Breaking Strength Test Procedures: A Complete Step-by-Step Laboratory Guide

Breaking strength testing measures the maximum tensile force a material can withstand before rupture, making it one of the most widely performed mechanical tests in textile, webbing, and material quality control. This guide explains the complete breaking strength test procedures in accordance with international standards such as ISO 13934-1, ASTM D5034, and GB/T 3923, covering sample preparation, equipment setup, step-by-step operation, and result evaluation. Whether you work in an accredited third-party laboratory or an in-house quality department, this article will help you understand how reliable breaking force data is generated.

Technical Overview

Breaking strength, also referred to as breaking force or tensile strength, is the maximum force a specimen sustains during a tensile test carried out to rupture. The test is performed on a constant-rate-of-extension (CRE) tensile testing machine, in which the specimen is gripped between two clamps, extended at a defined speed until it breaks, and the maximum force recorded by the load cell is reported as the breaking strength. The result is typically expressed in newtons (N) or kilonewtons (kN).

The most commonly applied standards include ISO 13934-1 and GB/T 3923.1 for the strip method, ISO 13934-2 and GB/T 3923.2 for the grab method, ASTM D5034 for the grab test of textile fabrics, and ASTM D5035 for the strip test. For narrow fabrics such as webbing, ASTM D6775 is often used, while ISO 2062 and ASTM D2256 cover yarn breaking force. Selecting the correct standard depends on the product type, the end-use requirement, and the specification referenced by the buyer or regulatory body.

Two principal test geometries are used. In the strip method, the full width of the specimen is clamped so that the entire cross-section carries the load. In the grab method, only the central portion of a wider specimen is gripped, which better reflects the performance of fabrics where the yarns away from the jaws contribute to strength, such as in seams or coated materials.

Testing Samples

Woven and Knitted Fabrics

For the strip method, specimens are cut with a working width of 50 mm (or 25 mm for narrow fabrics), and the yarns at both edges are removed until the width is accurate to within 0.5 mm. A minimum of five specimens in the warp direction and five in the weft direction are prepared, each with sufficient length to satisfy the gauge length plus the clamp allowance. Specimens should be free from creases, folds, selvedge areas, and visible defects, and no two specimens should contain the same set of lengthwise yarns.

Webbing, Strapping, and Narrow Fabrics

Narrow fabrics are tested at their full width without raveling. Specimens are cut to a length appropriate for a gauge length of typically 250 mm (or 100 mm for elastomeric webbing). The ends may require special treatment, such as protective tabs or resin dipping, to prevent jaw breaks caused by clamp damage on high-strength materials such as polyester or nylon lifting webbing.

Yarns and Sewing Threads

Yarn specimens are mounted on cardboard tabs or gripped with pneumatic capspline clamps to avoid slippage and jaw damage. The gauge length is usually 500 mm for ISO 2062 or 250 mm for ASTM D2256, and the pre-tension is set according to linear density.

Nonwovens, Coated Fabrics, and Geotextiles

These materials follow the same general cutting principles but often require wider specimens (for example 200 mm for geotextile wide-width tensile testing under ISO 10319) and higher-capacity load cells because of their greater breaking forces.

Testing Items

A complete breaking strength evaluation typically reports the following parameters:

• Maximum force (breaking strength), expressed in N or kN
• Elongation at maximum force, expressed as a percentage of the initial gauge length
• Force at rupture, when distinguishable from maximum force
• Breaking tenacity (for yarns), calculated as force divided by linear density
• Force-elongation curve characteristics, including initial modulus where required
• Mode of specimen failure (normal break, jaw break, slippage) for result validity assessment

Testing Methods

Step 1: Conditioning

Condition the specimens in a standard atmosphere of 20 ± 2 °C and 65 ± 4 % relative humidity (or 21 ± 1 °C and 65 ± 2 % for ASTM) until moisture equilibrium is reached. In most cases, at least 24 hours of conditioning is required before testing, because moisture content significantly influences the tensile behavior of hydrophilic fibers such as cotton, viscose, and wool.

Step 2: Instrument Preparation and Calibration

Verify that the tensile testing machine is within its calibration period and select a load cell whose capacity matches the expected breaking force, ideally so the result falls between 15 % and 85 % of the range. Confirm the force indicator reads zero with no load and check the extension indicator. Record the machine type (CRE), load cell capacity, and clamp type in the test report.

Step 3: Parameter Setting

Set the gauge length to 200 mm for the strip method or 100 mm for the grab method, unless otherwise specified. Set the speed of extension according to the standard: for ISO 13934, a rate of 20 mm/min with a 200 mm gauge length (or as determined to achieve rupture within the specified time), while ASTM D5034 specifies 300 mm/min for the grab test. For the grab method, mount the front jaws with a 25 mm × 25 mm clamping face and rear jaws of at least 50 mm × 75 mm.

Step 4: Specimen Mounting

Place the specimen in the clamps with its length direction parallel to the direction of force application and centered so that the same lengthwise yarns are gripped by both clamps. Apply appropriate pre-tension to remove slack without elongating the specimen. For grab specimens, ensure the central 25 mm strip is aligned with the jaw faces while the side wings remain unclamped. Tighten the clamps evenly to prevent slippage, using pneumatic pressure where available.

Step 5: Test Execution

Start the test and allow the moving clamp to extend at the set rate. Observe the specimen throughout the test: the material should break within the gauge section, and the force-elongation curve should rise smoothly to a distinct maximum. The machine records the maximum force and, where equipped, the elongation at that force.

Step 6: Result Validity and Rejection Criteria

Discard and replace any result in which the specimen slips in the jaws, breaks within 5 mm of the jaw line or at the jaw edge, or fails in an anomalous manner, provided the result lies clearly below the median of valid results. Document the reason for each rejection. If more than the permitted number of specimens (typically two out of five) fail in the jaws, the clamping method or jaw lining must be modified and the test repeated.

Step 7: Calculation and Reporting

Calculate the arithmetic mean of the valid breaking forces for each direction (warp, weft, or lengthwise and transverse) and express results to three significant figures. The test report should include the standard used, specimen dimensions, gauge length, test speed, conditioning details, individual and mean breaking force values, elongation at maximum force, number of rejected specimens, and any deviations from the standard procedure.

Applications

Breaking strength testing is applied across a broad range of industries:

• Apparel and home textiles: verifying fabric strength against product specifications and buyer requirements before bulk production
• Industrial textiles: quality control of lifting slings, cargo lashing webbing, safety harnesses, and straps
• Automotive textiles: evaluating seatbelt webbing, airbag fabrics, and interior trim materials
• Medical textiles: tensile assessment of bandages, surgical gowns, and wound dressings
• Geotextiles and construction: determining tensile strength for reinforcement applications in civil engineering
• Packaging: strength verification of woven bags, Flexible Intermediate Bulk Containers (FIBC), and strapping tapes
• Yarn and thread manufacturing: process control, lot acceptance, and comparison of fiber blends

FAQ

What is the difference between the strip method and the grab method?

The strip method clamps the entire cut width of the specimen, so all yarns across the width carry the load, giving a measure of the intrinsic strength of the fabric. The grab method grips only the central portion of a wider specimen, allowing adjacent yarns to contribute through yarn interaction, which better simulates localized loading such as a seam or a pin being pulled through the fabric.

Why do specimens sometimes break at the jaws, and how is this handled?

Jaw breaks occur when clamping pressure damages the yarns directly at the grip line, producing an artificially low result. They are addressed by adding protective jaw liners, reducing clamping pressure, using pneumatic clamps with compliant faces, or applying tab reinforcements. Results from jaw breaks are rejected and the test is repeated on a new specimen.

How many specimens should be tested?

Most standards require a minimum of five valid specimens per direction (warp and weft, or machine and cross direction). When a confidence interval on the mean is required or results show high variability, ten specimens per direction provide better statistical reliability.

Does moisture affect breaking strength results?

Yes. Natural and regenerated cellulosic fibers gain strength when wet or lose strength when dry relative to conditioned values, and all textile materials must reach moisture equilibrium in the standard atmosphere before testing. Testing without proper conditioning is one of the most common causes of disputed results between laboratories.

Can breaking strength results from different standards be compared directly?

Not directly. Gauge length, test speed, specimen width, and gripping geometry all influence the measured force. Comparisons should only be made between results obtained under the same standard and identical test parameters; otherwise, correlation studies must be performed by accredited third-party laboratories.