in accordance with ISO 527, ASTM D638, and GB/T 1040. This technical guide covers specimen geometry and preparation, test equipment, measurement parameters, step-by-step test procedures, and typical industrial applications. Whether for quality control, material qualification, or product certification in accredited third-party laboratories, understanding the fundamentals of tensile testing ensures reliable and comparable mechanical data.

Technical Overview

Tensile testing of plastic standard specimens is a quasi-static mechanical test in which a machined or molded dumbbell (dog-bone) specimen is gripped at both ends and stretched to failure at a controlled crosshead speed. The test generates a force-extension curve that is converted into an engineering stress-strain curve, from which key design and quality-control parameters are extracted, including tensile strength, tensile stress at yield, tensile strain at break, and tensile modulus.

The test is governed by a family of internationally recognized standards. ISO 527-1 specifies general principles, ISO 527-2 covers rigid molding and extrusion plastics, and ISO 527-3 applies to films and sheets. ASTM D638 is the dominant North American method, while GB/T 1040 is the Chinese national standard technically aligned with ISO 527. Japanese standard JIS K 7161 and JIS K 7162 are likewise harmonized with ISO 527. Because polymers are viscoelastic, results are strongly dependent on strain rate, temperature, and humidity, which is why standardized specimen geometry, conditioning, and test speed are essential for data comparability.

Typical stress-strain responses fall into several categories: brittle behavior with fracture before yield (for example, general-purpose polystyrene or unfilled thermosets), ductile behavior with a distinct yield point followed by cold drawing (for example, polypropylene or polyethylene), and ductile behavior with continuous strain hardening without a clear yield peak. The same material can shift between these categories as temperature or loading rate changes.

Testing Samples

ISO 527-2 Specimens

The preferred ISO specimen is type 1A, normally produced by injection molding. Type 1B has the same overall dimensions but a larger fillet radius, making it suitable for machining from sheets, rods, or finished parts.

• Type 1A/1B: 170 mm overall length, 10 mm narrow-section width, 4 mm thickness, 50 mm gauge length (L0), 115 mm initial distance between grips.
• Type 1BA and 1BB: scaled-down versions (75 mm and 30 mm overall length, 2 mm thickness) used when material quantity is limited.
• Type 5A and 5B: small multipurpose specimens for restricted sample volumes, tested at reduced speeds.
• ISO 3167 multipurpose specimens: the central section of these plates corresponds directly to type 1A or 1B geometry and supports multiple test types from one molding.

ASTM D638 Specimens

ASTM D638 defines five specimen types selected according to material form and thickness.

• Type I: preferred rigid specimen, 165 mm overall length, 13 mm narrow-section width, 3.2 mm preferred thickness, 50 mm gauge length, 115 mm grip separation.
• Type II: alternative geometry for materials that are difficult to machine.
• Type III: for plates and sheets between 7 mm and 14 mm thick.
• Type IV: for non-rigid plastics, elastomeric materials, and high-elongation compounds.
• Type V: small-scale specimen (63.5 mm overall length, 7.62 mm gauge length) used when only limited material is available, including coupons machined from parts or additive-manufactured samples.

Specimen Preparation and Conditioning

Specimen preparation has a decisive influence on results. Injection-molded specimens should be produced per ISO 294 or the molding annex of ASTM D638, with molding conditions documented. Machined specimens follow ISO 2818, with smooth polished edges free of machining marks, notches, or voids that act as stress concentrators. Flow direction must be recorded, since fiber-reinforced and highly oriented materials exhibit significant anisotropy and should be tested in both the machine and transverse directions.

Prior to testing, specimens are conditioned in a standard laboratory atmosphere:

• ISO 291, atmosphere 23/50: 23 °C ± 2 °C and 50 % ± 10 % relative humidity, minimum 88 hours.
• ASTM D618, Procedure A: 40 hours at 23 °C ± 2 °C and 50 % ± 5 % relative humidity.
• Hygroscopic materials such as polyamide 6 and polyamide 66 may require extended or accelerated conditioning per ISO 1110; dry-as-molded and moisture-equilibrated states must be clearly distinguished, as moisture can reduce yield stress by more than 30 %.

Testing Items

A standard tensile test report typically includes the following measured and calculated quantities:

• Tensile stress at yield: the first stress at which strain increases without a stress increase, or the stress at the yield peak.
• Tensile strength: the maximum stress sustained by the specimen up to the point of break.
• Tensile stress at break: the stress at the moment of rupture.
• Tensile strain at yield: strain corresponding to the yield point, expressed in percent.
• Tensile strain at break: extensometer strain within the gauge length at rupture.
• Nominal tensile strain at break: calculated from grip separation when extensometer measurement of break strain is not feasible.
• Tensile modulus: the chord modulus determined between strains of 0.05 % and 0.25 %.
• Poisson ratio: measured with transverse strain sensing, per the annex of ISO 527-1.
• Complete stress-strain curve and, optionally, the energy absorbed to break.

Testing Methods

Equipment and Calibration

Testing is performed on a screw-driven or servo-hydraulic universal testing machine verified to ISO 7500-1 class 1 or ASTM E4, with a load cell selected so the expected failure force lies within a favorable portion of its calibrated range. Because the modulus is calculated over a strain interval of only 0.2 %, machine crosshead displacement is not acceptable as a strain source; a contact extensometer, non-contact video extensometer, or digital image correlation system verified to ISO 9513 class 1 (or better) or ASTM E83 class B-1 is required. Self-tightening wedge grips or constant-pressure pneumatic side-action grips prevent slippage without crushing the specimen, and serrated or emery-lined faces are used where necessary. Environmental chambers permit testing at sub-ambient or elevated temperatures for automotive, aerospace, and packaging specifications.

Test Procedure

A representative test per ISO 527-2 on a type 1A specimen proceeds as follows:

• Verify conditioning, then measure the width and thickness of the narrow section at three positions to 0.02 mm or better and calculate the minimum cross-sectional area.
• Set the initial distance between grips to 115 mm and attach the extensometer with a 50 mm gauge length.
• Apply a small preload to remove slack and zero the force channel.
• Test at 1 mm/min to at least 0.3 % strain and calculate the tensile modulus between 0.05 % and 0.25 % strain.
• Change the speed to 50 mm/min without removing the specimen (the two-speed method is permitted) and continue to break.
• Record the yield point, maximum force, and force at break; observe and document the fracture location.
• Discard specimens that fail in the grips or at the fillets, then retest until five valid results are obtained.

For ASTM D638 Type I specimens, the modulus is measured at a crosshead speed of 5 mm/min and other properties at 50 mm/min; Types III and V use reduced speeds per the standard's speed table. Data evaluation uses engineering stress (force divided by the original cross-sectional area) and engineering strain (extension divided by the original gauge length). ISO 527-1 requires reporting the arithmetic mean for yield stress and modulus, and the median for stress and strain at break because fracture data scatter widely; ASTM D638 reports the arithmetic mean and standard deviation of all valid results. A minimum of five valid specimens is required, extended to ten for anisotropic materials tested in two directions.

Common Sources of Error

• Poor specimen edge quality, machining scratches, or molding voids causing premature brittle failure.
• Residual stresses or insufficient conditioning, particularly for moisture-sensitive resins.
• Grip slippage, grip-induced fracture, or misalignment producing bending loads.
• Using crosshead displacement for strain, which inflates modulus values due to machine compliance.
• Non-standard speeds or temperatures, which must always be reported with the data.

Applications

Tensile testing of standard specimens underpins nearly every stage of the plastics value chain:

• Quality control and lot release against datasheet limits or customer specifications in resin production and compounding.
• Incoming material verification and supplier qualification for injection molders and extruders.
• Research and development of new resins, blends, modifiers, recycled content levels, and filler loadings.
• Aging and durability studies, where retention of tensile properties after heat aging, hydrothermal exposure, UV weathering, or chemical contact is used as the acceptance criterion, including long-term property programs referenced by UL 746.
• Engineering design inputs and CAE material cards, where corrected true stress-strain curves feed structural and crash simulation for automotive components.
• Certification and compliance testing for automotive, electronics, packaging, medical device, and pipe sectors, often carried out by accredited third-party laboratories to ISO/IEC 17025.
• Additive manufacturing qualification, where Type V or type 1BA coupons printed in different build orientations reveal the anisotropy of 3D printed polymers.
• Failure analysis and competitive benchmarking of unknown or field-returned materials.

FAQ

Are ISO 527 and ASTM D638 results directly comparable?

No. The two standards use different specimen geometries (ISO type 1A is 10 mm wide and 4 mm thick; ASTM Type I is 13 mm wide and 3.2 mm thick) and, in some cases, different speeds and conditioning rules. Yield strength values are often numerically close, but modulus and elongation at break can differ systematically. Data should only be compared within the same standard and specimen type, and any cross-reference must be supported by an established correlation.

Why is an extensometer mandatory for modulus measurement?

The modulus is derived from the strain difference between only 0.05 % and 0.25 %, a range in which frame compliance, grip seating, and load-string deflection dominate the raw crosshead signal. A high-accuracy extensometer verified to ISO 9513 or ASTM E83 isolates the true gauge-section strain, and the low test speed of 1 mm/min ensures quasi-static loading conditions.

What should be done if the specimen breaks inside the grips?

A grip failure is not representative of the material and must be discarded. Corrective actions include reducing grip pressure, switching to pneumatic side-action grips, using emery cloth or tabs to distribute clamping stress, checking specimen alignment, and verifying that fillet radii are free of machining damage. Retest until the required number of valid mid-section breaks is achieved.

How many specimens are required for a valid test?

ISO 527 requires at least five valid results per condition, and ASTM D638 requires five specimens for isotropic materials, increasing to ten when testing anisotropic materials in both principal directions. Outliers from invalid breaks are replaced, not statistically deleted.

Why do polyamide results vary so much between laboratories?

Polyamides are strongly hygroscopic. Dry-as-molded specimens show high strength and low elongation, while specimens equilibrated at 23 °C and 50 % relative humidity show substantially lower yield stress and much higher elongation. Conditioning state per ISO 291 or ISO 1110 must always accompany the reported values.

Can datasheet tensile values be used directly for product design?

Datasheet values are single-point data measured at one speed and one temperature on ideal specimens. Because polymer strength decreases with loading duration and temperature, design allowables must be established with statistical treatment of data, temperature and rate conditions representative of service, and appropriate safety factors, ideally verified by testing of the actual molded part.