Tensile testing is the cornerstone mechanical evaluation for packaging materials, quantifying tensile strength, yield strength, elongation at break, and elastic modulus for plastic films, laminates, paper, and woven fabrics. Performed to internationally recognized standards such as ASTM D882, ISO 527-3, and GB/T 13022, these tests help manufacturers validate material quality, optimize converting processes, and guarantee package integrity during filling, transport, and storage. This article explains the applicable sample types, measured parameters, test procedures, and practical applications of tensile testing for packaging materials.
Technical Overview
Tensile testing, also referred to as tension testing, is the most widely applied method for evaluating the mechanical properties of packaging materials. In this test, a specimen of defined geometry is clamped between the grips of a universal testing machine and stretched at a controlled constant crosshead speed until it yields, necks, or fractures. The instrument continuously records force and extension data, producing a stress-strain curve from which tensile strength, yield strength, elongation at break, and elastic modulus are calculated.
Packaging materials are subjected to significant tensile stresses throughout their life cycle, including web tension during printing, laminating, and bag-making operations, mechanical loads during filling and sealing, and drop, vibration, and stacking forces during distribution. Tensile data therefore provides engineers with a scientific basis for material selection, converting parameter windows, and quality assurance. Manufacturers, brand owners, and accredited third-party laboratories all rely on tensile results to confirm that packaging performs reliably from production line to end user.
Typical instrumentation consists of a universal testing machine fitted with a suitably sized load cell (commonly 10 N to 5 kN for packaging substrates), pneumatic or capstan grips with rubber or serrated faces to prevent slippage without damaging thin films, and, where required, a contact or video extensometer for accurate strain measurement. Optional temperature chambers allow evaluation of performance under extreme cold or heat.
Testing Samples
Plastic Films and Sheets
Monolayer and coextruded films of PE, CPP, BOPP, BOPET, PA, PVC, EVOH, and other polymers are the most frequently tested category. Films with thickness below 1 mm are generally tested as rectangular strips per ASTM D882 or ISO 527-3, while thicker sheets use die-cut dumbbell specimens per ASTM D638, ISO 527-2, or GB/T 1040.
Composite and Laminated Films
Multi-layer structures such as PET/AL/PA/CPP retort laminates, metallized film structures, and aluminum-plastic composites are tested for overall tensile performance as well as interlayer bonding. Peel strength between laminate layers can be evaluated using related methods such as ASTM F904 or GB/T 8808.
Paper and Paperboard
Kraft sack paper, containerboard, coated papers, and paper-based flexible packaging are tested using constant-rate-of-elongation methods described in ISO 1924-2, ASTM D828, JIS P8113, and GB/T 12914. Machine direction (MD) and cross direction (CD) results differ substantially due to fiber orientation.
Woven and Nonwoven Packaging Fabrics
PP woven sacks, FIBC bulk bag fabrics, and nonwoven shopping bags are tested using strip or grab principles to verify weaving density, tape tenacity, and seam performance under load.
Stretch Films and Shrink Films
LLDPE stretch wrap and PE shrink films demand evaluation of both strength and high extensibility, so elongation at break is as critical as tensile strength for confirming pallet-load containment and package conformity.
Tapes, Strapping, and Lid Materials
Packaging tapes, PET and PP strapping, pharmaceutical blister films, and lidding foils are tested to verify break strength, bonding performance, and formability. Heat seal strength of sealed flexible materials is determined per ASTM F88 or QB/T 2358.
Testing Items
A full tensile test program for packaging materials typically reports the following parameters, separately for machine direction and transverse direction where applicable:
- Tensile strength (MPa): the maximum engineering stress sustained by the specimen before rupture.
- Yield strength (MPa): the stress at which the material exhibits the first departure from linear elastic behavior.
- Breaking force (N): the maximum force recorded, often specified for tapes, seals, and strapping.
- Elongation at break (%): the extension at rupture expressed as a percentage of the initial gauge length.
- Elongation at yield (%): the strain corresponding to the yield point, indicating the onset of plastic deformation.
- Elastic modulus (MPa): the chord or secant modulus between two specified strain points, reflecting material stiffness.
- Stress-strain curve: the complete deformation profile used to compare material toughness and ductility.
- Heat seal strength (N/15 mm): the force required to separate a sealed area, evaluated on sealed specimens.
- Lamination peel strength (N/15 mm): the interlayer adhesion force of composite structures.
Testing Methods
Specimen Preparation and Conditioning
Specimens are cut cleanly and free of nicks, wrinkles, or visible defects, using either sharp-blade strip cutting for films or matched steel dies for dumbbell specimens. Film strips are typically 10 mm to 25 mm wide with initial grip separations of 50 mm to 250 mm, selected according to the material's extensibility and the governing standard. Thickness and width are measured at several points along the gauge region to calculate the cross-sectional area. Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 40 hours in accordance with ISO 291, ASTM D618, or GB/T 2918.
Instrument Setup and Gripping
A load cell is selected so that the measured forces fall within its accurate working range, and grips are chosen to suit the material: smooth rubber-faced pneumatic grips for slippery films, serrated or wedge grips for rigid sheets and strapping, and capstan or wind grips for materials prone to jaw breaks. Careful axial alignment prevents premature failure from bending stresses.
Test Execution and Data Calculation
The crosshead speed is set according to the expected elongation of the material, ranging from around 5 mm/min for stiff foil-containing laminates up to 500 mm/min for highly extensible films such as LLDPE, as tabulated in the applicable standard. The test runs until the specimen ruptures, with at least five valid replicates tested in each direction. Specimens that fail within the grips are discarded and retested. Tensile strength is calculated as the maximum force divided by the original cross-sectional area, elongation at break as the extension at rupture divided by the initial gauge length, and the modulus as the slope of the stress-strain curve between the specified strain points. Results are reported as the mean, standard deviation, and, where required, coefficient of variation.
Applicable Standards
- ASTM D882: Tensile properties of thin plastic sheeting (below 1 mm) using strip specimens.
- ASTM D638: Tensile properties of plastics using dumbbell specimens.
- ASTM D828: Tensile properties of paper and paperboard at constant rate of elongation.
- ASTM F88: Seal strength of flexible barrier materials.
- ASTM F904: Ply adhesion comparison of similar flexible laminates.
- ISO 527-1 and ISO 527-3: Determination of tensile properties of plastics, including films and sheets.
- ISO 1924-2: Tensile properties of paper and board at constant rate of elongation.
- JIS P8113: Tensile strength and elongation of paper and board.
- GB/T 1040 series and GB/T 13022: Chinese national standards for plastics and plastic film tensile testing.
- GB/T 12914: Tensile properties of paper and board.
- GB/T 8808 and QB/T 2358: Peel force of flexible composite films and heat seal strength of flexible packaging.
Applications
Tensile testing data supports decision-making across the entire packaging value chain, from resin and film producers to converters, brand owners, and regulatory reviewers:
- Incoming material inspection and supplier qualification for films, foils, and papers.
- Process optimization for blown film, cast film, orientation, and lamination lines by correlating mechanical output with processing parameters.
- Converting window definition, ensuring web tensions during printing, coating, and slitting remain safely below material limits.
- Package structural design, including pouch format selection, seal width determination, and drop-impact performance prediction.
- Aging and stability studies, measuring tensile retention after heat, humidity, sterilization, or chemical exposure.
- Failure analysis of field breakage or splitting complaints, comparing retained specimens against specification limits.
- Compliance verification and batch release testing performed by accredited third-party laboratories against ISO, ASTM, EN, GB, and JIS requirements.
FAQ
What is the difference between tensile strength and breaking force?
Tensile strength is the maximum force normalized by the original cross-sectional area and expressed in MPa, which allows comparison between materials of different dimensions. Breaking force is the raw maximum load in N and is commonly specified for tapes, seals, strapping, and web applications where the actual load-carrying capacity matters.
Why must films be tested in both machine and transverse directions?
Manufacturing processes orient polymer chains preferentially along the machine direction, producing anisotropic behavior. A film may show high strength but low elongation in the machine direction and the opposite in the transverse direction, so both directions must be evaluated to predict real-world performance.
How many specimens should be tested?
Standards such as ASTM D882 and ISO 527 specify a minimum of five valid specimens for each direction. Specimens that fracture inside the grips are excluded, and additional specimens are tested until the required number of valid results is obtained.
What test speed should be used?
The crosshead speed is matched to the expected elongation of the material according to the tables in the governing standard. Lower speeds around 5 to 50 mm/min suit rigid laminates and foils, while highly extensible films are commonly tested at 200 to 500 mm/min. Changing the speed changes the measured strength, so consistency with the specified method is essential.
How is elastic modulus determined for flexible films?
Because very thin films make direct strain measurement difficult, the modulus is usually calculated as a chord or secant modulus between two specified strain points on the initial stress-strain curve, as defined by ISO 527 or ASTM D882. A video or contact extensometer improves accuracy when permitted.
Can the same equipment measure heat seal and peel strength?
Yes. A universal testing machine with appropriate fixtures and grips can perform seal strength tests per ASTM F88 or QB/T 2358 and laminate peel tests per ASTM F904 or GB/T 8808, making it a versatile platform for comprehensive mechanical characterization of packaging materials.