Tensile Strength Testing of Packing Strapping: Principles, Standards, and Procedures

Tensile strength testing of packing strapping is a fundamental mechanical evaluation that verifies whether PET, PP, composite, and steel straps can safely secure bundles, pallets, and heavy cargo during handling and transport. This article explains the testing principle, applicable international and national standards such as ISO 527 and ASTM D3953, specimen preparation, test procedures, data calculation, and typical applications in packaging quality control.

Technical Overview

Packing strapping (strapping band) is a flexible packaging material used to unitize, bundle, compress, and reinforce goods for storage and transport. The most common types include polypropylene (PP) strapping, polyester (PET) strapping, steel strapping, and composite or woven cord strapping. Because strapping is the primary load-bearing element in many packaging systems, its tensile performance directly determines cargo safety during lifting, stacking, and long-distance transportation.

Tensile strength determination measures the maximum tensile stress that a strapping specimen can withstand before it breaks, calculated from the maximum breaking force divided by the original cross-sectional area of the strap. The test is performed on a constant-rate-of-extension universal testing machine, following recognized methods such as the ISO 527 series and GB/T 1040 series for plastics, ASTM D638 for tensile properties of plastics, and ASTM D3953 for nonmetallic strapping products. Steel strapping is generally tested in accordance with metallic materials tensile standards such as ISO 6892-1 or ASTM E8. Accredited third-party laboratories routinely conduct these tests to support product certification, quality control, and export documentation.

• Verifies that the declared breaking force and tensile strength of the strapping are accurate and reproducible.
• Supports safe load design by matching strap strength to package weight and transport conditions.
• Enables comparison between different materials, manufacturers, and production batches.
• Provides objective evidence for incoming inspection, supplier qualification, and dispute resolution.

Testing Samples

Polyester (PET) Strapping

PET strapping is widely used as a high-strength, weather-resistant alternative to steel strap for securing steel coils, bricks, cotton bales, and heavy pallets. Specimens are cut from production coils at typical widths of 13 mm to 19 mm and thicknesses of 0.6 mm to 1.0 mm. Because PET is a oriented polymer material, specimens must be taken from consistent positions along the coil and free of visible defects, kinks, or crushed edges.

Polypropylene (PP) Strapping

PP strapping is a cost-effective option for light to medium-duty bundling, carton reinforcement, and palletizing. Specimens are cut from coils at common widths of 9 mm to 16 mm. PP is more sensitive to temperature and loading rate than PET, so conditioning and test speed must be strictly controlled to obtain comparable results.

Steel Strapping

Steel strapping is used for extremely heavy, rigid, or compression-sensitive loads. Tensile specimens are prepared with sufficient total length to accommodate the gauge length and grip sections, and testing follows metallic tensile standards such as ISO 6892-1 or ASTM E8 using appropriate flat or threaded wedge grips.

Composite and Woven Cord Strapping

Composite strapping (polyester fiber coated with polymer) and woven cord strapping require special attention to specimen gripping and termination, since failure at grips or buckles is common. Testing usually evaluates both the strap body and the complete joint system including buckles and seals.

Testing Items

A comprehensive tensile evaluation of packing strapping typically includes the following parameters:

• Breaking force (maximum tensile force at failure), expressed in kN or N.
• Tensile strength (breaking force divided by original cross-sectional area), expressed in MPa.
• Elongation at break, expressed as a percentage of the original gauge length.
• Stress at yield and elongation at yield, where applicable for polymer strapping.
• Tensile modulus (Young's modulus) for material characterization purposes.
• Joint or seal breaking strength and joint efficiency for closed strapping systems.
• Retained tensile strength after heat aging, UV exposure, or low-temperature conditioning when required by the specification.

Testing Methods

Specimen Preparation and Conditioning

Specimens are cut from the strapping coil with a total length sufficient for the gauge length plus clamping sections. At least five valid specimens are tested for each sample batch. Unless otherwise specified, polymer strapping is conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity for a minimum of 24 hours prior to testing, in accordance with conditioning standards such as ISO 291 or ASTM D618.

Equipment and Fixtures

The test is conducted on a calibrated universal testing machine with a force capacity appropriate to the expected breaking force and a load accuracy of at least class 1 or class 0.5. Grip selection is critical for strapping: serrated wedge grips, pneumatic grips, or wrap-around (capstan-type) fixtures are used to prevent slippage and stress concentration at the clamps. An extensometer or the machine crosshead displacement is used to determine elongation, depending on the accuracy requirements of the governing standard.

Test Procedure

The specimen is mounted centrally in the grips with a gauge length commonly between 100 mm and 500 mm, as specified by the applicable product or method standard. A small preload (for example 5 % of the expected maximum force) is applied to remove slack. The specimen is then extended at a constant crosshead speed, typically within the range of 50 mm/min to 300 mm/min as required by the standard, until rupture. The maximum force, force-elongation curve, and elongation at break are recorded. Specimens that fail within the grip section are invalid and are retested with replacement specimens.

Data Processing and Calculation

Tensile strength is calculated by dividing the maximum force recorded during the test by the original cross-sectional area of the strap (width multiplied by thickness), and is reported in MPa, where 1 MPa equals 1 N/mm². Elongation at break is calculated as the increase in gauge length at break divided by the original gauge length, expressed as a percentage. The arithmetic mean, standard deviation, and coefficient of variation of valid specimens are reported, and the median or mean value is used depending on the requirement of the governing standard.

Applications

Tensile strength determination of packing strapping is applied across the entire packaging supply chain:

• Manufacturer quality control: verification of every production batch against declared strength values and internal specifications.
• Incoming inspection: confirmation that purchased strapping meets contractual strength requirements before use on packaging lines.
• Cargo securing design: selection of strap type, width, and number of straps based on measured breaking force and required safety factors for rail, road, and sea transport.
• Heavy industry packaging: securing of steel coils, aluminum ingots, bricks, timber, cotton bales, paper reels, and machinery.
• Export packaging certification: provision of test reports from accredited third-party laboratories for customs, customers, and certification bodies.
• Research and development: evaluation of new resin formulations, orientation processes, joint designs, and seal technologies.

FAQ

What is the difference between breaking force and tensile strength of strapping?

Breaking force is the absolute maximum load (in kN or N) that a specific strap can carry before failure, and it depends on the strap dimensions. Tensile strength is the normalized value (in MPa) obtained by dividing the breaking force by the cross-sectional area, which allows fair comparison between straps of different widths and thicknesses.

What are typical tensile strength values for common strapping materials?

Indicative ranges are approximately 200 MPa to 350 MPa for PP strapping, 400 MPa to 600 MPa for PET strapping, and 700 MPa to 1,000 MPa or higher for high-tensile steel strapping. Actual values must always be confirmed by testing, since production processes and product grades vary significantly.

Which standards govern strapping tensile strength testing?

Polymer strapping is typically tested per the ISO 527 series, GB/T 1040 series, or ASTM D638, with product requirements covered by specifications such as ASTM D3953 for nonmetallic strapping. Steel strapping is tested per ISO 6892-1 or ASTM E8. The specific standard should be agreed between the purchaser and the testing laboratory before testing begins.

Why do specimens sometimes fail at the grips, and how is this prevented?

Strapping has high strength and a smooth surface, so stress concentration and slippage at the clamps can cause premature failure. This is prevented by using serrated, pneumatic, or wrap-around (capstan) fixtures, correct gripping pressure, adequate grip engagement length, and possibly reinforced grip tabs. Failures occurring within the grip section are treated as invalid and retested.

How many specimens are tested, and how are results reported?

A minimum of five valid specimens is the common requirement. The laboratory reports the individual values, mean or median tensile strength, breaking force, elongation at break, standard deviation, test speed, gauge length, conditioning conditions, and the testing standard applied.

Do temperature and humidity affect the test results?

Yes. Polymer strapping, especially PP, is sensitive to ambient temperature, humidity, and loading rate, which is why specimens must be conditioned at 23 °C and 50 % relative humidity before testing and evaluated at a controlled crosshead speed. Deviating from these conditions can produce results that are not comparable between laboratories.