Plastic tensile strength testing is the most widely used mechanical evaluation for determining how plastic materials perform under uniaxial pulling forces. This article provides a complete, step-by-step walkthrough of the plastic tensile testing process, covering specimen preparation, conditioning, equipment setup, test execution, and data analysis in accordance with ISO 527, ASTM D638, and GB/T 1040 standards.
Technical Overview
Tensile testing, also known as tension or pull testing, is the cornerstone of plastic material characterization. During the test, a standardized specimen is clamped between two grips of a universal testing machine and stretched at a constant speed until it yields or fractures. The resulting force-versus-elongation curve reveals critical mechanical properties that determine whether a material is suitable for a given application.
The test quantifies how much stress a plastic can withstand, how far it can stretch before breaking, and how stiff it behaves in its elastic region. Because plastics are strain-rate sensitive and sensitive to temperature and humidity, tensile testing must follow strictly standardized procedures to produce comparable, repeatable results. The principal governing standards include:
- ISO 527-1 and ISO 527-2 – Determination of tensile properties of plastics, general principles and test conditions for moulding and extrusion plastics
- ASTM D638 – Standard Test Method for Tensile Properties of Plastics
- GB/T 1040 series – Chinese national standard, technically aligned with ISO 527
- ISO 527-3 – Test conditions for films and sheets
- JIS K 7161 – Japanese standard for the determination of tensile properties of rigid plastics
Testing Samples
Rigid and Semi-Rigid Plastic Specimens
For injection-molded, compression-molded, or machined rigid plastics, dumbbell-shaped (dog-bone) specimens are used. The most common geometries are ISO 527-2 Type 1A (170 mm overall length, 10 mm narrow-section width, 4 mm nominal thickness) and ASTM D638 Type I (165 mm overall length, 13 mm narrow-section width, 3.2 mm thickness). The widened ends ensure that failure occurs in the narrow central gauge section rather than at the grips. Specimens must be free of voids, notches, scratches, and flash, as surface defects act as stress concentrators and distort results.
Film and Thin Sheet Specimens
For flexible films and sheets thinner than approximately 1 mm, ISO 527-3 and ASTM D882 apply. These tests use rectangular strip specimens or smaller dumbbell geometries cut with sharp dies, and often employ specialized low-force grips and lightweight extensometers to measure elongation accurately.
Specimen Conditioning
Plastic properties vary significantly with temperature and moisture content. Before testing, specimens must be conditioned in a standard atmosphere of 23°C ± 2°C and 50% ± 10% relative humidity for at least 24 hours, following ISO 291 or ASTM D618. Hygroscopic materials such as polyamide (nylon) may require extended conditioning or dry-as-molded testing, which must be clearly stated in the report. A minimum of five valid specimens per material and direction is required; ten or more are recommended when statistical confidence is important.
Testing Items
A single tensile test run yields a complete stress-strain curve from which the following parameters are extracted:
- Tensile strength (σM) – the maximum engineering stress sustained before fracture, calculated as maximum force divided by original cross-sectional area
- Tensile stress at yield (σY) – the stress at the first point where strain increases without a stress increase, critical for ductile plastics
- Tensile stress at break (σB) – the stress at the moment of specimen rupture
- Tensile strain (elongation) at yield (εY) – deformation at the yield point, expressed as a percentage of gauge length
- Tensile strain at break (εB) – total elongation at rupture, a key indicator of ductility versus brittleness
- Tensile modulus (Et) – the ratio of stress difference to strain difference between two specified strain points, typically 0.0005 and 0.0025 (0.05% and 0.25%) per ISO 527-1, representing material stiffness
- nominal strain at 50% elongation, where applicable, for materials that do not break within the machine travel limits
Testing Methods
Step 1: Specimen Measurement
Measure the width and thickness of each specimen at several points within the narrow gauge section using a calibrated micrometer or thickness gauge, and calculate the minimum cross-sectional area A0. Even small measurement errors propagate directly into every reported stress value, so dimensional measurement accuracy of 0.01 mm or better is recommended.
Step 2: Equipment Setup and Calibration
Configure a calibrated universal testing machine with a load cell whose capacity matches the expected breaking force, ideally keeping the measured force between 10% and 90% of the cell range. Select appropriate grips—wedge-action mechanical grips are common for rigid plastics, while pneumatic side-action grips with rubber-faced jaws are preferred for films. Verify machine alignment, because grip misalignment introduces bending stresses and premature failure.
Step 3: Specimen Mounting and Gauge Length Setting
Insert the specimen so that the wide ends are evenly gripped and the narrow section is centered between the grips. Set the initial grip separation, typically 115 mm for both ISO 527-2 Type 1A/1B and ASTM D638 Type I specimens. Attach the extensometer or activate the video extensometer to measure strain directly on the gauge length, which is 50 mm for these geometries.
Step 4: Test Speed Selection
Select the crosshead speed specified by the relevant standard, since plastics are highly strain-rate sensitive:
- 1 mm/min – used for tensile modulus determination under both ISO 527 and ASTM D638
- 50 mm/min – standard speed for tensile strength and elongation of rigid plastics (ISO 527-2 Type 1A/1B and ASTM D638 Type I)
- 5 mm/min to 500 mm/min – specified alternative speeds for smaller specimens, films, or specific material classes
Step 5: Test Execution and Data Acquisition
Start the test and allow the machine to pull the specimen at constant speed. The data acquisition system continuously records force and elongation, generating the stress-strain curve. Observe and mark the yield point, maximum force point, and break point. For materials that stretch beyond the machine travel limits without breaking, terminate at 50% or another specified strain and report accordingly.
Step 6: Calculation and Reporting
Calculate engineering stress as force divided by original cross-sectional area and strain as elongation divided by initial gauge length. Determine the tensile modulus from the slope of the stress-strain curve between the two specified strain limits. Report the mean value, standard deviation, and number of specimens for each parameter, together with complete test details: material identification, specimen type and dimensions, conditioning atmosphere, test speed, grip type, and date of test. Specimens that fail in the grip section or at a flaw should be discarded and retested, as required by both ISO and ASTM methodologies.
Applications
Tensile testing data supports nearly every stage of the plastic product lifecycle, from material development to end-product verification. Typical application scenarios include:
- Incoming material inspection – verifying that resin lots and compounds meet purchase specifications and datasheet values
- Quality control during production – monitoring injection molding and extrusion processes for degradation or batch inconsistency
- Research and development – comparing candidate materials, evaluating filler and reinforcement effects, and validating simulation input data
- Failure analysis – determining whether brittle fracture, insufficient strength, or excessive ductility contributed to field failures
- Regulatory compliance and certification – generating data for automotive, electrical, medical device, packaging, and construction approvals, including UL flammability and property certification programs and CE marking support under EN standards
- Verification testing at accredited third-party laboratories for customer declarations, export documentation, and dispute resolution
FAQ
How many specimens are needed for a valid tensile test?
Both ISO 527 and ASTM D638 require a minimum of five valid specimens per condition. Specimens that break inside the grips, slip, or fail at visible flaws must be discarded and replaced. For statistically robust results, particularly in R&D or certification work, testing ten or more specimens is recommended.
What are the main differences between ISO 527 and ASTM D638?
The two standards differ in specimen geometry, gauge length definition, modulus strain limits, and preferred test speeds. ISO 527-2 Type 1A uses a 10 mm wide, 4 mm thick dumbbell while ASTM D638 Type I uses a 13 mm wide, 3.2 mm thick dumbbell. Results from the two methods are not directly interchangeable, so the required standard must be confirmed with the customer or specification before testing begins.
Why do tensile results sometimes show large scatter?
Common causes include insufficient conditioning, specimen surface defects, poor machining or die-cutting quality, moisture absorption in hygroscopic resins, incorrect test speed, and grip-induced stress concentrations. Careful control of specimen preparation, environment, and machine setup typically brings the coefficient of variation below 5% for rigid plastics.
Can results obtained at different test speeds be compared?
No. Plastics are viscoelastic and strain-rate sensitive: higher test speeds generally produce higher strength values and lower elongation at break. All comparisons must be made at identical speeds, temperatures, and humidity conditions, and every report should state the exact test parameters used.