A plastic tensile test report documents how plastic materials perform under uniaxial tensile loading, presenting key mechanical parameters such as tensile strength, yield stress, tensile modulus, and elongation at break. Testing is typically conducted by accredited third-party laboratories in accordance with internationally recognized standards such as ISO 527, ASTM D638, and GB/T 1040. This technical guide explains the specimen requirements, testing items, test methods, data interpretation, and practical applications of plastic tensile test reports for engineers and quality professionals.
Technical Overview
Plastic tensile testing is one of the most widely performed mechanical tests for polymeric materials. A standard dumbbell-shaped specimen is clamped in a universal testing machine and pulled along its axis at a controlled speed until yielding or fracture occurs. The force and extension data are converted into a stress-strain curve, from which the characteristic mechanical properties of the material are derived.
A plastic tensile test report is the formal deliverable of this test. It records the complete test chain, including material and specimen identification, conditioning history, test atmosphere, equipment details, crosshead speed, individual and statistical results, stress-strain curves, and conclusions regarding conformity. The dominant international standards are ISO 527 (Plastics — Determination of tensile properties), ASTM D638 (Standard Test Method for Tensile Properties of Plastics), GB/T 1040 (the Chinese national standard, technically aligned with ISO 527), and JIS K 7161/7162 (the Japanese standards, also harmonized with ISO 527).
Because test results depend heavily on specimen geometry, preparation method, and test speed, a valid report must state all of these conditions explicitly. Only reports issued under a documented quality system, ideally with ISO/IEC 17025 accreditation of the laboratory, are suitable for supplier qualification, certification, and regulatory submission purposes.
Testing Samples
Injection-Molded Dumbbell Specimens
The preferred specimen for rigid and semi-rigid plastics is the multi-purpose dumbbell specimen defined in ISO 527-2. Type 1A (overall length 170 mm, gauge length 50 mm, width of parallel section 10 mm, thickness 4 mm) is normally produced by direct injection molding, while Type 1B (overall length 150 mm) is typically machined from sheets, laminates, or molded plates. The ASTM D638 equivalent is the Type I specimen (overall length 165 mm, gauge length 50 mm, width of narrow section 13 mm, thickness 3.2 mm). Film and sheet materials are tested with the smaller specimen types defined in ISO 527-3 or ASTM D882.
Specimens from Finished Products
When testing pipes, profiles, films, or molded parts, specimens may be machined or die-cut directly from the product in accordance with ISO 2818. In these cases, the report must clearly document the sampling location and orientation, since properties such as elongation at break can vary significantly with processing direction, weld lines, and flow-induced molecular orientation.
Specimen Conditioning
Plastic properties are sensitive to temperature and moisture. Prior to testing, specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 10 % relative humidity in accordance with ISO 291 or ASTM D618, typically for at least 24 hours and longer for hygroscopic materials such as polyamide. The conditioning state, duration, and any special drying treatments must be recorded in the report.
Testing Items
A complete plastic tensile test report typically includes the following measured and calculated parameters:
- Tensile stress at yield: the first stress at which strain increases without a further increase in stress, reported in MPa.
- Tensile strength: the maximum tensile stress sustained by the specimen before rupture or yielding, reported in MPa.
- Tensile stress at break: the stress recorded at the moment of specimen separation.
- Tensile strain at yield: the strain corresponding to the yield point, expressed in %.
- Tensile strain at break (elongation at break): the strain at specimen rupture relative to the initial gauge length, expressed in %.
- Tensile modulus (Young's modulus): the slope of the stress-strain curve, defined in ISO 527-1 as a secant modulus between strains of 0.05 % and 0.25 %, reported in MPa.
- Complete stress-strain curves for each specimen, which characterize the material behavior as brittle, ductile with cold drawing, or ductile without yield.
- Statistical evaluation: individual values, arithmetic mean, and standard deviation for each property, based on a minimum of five valid specimens per ISO 527-1.
For specific applications, optional items such as Poisson's ratio (ISO 527-1 Annex), nominal strain at a defined stress, or secant moduli at extended strain ranges may be added by agreement between the parties.
Testing Methods
Apparatus and Calibration
The test is performed on a screw-driven or servo-hydraulic universal testing machine equipped with suitable grips that apply force along the specimen axis without slippage or jaw breaks. The force measuring system must satisfy ISO 7500-1 class 1 or better, and the extension measuring device must meet ISO 9513 class 1. Modulus determination requires a contact or non-contact extensometer with gauge length accuracy within ±1 %; crosshead displacement alone is not acceptable for modulus measurement.
Test Procedure
The specimen width, thickness, and gauge length are measured at several points to the accuracy required by the standard, and the cross-sectional area of the narrow parallel portion is calculated. The specimen is then mounted in the grips, the extensometer is attached, and a constant crosshead speed is applied. Typical speeds are 1 mm/min for modulus determination on ISO Type 1A/1B specimens, followed by 50 mm/min for the determination of strength and elongation; ASTM D638 Type I specimens are commonly tested at 50 mm/min for strength and 5 mm/min for modulus. The test is continued until the specimen breaks, with force and extension recorded continuously.
Data Processing and Calculations
Tensile stress is calculated as σ = F / A₀, where F is the measured force and A₀ is the original cross-sectional area of the narrow section. Tensile strain is calculated as ε = ΔL / L₀, where ΔL is the increase in gauge length and L₀ is the initial gauge length (50 mm for Type 1A/1B and ASTM Type I). The tensile modulus is determined as the slope of the secant line between the stress values at 0.05 % and 0.25 % strain, a range chosen because plastics rarely exhibit a truly linear elastic region at very small strains.
Content of a Compliant Report
The report shall identify the testing standard and its edition, the material and specimen type with dimensions, the conditioning and test atmospheres, the testing machine and extensometer details, the crosshead speeds, the date of testing, the individual and mean results with standard deviations, the stress-strain curves, and a statement of test conclusions. Any deviations from the standard, invalid specimens, or jaw-break events must be reported transparently.
Applications
Plastic tensile test reports support decision-making across the entire product life cycle, from material development to end-of-life failure analysis. Typical applications include:
- Incoming material inspection and supplier qualification for resins, compounds, and masterbatches.
- Production quality control for injection-molded, extruded, and thermoformed parts.
- Generation and verification of material datasheet values used in design and simulation.
- Research and development of new formulations, blends, filled and reinforced compounds, and recycled materials.
- Process optimization, for example evaluating the effect of mold temperature, annealing, or regrind content on mechanical performance.
- Compliance demonstration for product certification, customer specifications, and regulatory requirements.
- Failure analysis, where comparing the tensile properties of a failed part against reference material helps identify degradation, embrittlement, or processing defects.
FAQ
What is the difference between ISO 527 and ASTM D638?
Both standards determine the same fundamental properties, but they use different specimen geometries, modulus strain ranges, and preferred test speeds. ISO 527-2 Type 1A specimens are 4 mm thick with a 10 mm wide parallel section, while ASTM D638 Type I specimens are 3.2 mm thick with a 13 mm narrow section. ISO 527 defines the modulus between 0.05 % and 0.25 % strain, whereas ASTM D638 typically uses a strain range of 0.05 % to 0.25 % via extensometer or an alternative machine-compliance-corrected range. Because of these differences, results from the two standards are not directly interchangeable.
How many specimens are required for a valid test?
ISO 527-1 requires a minimum of five valid specimens per material and condition. If results show excessive scatter, additional specimens should be tested. The report must present individual values together with the arithmetic mean and standard deviation, not the mean alone.
What is the difference between tensile strength at yield and at break?
Tensile strength at yield marks the onset of plastic deformation, where the stress-strain curve reaches a local maximum and the material begins to flow. Stress at break is recorded when the specimen ultimately fractures. For ductile materials the yield stress is usually higher than the break stress due to cold drawing, whereas for brittle materials the two values coincide closely, and only a break value is reported.
Why is a crosshead speed of 50 mm/min commonly used?
The standard test speed of 50 mm/min for ISO Type 1A/1B and ASTM Type I specimens provides a practical test duration while keeping results comparable across laboratories. Because plastics are viscoelastic, higher speeds generally increase measured strength and modulus while reducing elongation, so the applied speed must always be stated in the report.
Can results from different laboratories be directly compared?
Meaningful comparison requires that both reports use the same standard edition, specimen type and preparation method, conditioning state, and test speed. Accredited third-party laboratories operating under ISO/IEC 17025 with calibrated equipment provide the highest level of confidence in interlaboratory comparability.