PET strapping bands are widely used as a strong, safe, and cost-effective alternative to steel strapping, but their long-term performance depends heavily on resistance to creep under sustained tension. Creep resistance testing measures how much a PET strapping band elongates and how much tension it retains over time when subjected to constant loads. This article provides a complete technical guide covering test samples, testing items, test methods, data evaluation, industry applications, and frequently asked questions.
Technical Overview
PET strapping bands, manufactured from high-strength polyethylene terephthalate, combine high tensile strength with flexibility, chemical stability, and resistance to rust and corrosion. They are commonly used to secure heavy industrial loads such as steel coils, bricks, cotton bales, and palletized goods. Unlike a short-term tensile failure, the most critical long-term risk for PET strapping is creep, defined as the time-dependent, progressive deformation of the material under a sustained load below its yield point.
When a PET strapping band is tensioned around a package, it carries a constant working load for weeks or months during storage and transport. Under this sustained stress, polymer molecular chains gradually relax and slip past one another, causing the strap to elongate. As elongation accumulates, the applied tension decays, the package loosens, and load stability is compromised. Creep resistance testing therefore focuses on quantifying elongation over time, tension retention, and, in extreme cases, the time to creep rupture.
The core reference methods for creep testing of plastics include ASTM D2990 (Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics) and ISO 899-1 (Determination of creep behaviour under tensile loading). Base tensile properties are determined according to ASTM D638 or ISO 527, while product-level requirements for strapping may reference ASTM D3953 and related purchase specifications. Conditioning is performed in accordance with ISO 291 or ASTM D618.
Testing Samples
Standard Plain-Surface PET Strapping Bands
These samples cover the most common commercial configurations, typically with widths from 13 mm to 32 mm and thicknesses from 0.5 mm to 1.0 mm. Breaking strengths generally range from approximately 3,000 N to more than 9,000 N depending on cross-section and grade. Plain green strapping is the dominant industrial type and serves as the baseline sample for creep evaluation.
Embossed and Colored Bands
Embossed bands feature a textured surface pattern that improves grip in seals, buckles, and tool grippers. Black or gray bands often contain UV stabilizers or carbon-based additives for outdoor or extended-storage exposure. Because embossing geometry and additive loading can influence crystallinity and long-term deformation, these variants are tested separately from plain bands.
Jointed Strap Specimens
In real packaging systems, the joint is frequently the governing element for tension retention. Specimens joined by friction-weld (heat-seal) joints, notched seals, or wire buckles are tested in the assembled condition to evaluate creep of both the strap body and the joint interface.
Specimen Conditioning and Preparation
All samples are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours prior to testing. Specimens are cut to a total free length of at least 500 mm, with a marked gauge length of typically 100 mm to 200 mm. Kinked, twisted, or damaged sections are excluded, and width and thickness are measured at three positions along the gauge length to calculate the nominal cross-sectional area.
Testing Items
The creep resistance evaluation of PET strapping bands typically includes the following measured items:
- Instantaneous elastic elongation immediately after full load application
- Total creep elongation at standard checkpoints of 1 h, 24 h, 72 h, and 168 h under constant load
- Steady-state creep rate determined from the linear region of the creep curve
- Retained tension ratio, expressed as the percentage of initial tension maintained after a specified hold period
- Creep rupture time under elevated load levels approaching the short-term breaking strength
- Elastic and delayed recovery elongation measured after unloading
- Elevated-temperature creep performance at 40 °C, 60 °C, or 70 °C to simulate containerized export transport
- Optional stress relaxation behavior at fixed elongation for package-load interaction analysis
Testing Methods
Test Equipment and Setup
Testing is performed using dedicated constant-load creep frames with calibrated dead-weight or lever systems, or universal testing machines equipped with closed-loop load-hold capability. Grips use serrated or pressure-die surfaces designed to prevent slippage and stress concentration at the jaws. Elongation is measured with clip-on extensometers, long-travel LVDT sensors, or non-contact video extensometry, connected to a continuous data acquisition system. Temperature-controlled chambers are used for elevated-temperature test series.
Test Procedure
The standard constant-load tensile creep test follows these steps:
- Determine the reference breaking strength of the batch by a standard tensile test on at least five specimens
- Select creep load levels, typically 25%, 50%, and 65% to 70% of the measured breaking strength, reflecting realistic working tension ranges
- Install the specimen, apply a small preload of 1% to 2% of the test load, and zero the extension measurement
- Apply the target load smoothly and without shock, recording the instantaneous elastic elongation
- Record elongation at logarithmic intervals, for example at 1, 2, 5, 10, 30, and 60 minutes, then hourly and daily thereafter
- Maintain the constant load for the specified duration: 24 h for routine quality control, 72 h to 168 h for qualification, and up to 1,000 h or longer for long-term prediction
- For rupture tests, hold the load until failure occurs and record the elapsed time
- For recovery evaluation, remove the load and measure residual elongation after 1 h and 24 h of unloaded rest
Data Processing and Evaluation
Creep strain is calculated as elongation divided by initial gauge length and plotted against logarithmic time. The Findley power-law model, expressed as strain equals initial strain plus a coefficient multiplied by time raised to an exponent n, is fitted to the data to extrapolate long-term behavior from shorter tests. Steady-state creep rate is obtained from the slope of the secondary creep region, and time-temperature superposition may be applied to construct master curves for service-life prediction.
Typical acceptance criteria used across the industry include a total additional creep elongation of no more than 1.5% to 2% after 24 hours at 50% of breaking strength, and a retained tension ratio of at least 60% to 70% after the specified hold period. Results are reported as the mean and standard deviation of at least three valid specimens per condition, along with test temperature, humidity, load level, and failure mode where applicable.
Applications
Creep resistance test data for PET strapping bands directly support safety and quality decisions across many industries:
- Bundling of steel coils, aluminum ingots, and metal profiles where long-term tension retention prevents coil loosening
- Packaging of bricks, blocks, tiles, and refractory materials stored outdoors or in yards for extended periods
- Cotton, fiber, and textile baling, where strap tension must counteract the internal expansion of compressed bales
- Palletized consumer and industrial goods in warehouse logistics and long-distance land transport
- Export container shipping, in which internal container temperatures can reach 60 °C to 70 °C and significantly accelerate creep
- Securing glass, ceramics, and stone slabs, where even modest tension loss can lead to abrasive product damage
- Manufacturer quality control, incoming material inspection, new resin grade development, and optimization of heat-setting and orientation processes
- Engineering input for cargo-securing calculations and load-securing plans required by freight regulations
FAQ
What causes creep in PET strapping bands? Creep results from molecular chain relaxation and slippage within the oriented polymer structure under sustained stress. Higher temperatures and higher stress ratios accelerate the process, which is why elevated-temperature testing is essential for tropical and containerized shipping routes.
What load level is used in the creep test? The most common level is 50% of the short-term breaking strength, because field tensioning typically sets working tension between 25% and 50% of break strength. Additional levels at 25% and 65% to 70% define the full stress dependence of creep behavior.
How long does a creep test take? Routine quality control tests run 24 hours. Qualification testing commonly extends to 72 or 168 hours, and predictive studies may run 500 to 1,000 hours or longer to support reliable extrapolation of service life.
Which standards apply to PET strapping creep testing? The primary methods are ASTM D2990 and ISO 899-1 for creep and creep rupture of plastics, supported by ASTM D638 and ISO 527 for base tensile properties, ASTM D3953 for nonmetallic strapping specifications, and ISO 291 or ASTM D618 for conditioning.
How do temperature and humidity affect results? As a practical rule, creep deformation increases markedly with temperature, with a significant acceleration for every 10 °C to 15 °C rise. Humidity has a smaller but measurable plasticizing effect on PET, so strict conditioning control at 23 °C and 50% relative humidity is required for comparable data.
How can creep resistance of PET strapping be improved? Effective measures include using higher intrinsic-viscosity resin, increasing molecular orientation during production, optimizing heat-setting to raise crystallinity, refining embossing geometry, and ensuring high-quality friction-welded joints, since joint relaxation is often the limiting factor in overall tension retention.
