PET strapping band breaking strength testing measures the maximum tensile force a polyester (PET) strap can withstand before fracture, making it the single most important quality indicator for load-securing performance. This technical guide covers the applicable standards, sample preparation, test equipment, step-by-step procedures, and typical result ranges for PET strapping used in packaging, logistics, and industrial load securing. Whether you operate a strap extrusion line or a packaging quality-control laboratory, the methods described below will help you obtain accurate, repeatable, and comparable breaking force data.
Technical Overview
PET strapping band, commonly known as plastic steel strapping or polyester strapping, is produced from polyethylene terephthalate resin through extrusion, molecular orientation, and surface embossing or texturing. It has become a widely adopted alternative to steel strapping because it combines high tensile performance with low weight, corrosion resistance, rust-free handling, and improved operator safety.
Breaking strength, also referred to as breaking force or breaking tensile force, is the maximum force a strap specimen sustains during a tensile test carried out to complete rupture. Because the strap and its joints are typically the weakest links in a strapping system, the measured breaking force directly determines the maximum permissible working tension and, ultimately, the security of a strapped load during storage, handling, and transport.
Breaking strength testing is therefore performed at several stages of the product life cycle, including in-process quality control during extrusion, final release testing before shipment, incoming inspection by end users, and evaluation of joints produced by friction-weld tools or mechanical seals.
Testing Samples
Plain and Embossed PET Strapping
Standard specimens are cut directly from production coils at common widths of 13 mm, 16 mm, 19 mm, and 25.4 mm, with thicknesses typically ranging from 0.5 mm to 1.2 mm. Specimens are tested at full width without any machining, since cutting or narrowing the strap would change its true load-bearing capacity. Care must be taken to avoid nicks, folds, or edge damage when sampling, as surface defects can cause premature failure and artificially low results.
Corded and High-Tensile PET Strapping
Heavily oriented or corded strapping, designed for the most demanding steel and brick applications, is tested using the same full-width specimen approach. Because these grades exhibit higher stiffness and lower elongation, particular attention is paid to gripping technique to prevent slip or grip-line fracture during loading.
Jointed Specimens
To evaluate the complete strapping system, specimens are prepared with joints created by battery-powered friction-weld tools, pneumatic welders, wire buckles, or metal seals. The joint breaking force is compared against the breaking force of the base strap to calculate joint efficiency, which expresses how much of the strap strength is retained at the connection point.
Testing Items
A complete breaking strength evaluation of PET strapping typically includes the following measured and calculated parameters:
- Breaking tensile force, expressed in newtons (N) or kilonewtons (kN), recorded as the peak force during the test
- Breaking strength per unit width, expressed in N/mm, calculated by dividing the breaking force by the nominal strap width
- Tensile breaking stress, expressed in MPa, calculated by dividing the breaking force by the original cross-sectional area
- Elongation at break, expressed as a percentage of the initial gauge length at the moment of rupture
- Joint breaking force and joint efficiency, expressed as the percentage ratio of joint force to base strap breaking force
- Statistical evaluation, including mean value, standard deviation, and coefficient of variation across all valid specimens
Testing Methods
Testing is generally performed with a constant-rate-of-extension universal testing machine in accordance with recognized strapping and plastics standards. Commonly referenced specifications include ASTM D3953 for nonmetallic strapping, GB/T 22340 for packaging strapping band, and tensile testing principles adapted from ISO 527 and ASTM D638. Where a customer specification exists, its sample dimensions, gauge length, and test speed always take precedence.
Equipment Requirements
- A floor-standing or benchtop universal testing machine with a capacity of 10 kN, 20 kN, or 50 kN depending on strap grade, with force measurement accuracy of grade 0.5 or grade 1
- Wind-on (capstan) grips or wedge-action grips with serrated insert faces designed specifically for flexible strapping materials
- An extensometer or crosshead displacement measurement system for determining elongation at break
- Test software capable of recording the force-elongation curve, peak force, and automatic statistical reporting
Specimen Preparation and Conditioning
- Cut a minimum of five specimens from different positions along the coil, spaced at least 1 m apart to represent the full batch
- Condition specimens at a temperature of 23 ± 2 °C and relative humidity of 50 ± 10 percent for at least 24 hours before testing, unless otherwise specified
- Use a free length between grips of approximately 500 mm, or the gauge length specified by the applicable standard
- Verify that specimen ends are clean, undamaged, and axially aligned in the grips before starting the test
Test Procedure
- Mount the specimen symmetrically in the grips and apply light pre-tension, typically around 10 N, to remove slack
- Set the crosshead speed to the standard value, commonly within the range of 100 to 300 mm/min, and keep it constant across all comparative tests
- Run the test to complete rupture while continuously recording force and elongation
- Record the breaking force at peak load and the elongation at the moment of break
- Discard any specimen that slips in the grips or fractures within the grip jaws, and repeat the test with a new specimen
- Calculate the mean breaking force, breaking strength, and coefficient of variation from all valid results
Factors Affecting Test Results
- Ambient temperature and humidity, since PET properties vary noticeably with environmental conditions
- Grip type and clamping pressure, which influence slip and premature grip-line failure
- Test speed, since higher speeds generally produce higher apparent breaking forces
- Embossing depth and orientation degree of the strap, which directly affect strength and elongation
- Specimen edge quality and surface damage introduced during sampling or handling
Applications
Breaking strength testing of PET strapping supports quality assurance and safety across a wide range of industries:
- Steel service centers and metal processors securing coils, sheets, and bundles
- Brick, block, tile, and timber producers palletizing heavy construction materials
- Paper mills and printing houses strapping reels of paper and board
- Textile and fiber producers compressing and securing cotton, polyester, and recycling bales
- Glass, ceramics, and machinery manufacturers protecting high-value finished goods
- Export packaging and cargo securing audits, where documented strap strength is required for load plans
- Research and development of new strap formulations, joint designs, and tooling parameters
FAQ
What is the typical breaking force of PET strapping?
Breaking force depends mainly on width and thickness. As an indication, 16 mm by 0.8 mm strap typically reaches approximately 4,000 to 5,500 N, 19 mm by 1.0 mm strap approximately 6,000 to 8,000 N, and 25.4 mm strap 9,000 N or more. Always confirm the declared values of the specific manufacturer and verify them by testing.
Which grips prevent specimen slip or premature fracture at the jaws?
Wind-on or capstan grips are preferred because they distribute the clamping load over a wrapped length of strap, avoiding stress concentration at the jaw line. If wedge grips are used, serrated faces with controlled pneumatic pressure and protective liners usually provide acceptable results.
What is the difference between breaking force and working tension?
Breaking force is the maximum one-time load the strap can resist in a laboratory test, while working tension is the tension applied in service. To account for shock loads, joint efficiency, tension relaxation, and long-term degradation, the working tension is usually limited to roughly 25 to 35 percent of the measured breaking force.
How is joint efficiency calculated, and what is acceptable?
Joint efficiency equals the joint breaking force divided by the base strap breaking force, multiplied by 100. Well-executed friction welds commonly achieve 70 percent or higher, while buckle joints vary with seal and strap combination. Minimum requirements should be agreed between the strap supplier and the end user based on the load securing calculation.
How many specimens should be tested for a valid result?
At least five valid specimens per batch is standard practice, and ten specimens are recommended for certification or dispute testing. Individual values that differ abnormally from the mean should be investigated for grip slip, jaw fracture, or sampling defects before being excluded.
How often should breaking strength testing be performed?
Production lines normally test each extrusion batch during in-process control and again at final release. End users should perform incoming inspection on each new supplier lot and re-test straps that have been stored for extended periods or exposed to heat, moisture, or UV radiation.
