PET Strapping Break Strength Testing: A Complete Technical Guide to Tensile Force Evaluation

PET strapping break strength testing is the essential quality control procedure for measuring the maximum tensile force that polyester strapping bands can withstand before fracture. This technical guide explains the testing standards, sample preparation, equipment requirements, calculation methods, and typical performance values for PET plastic steel strapping, helping manufacturers and packaging engineers ensure reliable load securing performance.

Technical Overview

PET strapping, commonly known as plastic steel strapping, is a high-strength polyester packaging band widely used as a safe and economical alternative to traditional steel strapping. Because the strapping serves as the primary load-securing element in palletizing, bundling, and containerized shipping, its mechanical reliability directly affects cargo safety. Break strength testing, also referred to as breaking tensile force testing, measures the maximum tensile load a strapping specimen can sustain before it fractures under uniaxial tension.

The test is performed using a universal testing machine equipped with suitable gripping fixtures. Key outputs include breaking force, tensile strength, and elongation at break. These results allow manufacturers to verify compliance with product specifications, enable buyers to compare supplier quality, and help engineers calculate the correct strapping size and number of straps required for a given package weight and transport condition.

Testing Samples

Standard Plain PET Strapping

Smooth-surface PET strapping produced from virgin or virgin-dominant resin, supplied in common widths of 13 mm, 16 mm, and 19 mm with thicknesses from 0.6 mm to 1.0 mm, represents the baseline sample category for break force verification.

Embossed and Textured PET Strapping

Embossed strapping features a patterned surface that improves grip in manual and battery-operated tensioners. Because embossing can slightly reduce the effective solid cross-section, these samples are tested separately to confirm that break strength remains within specification.

Colored and Recycled-Content Strapping

Black, green, blue, or gray strapping, often manufactured with recycled PET content or color masterbatch, requires dedicated testing since pigments and recycled material can reduce molecular weight and lower tensile performance.

Joint Specimens

Specimens prepared with friction-welded, seal-lock, or buckle joints are tested to evaluate joint strength and joint efficiency, which is typically lower than the base strap strength and often determines the true working strength of the strapping system.

Aged and Environmental Exposure Samples

Samples subjected to ultraviolet exposure, elevated temperature, or humidity aging may be tested to assess strength retention over storage and service life.

Testing Items

A comprehensive PET strapping evaluation program covers the following measured and calculated parameters:

• Breaking force (kN): the maximum tensile load reached before specimen fracture.
• Tensile strength (MPa): breaking force divided by the original cross-sectional area (width multiplied by thickness).
• Breaking strength per unit width (kN/cm): a practical comparison index between different strap widths.
• Elongation at break (%): the percentage increase in gauge length at the moment of fracture.
• Elongation at a defined load: the strain measured at a specified applied force, useful for assessing strap stiffness.
• Joint breaking force and joint efficiency (%): the ratio of joint strength to base strap breaking force.
• Complete stress-strain curve characteristics, including initial modulus and yield behavior.

Testing Methods

Reference Standards

Testing is generally conducted with reference to ASTM D3953 for nonmetallic strapping, ISO 527 for plastics tensile properties, GB/T 22340 for plastic packaging strapping, and conditioning atmospheres defined in ASTM D618 or ISO 291. The applicable customer specification or purchase contract always takes precedence.

Specimen Preparation and Conditioning

Specimens are cut from production coils at least one full wrap below the outer layer to avoid surface damage. A typical free length between grips of 200 mm to 400 mm is used, with total specimen length of approximately 500 mm to 600 mm. At least five valid specimens per sample lot are tested. Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 24 hours before testing.

Equipment and Gripping

A universal testing machine with a load capacity appropriate to the expected break force, typically 10 kN for standard straps and up to 50 kN for heavy-duty grades, is used. Wedge action grips, roller grips, or pneumatic grips lined with serrated or rubber-faced inserts prevent slippage, while a protective shield guards the operator against recoil when the strap snaps.

Test Procedure

• Measure and record the width and thickness of each specimen at three positions and calculate the average cross-sectional area.
• Install the specimen in the grips, ensuring axial alignment without twist or pre-tension beyond a small seating load.
• Set the crosshead speed, commonly 100 mm/min, unless otherwise specified by the governing standard.
• Run the test to complete fracture, recording force and displacement continuously.
• Read the maximum force as the breaking force and the corresponding extension as elongation at break.
• Reject and replace any specimen that fails by slippage in the grips or fracture within the grip faces.

Data Calculation

Tensile strength is calculated by dividing the breaking force by the original cross-sectional area. Elongation at break equals the extension at maximum force divided by the original gauge length, multiplied by 100. Joint efficiency equals the joint breaking force divided by the base strap breaking force, multiplied by 100. The arithmetic mean of valid specimens is reported together with the standard deviation.

Typical Results

Representative breaking force values for quality virgin PET strapping are approximately 2,800 N to 3,200 N for 13 mm by 0.7 mm, 4,500 N to 5,500 N for 16 mm by 0.8 mm, and 7,000 N to 8,500 N for 19 mm by 1.0 mm, with elongation at break typically between 8% and 25%. Friction-welded joints generally achieve 70% to 85% joint efficiency.

Applications

Break strength test data support safe strap selection across a wide range of industries:

• Steel and metal service centers for bundling coils, pipes, bars, and sheets.
• Brick, block, tile, and refractory plants for unitizing heavy palletized loads.
• Cotton, fiber, and textile baling and compress-packaging operations.
• Glass, ceramics, and building material packaging requiring controlled tension.
• Paper mills and printing houses for securing paper reels and stacked sheets.
• Timber and panel products for export-grade bundling.
• General logistics, warehousing, and container loading of palletized goods.
• Inbound quality inspection by purchasing departments and third-party inspection agencies.

FAQ

What is the difference between breaking force and tensile strength?

Breaking force is the absolute maximum load in newtons or kilonewtons at fracture, while tensile strength is that load normalized by the cross-sectional area and expressed in megapascals, allowing fair comparison between straps of different dimensions.

Why does PET strapping sometimes break near the joint instead of the middle?

Joint efficiency is inherently below 100% because welding, seals, or buckles create local stress concentrations and partial thickness loss. A properly made friction weld should still deliver 70% to 85% of the base strap strength.

How many specimens should be tested per batch?

At least five valid specimens are recommended for routine quality control, and ten or more for type testing, statistical certification, or dispute resolution.

What machine capacity is required?

A 10 kN universal testing machine covers most standard PET strapping up to 19 mm width, while a 20 kN to 50 kN machine is preferred for heavy-duty or composite grades with higher break forces.

Why is specimen conditioning necessary?

PET is mildly sensitive to temperature and moisture. Conditioning at 23 °C and 50% relative humidity ensures repeatable, comparable results and prevents moisture or temperature extremes from skewing the measured break strength.