Tensile testing of packaging materials is a fundamental mechanical test that measures tensile strength, elongation, and modulus of films, foils, papers, laminates, and other packaging substrates. Performed in accordance with international standards such as ISO 527, ASTM D882, and GB/T 1040, the test verifies that packaging materials can withstand the stresses of filling, sealing, transportation, and storage. This article explains the sample types, test items, methods, and typical applications of tensile testing for packaging quality control.
Technical Overview
Tensile testing is one of the most widely performed mechanical tests for packaging materials. A specimen of defined dimensions is clamped between the grips of a universal testing machine and pulled at a constant speed until it breaks. During the test, the force and extension are continuously recorded, producing a stress-strain curve from which key mechanical properties are calculated.
For packaging manufacturers, converters, and brand owners, tensile data provides direct evidence of whether a material can survive the mechanical demands of converting, filling, sealing, handling, and distribution. The test supports incoming material inspection, product development, process optimization, and failure analysis. Accredited third-party laboratories routinely conduct tensile testing according to internationally recognized methods published by standards organizations such as ISO, ASTM, EN, GB, and JIS.
- Applicable materials: plastic films, laminates, paper, paperboard, aluminum foil, woven fabrics, tapes, and strapping
- Core output: tensile strength, elongation at break, elastic modulus, and full stress-strain behavior
- Common standards: ISO 527-3, ASTM D882, ASTM D638, ISO 1924-2, GB/T 1040, GB/T 13022
Testing Samples
Plastic Films and Sheets
Monolayer and multilayer films made of PE, PP, PET, PA, PVC, EVOH, and CPP are tested as rectangular strips or dumbbell specimens. Thin films below 1 mm in thickness are typically tested to ASTM D882 or ISO 527-3, while thicker sheets are tested to ASTM D638 or ISO 527-2 using dumbbell-shaped specimens.
Laminated Films and Flexible Packaging
Composite structures such as PET/AL/PE, BOPP/CPP, and NY/PE laminates used for pouches, sachets, and bags are tested to evaluate the combined strength of the laminate. Testing is performed in both machine direction (MD) and transverse direction (TD) because laminates frequently exhibit anisotropic behavior.
Paper and Paperboard
Packing paper, kraft sack paper, and carton board are tested to ISO 1924-2 or TAPPI-aligned methods such as ASTM D828. Results include tensile strength, tensile stiffness, and tensile energy absorption, which are critical indicators of paper performance in bag-making and printing.
Aluminum Foils and Foil Laminates
Foil used in blister packs, lidding foils, and barrier laminates is tested to assess strength and ductility. Because foils are thin and easily damaged, specimen preparation and careful gripping are essential to obtain valid results.
Woven and Nonwoven Packaging Fabrics
PP woven sacks, FIBC fabrics, and nonwoven wrapping materials are tested to ASTM D5034, ASTM D5035, or relevant GB standards to determine breaking force and elongation, which reflect the load-bearing capacity of bulk packaging.
Tapes, Strapping, and Seals
Pressure-sensitive adhesive tapes are tested for tensile strength and elongation to ASTM D882 or ASTM D3759, while plastic strapping (PET, PP) is tested to verify breaking strength for palletizing and unitizing applications. Heat-sealed joints may also be evaluated in tension using seal-strength-oriented methods such as ASTM F88.
Testing Items
Depending on the material and the applicable standard, the following parameters are typically reported:
- Tensile strength: the maximum stress the specimen withstands before rupture, expressed in MPa or N/mm²
- Breaking force: the maximum force at rupture, expressed in N or N per unit width (N/15 mm) for films
- Yield strength: the stress at which the material begins to deform plastically
- Elongation at break: the strain at rupture expressed as a percentage of the original gauge length
- Elongation at maximum force: the strain corresponding to the peak force
- Tensile modulus (Young's modulus) and secant modulus: measures of material stiffness within the elastic region
- Tensile stiffness: particularly important for paper and board materials
- Tensile energy absorption (TEA): the total energy absorbed up to rupture, used mainly for sack paper
- Stress-strain curve: the complete force-extension profile characterizing material behavior
Testing Methods
Testing is performed on a calibrated universal testing machine equipped with appropriate load cells, grips, and, where required, a contact or video extensometer. Typical pneumatic side-action grips, capstan grips, or extended-length grips are selected according to the specimen material to prevent slippage and premature failure at the jaw faces.
Specimen Preparation
Specimens are die-cut with sharp, dedicated cutting tools to ensure clean, nick-free edges. Films are typically cut into strips of defined width (for example, 15 mm or 25 mm) and adequate length to achieve the specified gauge length. Thicker plastics are molded or machined into dumbbell specimens. A minimum of five valid specimens per direction is generally required, with more specimens tested when variability is high.
Conditioning
Prior to testing, specimens are conditioned in a standard atmosphere, typically 23 °C ± 2 °C and 50 % ± 10 % relative humidity, in accordance with ISO 291, ISO 187, or ASTM D618. The conditioning period depends on material thickness and type, and testing is carried out under the same controlled conditions whenever possible.
Test Execution
- Measure and record specimen width and thickness at multiple points to calculate cross-sectional area
- Mount the specimen in the grips, aligning it precisely to avoid bending or twisting loads
- Apply the specified preload to remove slack from the specimen
- Select the crosshead speed according to the standard, for example 200 mm/min to ISO 527-3 or 500 mm/min (or elongation-based speed) to ASTM D882
- Run the test to rupture and record force-extension data continuously
- Reject results from specimens that fail in or slip at the grips, and retest to obtain the required number of valid results
Data Processing and Reporting
Calculated values are reported as the arithmetic mean, standard deviation, and coefficient of variation for each property. Reports identify the material, specimen geometry, testing direction (MD or TD), conditioning regime, test speed, standard applied, and the full stress-strain curves. Comparisons against customer specifications or product standards are provided upon request.
Applications
- Incoming inspection: verifying that purchased films, foils, and papers meet agreed strength specifications before production
- Product development: comparing candidate materials, laminate structures, and thickness reductions during packaging design
- Process control: monitoring the effect of extrusion, orientation, lamination, printing, and curing on mechanical performance
- Regulatory and customer compliance: generating test reports required by food, pharmaceutical, and export packaging specifications
- Failure analysis: determining whether package breakage, tearing, or seal failure during transport is related to insufficient tensile properties
- Supplier qualification: benchmarking materials from different suppliers using a consistent, standardized method
FAQ
What specimen sizes are required for film tensile testing?
For strip-method standards such as ASTM D882 and ISO 527-3, films are typically cut into strips of 15 mm or 25 mm width with a length sufficient to provide the specified initial grip separation. Dumbbell specimens are required for thicker plastic materials tested to ASTM D638 or ISO 527-2. Exact dimensions are defined by the applicable standard.
Why must specimens be tested in both machine and transverse directions?
Films, laminates, and papers are manufactured with molecular or fiber orientation, so tensile properties differ significantly between the machine direction (MD) and transverse direction (TD). Testing both directions provides a complete picture of how the packaging will perform during forming, filling, and distribution.
What is the difference between ISO 527-3 and ASTM D882?
Both standards cover tensile testing of thin plastic sheeting and film, but they differ in specimen dimensions, grip separation, test speeds, and some calculation details. The choice depends on the market, customer specification, or regulatory requirement; ISO methods are common in Europe and internationally, while ASTM methods are widely used in North America.
How long does a typical tensile test take?
The test itself usually completes within one to several minutes per specimen depending on the specified speed and material elongation. Total project time also includes specimen preparation and conditioning, so accredited third-party laboratories typically report results within three to seven working days for standard materials.
Can tensile testing evaluate heat seals on finished pouches?
Standard tensile testing evaluates the bulk material, while seal integrity of finished packages is assessed using seal-strength methods such as ASTM F88 or burst-oriented tests. Many laboratories offer both material tensile testing and seal-strength testing as complementary services for flexible packaging qualification.
What factors most commonly cause invalid tensile results?
Common causes include specimen slippage in the grips, jaw breaks caused by unsuitable grip pressure or jaw faces, nicks introduced during poor specimen cutting, insufficient conditioning, and misalignment of the specimen. Careful method control and experienced operators are essential for reproducible results.