PET Strapping Break Force Testing: Methods, Standards, and Equipment

PET strapping break force testing is the core quality control procedure used to verify the tensile performance of polyester strapping bands before they are used to secure heavy industrial loads. This technical guide explains the test principle, specimen preparation, equipment requirements, applicable standards, and typical break force values for PET strapping of different sizes.

Technical Overview

Polyethylene terephthalate (PET) strapping, widely known as plastic steel strapping or polyester strapping, is a high-strength packaging band produced by uniaxially orienting PET polymer during the extrusion process. Because the molecular chains are highly aligned along the strap axis, the finished product achieves a tensile strength that approaches that of low-carbon steel strapping, while offering superior elasticity, shock absorption, and complete resistance to rust and corrosion.

Break force testing, also referred to as tensile break force testing, measures the maximum tensile load that a strap specimen can withstand before it fractures. This result is the single most important performance indicator for PET strapping because it directly determines the maximum bundling, palletizing, and securing load the strap can handle. The test is performed on a universal testing machine equipped with specialized grips that prevent specimen slippage and premature failure at the jaws.

Typical break force values for commercial PET strapping depend on the cross-section and material grade, and generally fall within the following ranges:

• 13 mm x 0.6 mm: approximately 2,000 to 2,800 N
• 16 mm x 0.8 mm: approximately 4,000 to 5,000 N
• 19 mm x 1.0 mm: approximately 6,500 to 8,000 N
• 19 mm x 1.27 mm: approximately 8,500 to 10,000 N
• 25 mm x 1.0 mm: approximately 8,000 to 9,500 N
• 32 mm x 1.0 mm: approximately 10,000 to 12,000 N

Testing Samples

Strap Grades and Dimensions

Test samples should be representative of the production batch. PET strapping is commonly supplied in widths from 13 mm to 32 mm and thicknesses from 0.5 mm to 1.3 mm, in embossed, smooth, or printed surface finishes. Both virgin-grade and recycled-content straps can be evaluated, although recycled or blended materials typically show lower and more variable break force values.

Specimen Preparation

Samples shall be taken at random from the batch, cutting at least 1 m from the outer end of the coil to avoid damaged or weathered material. A minimum of five valid specimens, each approximately 600 mm to 1,000 mm in length, is recommended for statistical reliability.

Environmental Conditioning

Since PET properties are sensitive to temperature and moisture, specimens must be conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours prior to testing, in accordance with ISO 291 or ASTM D618.

Testing Items

The principal parameters evaluated in a PET strapping break force test include:

• Break force: the maximum tensile load reached at fracture, expressed in N or kN; this is the primary acceptance criterion.
• Tensile strength: the break force divided by the original cross-sectional area (width x thickness), expressed in MPa; typical values range from 300 MPa for standard grades up to about 500 MPa for premium oriented grades.
• Elongation at break: the percentage increase in gauge length at fracture, typically between 8% and 25%; higher elongation indicates better shock absorption and tension retention.
• Joint efficiency (optional): the break force of sealed or friction-welded joints expressed as a percentage of the strap break force, used to evaluate overall system strength.

Testing Methods

Equipment Requirements

A universal testing machine with a capacity selected so that the expected break force falls between 20% and 80% of full scale is required. For straps up to 19 mm wide, a 10 kN machine is generally sufficient; for 25 mm to 32 mm straps, a 20 kN to 50 kN machine is recommended. The force measuring system shall be verified and calibrated in accordance with ISO 7500-1 or ASTM E4, with accuracy class 0.5 or better preferred. An extensometer or crosshead displacement measurement is used to record elongation.

Grip Selection

PET strapping is smooth, strong, and prone to slippage or jaw-breaks when held in ordinary flat grips. Roller (capstan) grips or wrap-around bollard grips, which allow the specimen to bend 90 to 180 degrees around polished rollers, are strongly preferred. Alternatively, serrated wedge grips with protective liners such as emery cloth or thin leather may be used to distribute clamping stress and encourage failure within the gauge section.

Test Procedure

• Condition the specimens, then measure the width and thickness at three positions along each specimen using a micrometer, and calculate the average cross-sectional area.
• Set the gauge length (grip separation) to 200 mm to 300 mm, or as specified by the applicable standard.
• Mount the specimen centered and straight in the grips, then apply a small preload of approximately 10 N to 50 N to remove slack.
• Start the test at a constant crosshead speed, typically 100 mm/min unless the governing standard specifies otherwise.
• Continue loading until the specimen fractures, recording the force-elongation curve, the maximum force, and the elongation at break.
• Repeat for at least five valid specimens and report the individual values, the arithmetic mean, and the minimum value.

Validity Criteria

A test is considered valid only when fracture occurs within the free gauge region between the grips. Specimens that slip in the jaws, fail partially at the grip line, or break at the roller contact point shall be discarded and replaced with new specimens. The reported break force is normally the average of all valid results.

Referenced Standards

• ISO 527-1 and ASTM D638: general tensile testing of plastics, adapted for strapping geometry.
• ASTM D4675: standard guide for the selection and use of flat strapping materials.
• QB/T 4010: widely referenced industry specification covering classification and technical requirements for PET strapping bands.
• ISO 7500-1 and ASTM E4: verification of the force measuring system of the testing machine.

Applications

Break force testing of PET strapping supports quality assurance and safe packaging across many industries, including:

• Manufacturing quality control: routine batch release testing by strap producers to confirm that break force, tensile strength, and elongation meet declared specifications.
• Incoming inspection: verification by end users in the steel, brick, timber, cotton, fiber, paper, glass, and building-material industries before straps are committed to production lines.
• Research and development: evaluation of resin formulations, recycled-content blends, embossing patterns, and orientation process parameters.
• System validation: comparison of strap body strength versus seal or weld joint strength to optimize overall load security.
• Third-party certification: independent laboratory verification for supplier qualification, tender compliance, and export documentation.

FAQ

What is the typical break force of PET strapping?

It depends on the width and thickness. Common commercial straps range from about 2,000 N for a 13 mm x 0.6 mm band to more than 10,000 N for a 32 mm x 1.0 mm band. Premium fully oriented straps achieve tensile strengths of 400 MPa to 500 MPa.

Can PET strapping really replace steel strapping?

For most bundling and palletizing applications, yes. High-grade PET strap approaches the break force of lighter-gauge steel strap, and its higher elongation and elastic recovery allow it to retain tension when loads settle, shrink, or receive shocks during transport. It is also non-rusting, non-staining, and far safer to cut open than steel.

What testing machine capacity do I need?

Select a capacity so the expected break force lies between 20% and 80% of full scale. A 10 kN universal testing machine suits straps up to 19 mm wide, while 20 kN to 50 kN machines are recommended for wider or heavier-duty straps.

Why do specimens keep breaking at the jaws, and how can this be avoided?

Jaw breaks are caused by stress concentration from the clamping teeth and indicate an invalid result. Using roller or wrap-around grips, adding a protective liner between the specimen and serrated jaws, reducing clamping pressure, and carefully aligning the specimen axially all help move the failure point into the gauge region.

How many specimens should be tested per batch?

A minimum of five valid specimens is standard practice. Reporting the average together with individual values and the minimum result provides a reliable picture of batch consistency and guards against weak outliers.