Plastic tensile strength at break testing determines the maximum stress a plastic material can sustain at the moment of fracture, making it one of the most fundamental indicators of mechanical performance in material specification, product design, and quality control. Performed with a universal testing machine in accordance with internationally recognized standards such as ISO 527 and ASTM D638, the test also yields tensile modulus, yield strength, and elongation at break. This article provides a comprehensive technical overview of the specimen requirements, test procedures, data calculations, and typical industrial applications of tensile fracture strength testing for plastics.
Technical Overview
Tensile strength at break (also called tensile fracture strength or breaking strength) is defined as the maximum tensile stress a plastic specimen carries at the instant of rupture, calculated by dividing the breaking force by the original cross-sectional area of the parallel section of the specimen. Together with tensile strength at yield, tensile modulus, and elongation at break, it forms the core set of tensile properties used to characterize plastics.
The test is conducted by clamping a standardized dumbbell-shaped or strip specimen between the grips of a universal testing machine and pulling it at a constant crosshead speed until fracture occurs. During the test, the force and extension data are continuously recorded, generating a stress-strain curve from which all tensile properties are derived. Brittle materials such as general-purpose polystyrene or unfilled thermosets break at low elongation with little or no yielding, while ductile materials such as polyethylene, polypropylene, and polyamide may yield, draw, and strain-harden before final rupture.
The principal standards governing this test include ISO 527-1 and ISO 527-2 (plastics – determination of tensile properties), ASTM D638 (tensile properties of plastics), GB/T 1040 (the Chinese national standard, technically aligned with ISO 527), and the JIS K 7161/7162 series. Although these standards are conceptually equivalent, they differ in specimen geometry, test speeds, and modulus calculation, so results obtained under different standards are not directly interchangeable.
Key equipment and conditions for a valid test include:
- A calibrated universal testing machine with a load cell appropriate to the expected breaking force (typically with accuracy class 1 or better per ISO 7500-1)
- Grips (manual, pneumatic, or wedge action) that transmit force without slippage or premature failure at the jaw faces
- A contact or non-contact extensometer for accurate strain measurement, particularly for modulus determination
- A conditioned laboratory atmosphere of 23 ± 2 °C and 50 ± 10 % relative humidity per ISO 291 or ASTM D618
Testing Samples
Specimen geometry and preparation have a decisive influence on tensile test results. The standards define several specimen types covering the full range of plastic product forms.
Injection-Molded Dumbbell Specimens
Preferred for rigid and semi-rigid thermoplastics, these specimens are molded directly in a dumbbell cavity. ISO 527-2 specimen types 1A and 1B (gauge length 50 mm, thickness 4 mm) and ASTM D638 Type I are the most widely used geometries. Direct molding eliminates machining marks and best represents the material in its as-molded state, provided molding conditions are tightly controlled.
Machined Specimens from Sheets, Plates, and Finished Parts
When the material is supplied as sheet, plate, pipe, laminate, or a finished component, specimens are cut or milled to the standard contour (for example ASTM D638 Type IV for sheet materials). Edges must be smoothed to remove nicks and machining marks, since surface imperfections act as stress concentrators and can significantly reduce the measured breaking strength.
Film and Thin-Sheet Specimens
Plastic films below approximately 1 mm thickness are tested with dedicated geometries: ISO 527-3 and ASTM D882 specify parallel strip specimens tested with extended gauge lengths and appropriate grip lining. Film testing is especially sensitive to specimen alignment, edge quality, and grip pressure.
Small and Limited-Material Specimens
For micro-molded parts, 3D-printed coupons, or when only a small amount of material is available, reduced-size geometries such as ISO 527-2 type 5A/5B or 1BA/1BB and ASTM D638 Type V are used. Results from reduced-scale specimens are indicative but should not be compared directly with full-size specimen data.
Testing Items
A single tensile fracture test campaign typically reports the following properties:
- Tensile strength at break – maximum stress sustained at the moment of fracture (MPa)
- Tensile strength at yield – stress at the first point where strain increases without a stress increase, for ductile materials (MPa)
- Tensile stress at 50 % strain (or another agreed strain value) – for highly extensible materials
- Elongation at break – strain at the moment of fracture, expressed as a percentage of the initial gauge length
- Elongation at yield – strain at the yield point for ductile materials
- Tensile modulus (Young's modulus) – the slope of the stress-strain curve in the initial linear region, in ISO 527 calculated as a chord modulus between strains of 0.05 % and 0.25 %
- Nominal tensile strain at break – based on crosshead separation or grip travel when an extensometer cannot be used up to rupture
Testing Methods
The test procedure, adapted from ISO 527-1/-2 and ASTM D638, comprises the following steps:
- Specimen conditioning: Specimens are conditioned at 23 ± 2 °C and 50 ± 10 % relative humidity for at least 40 hours before testing, since moisture content and temperature strongly affect the ductility of many plastics, notably polyamides.
- Dimensional measurement: The width and thickness of the narrow parallel section are measured at several points with a calibrated micrometer, and the minimum cross-sectional area is used in stress calculations.
- Grip installation and gauge length setting: The specimen is mounted symmetrically in the grips, an extensometer is attached within the gauge length (50 mm for ISO type 1A/1B and ASTM Type I), and a small preload removes slack.
- Speed selection: Test speed is specified by the standard and specimen type. Modulus is measured at a low speed (typically 1 mm/min), while strength and elongation for ISO type 1A and ASTM Type I specimens are commonly measured at 50 mm/min. Higher speeds are permitted for thin or extensible specimens and lower speeds for reinforced or brittle materials.
- Loading to fracture: The specimen is pulled at constant speed while force and extension are recorded continuously until it breaks completely. At least five valid specimens are tested for each material and condition; breaks occurring in or at the grips are normally rejected and retested.
- Calculation and reporting: Stress is calculated as force divided by the original cross-sectional area, strain as extension divided by the initial gauge length, and the reported value for each property is the arithmetic mean with its standard deviation. The test report must state the material identification, specimen type and preparation method, test speed, conditioning, and the complete set of measured properties.
Applications
Tensile fracture strength data support a wide range of engineering and commercial activities:
- Incoming material inspection: Verifying that purchased resins, compounds, and masterbatches meet the mechanical properties declared in datasheets or purchase specifications.
- Production quality control: Monitoring molding or extrusion processes by periodically testing specimens taken from production runs to detect degradation, contamination, or process drift.
- Material selection and design: Providing input data for finite element analysis, wall-thickness calculations, and safety factor determination in automotive, appliance, electronics, and construction applications.
- Development of new formulations: Comparing the effect of fillers, reinforcements, plasticizers, impact modifiers, or stabilizers on strength and ductility.
- Aging and environmental studies: Quantifying the retention of tensile properties after heat aging, UV or weathering exposure, chemical contact, or humidity conditioning.
- Recycled and bio-based plastics qualification: Assessing how recycled content or bio-based feedstocks influence mechanical performance.
- Additive manufacturing: Evaluating printed test coupons, including the strong anisotropy between different print orientations.
- Regulatory and customer compliance: Submitting mechanical test reports to accredited third-party laboratories for product certification, customer approval, or entry into regulated supply chains.
FAQ
What is the difference between tensile strength at break and tensile strength at yield?
Tensile strength at yield is the stress at which a ductile plastic begins to deform permanently, while tensile strength at break is the stress at the actual moment of fracture. In ductile materials that strain-soften, the breaking stress may be lower than the yield stress; in brittle materials with no yield point, the two are effectively identical to the maximum stress.
How many specimens are required for a valid test?
Both ISO 527 and ASTM D638 require a minimum of five valid specimens per material and test condition. Additional specimens should be tested if the scatter is unusually high or if breaks occur outside the gauge section.
Are ISO 527 and ASTM D638 results directly comparable?
No. Although the test principle is identical, the two standards use different specimen geometries, preferred thicknesses, and test speeds, which leads to systematically different results. Comparisons should only be made within the same standard, or conversion between them should be established experimentally.
Why do specimens sometimes break inside the grips?
Breaks at the grips are usually caused by excessive clamping pressure, stress concentration from jaw serrations, specimen misalignment, or flaws introduced during specimen preparation. Such breaks are considered invalid, and the remedy is to adjust grip pressure, use felt or emery liners, correct alignment, or improve edge finish.
How does temperature affect tensile strength at break?
Most plastics become weaker and more ductile as temperature rises and stronger but more brittle as temperature falls. For this reason, tests are performed under tightly controlled laboratory conditions, and additional tests at elevated or sub-zero temperatures are specified when the product operates in such environments.
How are thin plastic films tested for breaking strength?
Films are tested as strips according to ISO 527-3 or ASTM D882, using longer gauge lengths, lower forces, and specialized film grips. Averaging of thickness measurements across the specimen is critical because the very small cross-section amplifies measurement uncertainty.
