Tensile creep testing measures how packaging materials such as plastic films, stretch wraps, strapping, and laminates deform under sustained tensile loads over extended periods of time. This article provides a comprehensive technical overview of tensile creep performance testing, covering test principles, applicable international standards, sample preparation, testing procedures, data analysis, and typical industrial applications. It serves as a practical reference for packaging manufacturers, quality engineers, and material developers seeking to validate long-term load-bearing performance.
Technical Overview
Tensile creep is defined as the progressive, time-dependent deformation of a material when subjected to a constant tensile stress below its short-term yield or tensile strength. While conventional tensile testing captures the instantaneous mechanical response of a packaging material, creep testing reveals its behavior under sustained loading, which is frequently the governing condition in real-world packaging scenarios such as palletized goods restrained by stretch film, heavy bundles secured with plastic strapping, and stacked flexible sacks under static compression and tension.
A typical creep curve consists of three distinct stages: primary creep, in which the strain rate decreases rapidly as the material undergoes initial molecular rearrangement; secondary creep, characterized by a near-constant minimum strain rate; and tertiary creep, in which strain accelerates due to localized necking or damage accumulation, ultimately leading to creep rupture. For packaging materials, even if creep rupture never occurs, excessive creep strain can cause a critical loss of containment force, film sagging, load shifting, or package distortion during storage and distribution.
Key parameters evaluated in tensile creep testing include:
- Creep strain as a function of time at a specified stress and temperature
- Creep modulus and apparent modulus decay over time
- Creep compliance and its time dependence
- Time to creep rupture and creep rupture strength at various stress levels
- Isochronous stress-strain curves for design purposes
- Creep recovery behavior after load removal
Creep behavior is strongly influenced by temperature, humidity, stress level, polymer morphology (crystallinity and orientation), and the molecular weight of the material. For example, semi-crystalline polypropylene and polyethylene films generally exhibit lower creep resistance at elevated temperatures, while hygroscopic materials such as nylon films and paper-based packaging show dramatically increased creep under high humidity conditions. These dependencies make standardized, controlled-condition creep testing essential for reliable material qualification.
Testing Samples
Plastic Packaging Films
Monolayer and multilayer films made of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), and their coextruded or laminated combinations are commonly tested. Specimens may be cut in both the machine direction (MD) and transverse direction (TD) because film orientation produces strongly anisotropic creep behavior.
Stretch Wrap and Stretch Hood Films
Linear low-density polyethylene (LLDPE) stretch films used for pallet unitization are evaluated for their ability to maintain containment force over weeks or months. Testing may be performed on the film as supplied or after controlled pre-stretching to simulate actual application conditions.
Strapping and Banding Materials
Polypropylene strapping, PET strapping, polyester textile banding, and composite strapping are tested to determine tension retention and creep rupture characteristics, which are critical for securing heavy loads such as steel coils, lumber, bricks, and machinery during long shipments.
Adhesive Tapes
Pressure-sensitive tapes used for carton sealing and load stabilization undergo creep testing to assess long-term holding power, shear creep resistance, and elongation retention under sustained stress.
Woven Fabrics and Heavy-Duty Sacks
Woven polypropylene fabrics, heavy-duty shipping sacks, and flexible intermediate bulk container (FIBC) webbing and lifting loops are tested because they experience sustained tensile loading during filling, lifting, stacking, and extended storage.
Paper and Paper-Based Packaging
Paper sacks, kraft paper, and paper-plastic laminates exhibit significant moisture-accelerated creep and are typically tested under multiple humidity conditions to establish practical safety limits for stacked storage and transport.
Testing Items
Standard tensile creep test programs for packaging materials typically include the following measurable items:
- Tensile creep strain versus time at constant stress (typically 10% to 50% of short-term tensile strength)
- Time-dependent creep modulus and apparent creep modulus
- Creep rupture time at a series of stress levels, generating a stress-rupture envelope
- Creep rupture strength at defined lifetimes (for example, 1 hour, 100 hours, and 1,000 hours)
- Isochronous stress-strain curves constructed from multi-stress creep data
- Total strain decomposition into instantaneous elastic strain, delayed elastic strain, and permanent plastic strain
- Creep recovery and residual strain after unloading
- Strain rate in the secondary creep stage as an indicator of long-term stability
Testing Methods
Applicable Standards
The most widely applied standards for tensile creep testing of polymeric packaging materials include:
- ASTM D2990, covering tensile, compressive, and flexural creep and creep-rupture of plastics
- ISO 899-1, which specifies the determination of tensile creep behavior of plastics
- GB/T 11546.1, the Chinese national standard technically aligned with ISO 899-1
- ASTM D638 and ISO 527-1/527-3, referenced for specimen geometry and short-term tensile characterization of rigid plastics and films respectively
- ASTM D882, referenced for specimen preparation of thin plastic film strips
- ASTM D618 and ISO 291, governing specimen conditioning prior to testing
Specimen Preparation and Conditioning
Dumbbell-shaped specimens are typically used for strapping, tapes, and heavier materials, following the type geometries defined in ASTM D638 or ISO 527. For thin films, rectangular strip specimens are preferred in accordance with ASTM D882 or ISO 527-3. Specimens must be cut cleanly without nicks, burrs, or edge defects that would initiate premature failure. Unless otherwise specified, conditioning is performed at 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 40 hours, and testing is conducted under the same standard atmosphere. For hygroscopic materials such as nylon film or paper, extended conditioning to reach moisture equilibrium is required, and testing at elevated humidity levels (for example, 85% RH or 90% RH) may be specified.
Equipment and Instrumentation
Tensile creep testing is performed using dead-weight creep frames, lever-arm loading systems, or electromechanical universal testing machines equipped with static load-hold capability. Elongation is measured continuously or at programmed intervals using clip-on extensometers, LVDT displacement transducers, or non-contact optical and video extensometry, with the latter being particularly advantageous for thin films and high-elongation materials. Environmental chambers enable testing at controlled elevated or reduced temperatures to simulate tropical climates, cold-chain distribution, or warehouse conditions. A minimum of three replicates per stress level is recommended to account for material variability.
Test Procedure
The general test sequence is as follows:
- Measure and record specimen dimensions at multiple locations and calculate the cross-sectional area
- Mount the specimen in the grips, ensuring axial alignment to eliminate bending stresses
- Apply a small preload (typically less than 1% of the test load) and zero the strain measurement
- Apply the target constant stress smoothly and rapidly, without overshoot, and start the timing clock at full load application
- Record strain at logarithmically spaced intervals (for example, 1, 10, 100, 1,000, 10,000 seconds, and onward) until the scheduled test duration, such as 1,000 hours, or until creep rupture
- Repeat at multiple stress levels and, where required, multiple temperatures to construct creep curves and rupture envelopes
- For recovery studies, remove the load and record the strain decay over time
Data Analysis
Creep strain is plotted against time, usually on a logarithmic time axis, and creep modulus is calculated as the applied stress divided by the total strain at each time point. Power-law models, such as the Findley equation, are commonly fitted to the primary and secondary creep regions to enable cautious extrapolation beyond the measured time window. Stress-rupture data are plotted as applied stress versus logarithm of time to failure, allowing engineers to determine allowable long-term design stresses with appropriate safety factors. Extrapolation beyond one decade of time beyond the experimental data is generally discouraged, and accredited third-party laboratories typically report all raw strain-time data alongside fitted parameters for full transparency.
Applications
Tensile creep performance testing directly supports numerous packaging engineering decisions:
- Pallet unitization: validating that stretch films and stretch hoods maintain sufficient containment force throughout the planned storage and distribution cycle without excessive stress relaxation or creep elongation
- Strapping qualification: selecting between polypropylene, PET, and composite strapping based on tension retention over multi-week ocean shipments and long-term warehouse storage
- Heavy-duty sacks: confirming that woven polypropylene and kraft paper sacks withstand sustained loads from stacked pallets in static storage for months
- Suspended and hanging packaging: ensuring retail blister cards, hanging bags, and display packaging do not creep to failure under the constant weight of the contained product
- Cold chain and high-temperature logistics: comparing material performance at low temperatures and in hot, humid climates to select suitable film structures
- Material development: benchmarking new resin grades, film structures, and strapping formulations against incumbent materials during research and development
- Regulatory and customer compliance: providing objective evidence of long-term performance for product specifications, supplier approvals, and failure investigations
FAQ
What is the difference between creep and stress relaxation?
In creep testing, a constant stress is applied and the increase in strain over time is measured. In stress relaxation testing, a constant strain is imposed and the decay of stress over time is measured. Both phenomena reflect the same underlying viscoelastic behavior, and for packaging applications such as stretch film and strapping, the two are often studied together because containment force loss results from their combined effects.
How long should a tensile creep test run?
Common durations range from 1 hour to 1,000 hours depending on the objective. Short-term screening tests of 24 hours are useful for quality control, whereas qualification testing for long-life applications typically extends to 1,000 hours or longer. Mathematical models fitted to the measured data can support limited extrapolation, but direct measurement remains the most defensible basis for critical applications.
How are stress levels for the test selected?
Stress levels are normally chosen as defined fractions of the material's short-term tensile strength, commonly between 10% and 50%, so that the resulting creep curves span from low-strain stable behavior to eventual creep rupture. Testing at multiple stress levels allows construction of an isochronous curve set and a stress-rupture envelope suitable for design use.
Which standard should be used for films versus strapping?
ASTM D2990 and ISO 899-1 form the core creep methodology for plastic materials. Thin films are prepared and dimensioned according to ASTM D882 or ISO 527-3, while strapping specimens usually follow ASTM D638 or ISO 527-1 geometries or product-specific strapping standards. An accredited third-party laboratory can confirm the most appropriate combination for a given product and regulatory market.
Why do temperature and humidity have such a strong effect on creep results?
Polymer chain mobility increases with temperature, accelerating creep rates and shortening rupture times, with effects becoming dramatic as the glass transition temperature of amorphous polymers is approached. For hygroscopic materials such as nylon and paper, absorbed moisture acts as a plasticizer, softening the material and greatly increasing creep deformation. Testing must therefore replicate the intended service environment, and accelerated testing at elevated temperature must be interpreted carefully.
What are typical acceptance criteria for packaging applications?
Acceptance criteria depend on the application but commonly specify a maximum permissible creep strain or a minimum retained containment force after a defined period, such as no more than a stated percentage of elongation after 28 days at a given stress and temperature. These limits are established by the brand owner or packaging engineer based on load stability requirements and are verified through the creep test data reported by the testing laboratory.