A breaking force test plan (tensile breaking force test scheme) is a structured laboratory procedure used to determine the maximum tensile load a material, wire, cable, textile, or mechanical joint can withstand before fracture. This guide explains the testing principles, applicable standards, sample preparation, equipment requirements, and step-by-step methodology required to produce accurate and repeatable breaking force results. It is intended for quality engineers, laboratory technicians, and procurement specialists evaluating product reliability and compliance.
Technical Overview
The breaking force test, also known as the ultimate tensile force test or pull-to-failure test, measures the maximum axial tensile load that a specimen can sustain before it ruptures. The result is typically expressed in newtons (N) or kilonewtons (kN). When combined with the specimen's cross-sectional area, the breaking force can be converted into tensile strength (MPa), and when combined with the original gauge length, it can be used to calculate elongation at break (%).
A well-designed breaking force test plan defines the test objective, applicable standards, specimen geometry and conditioning, equipment calibration requirements, test speed, gripping method, acceptance criteria, data recording rules, and reporting format. The purpose is to ensure that results are accurate, repeatable, and comparable between laboratories, whether the test is performed internally or at an accredited third-party laboratory.
Typical standards organizations publishing breaking force test methods include ISO, IEC, ASTM, EN, GB, JIS, and UL. The selection of the correct standard depends on the product category, as shown in the sections below.
Testing Samples
Metal Wires and Conductors
Bare copper, aluminum, copper-clad aluminum, and alloy conductors used in wire and cable manufacturing are tested to verify conductor tensile strength and elongation. Relevant methods include IEC 60811-501, GB/T 4909.3, and ASTM B1/B3 related conductor specifications.
Enameled and Insulated Winding Wires
Fine magnet wires used in motors, transformers, and coils are tested for conductor breaking force and elongation. Typical references include IEC 60851 and GB/T 6109 series standards.
Wire Harnesses, Crimped Terminals, and Connectors
Crimped and soldered connections in automotive and appliance harnesses are subjected to pull force tests to verify mechanical integrity of the termination. Common references include IEC 60352-2, UL 2556, and OEM harness specifications.
Yarns, Threads, and Textile Fabrics
Spun and filament yarns, sewing threads, webbings, and woven fabrics are tested for breaking force and elongation using methods such as ASTM D2256, ISO 2062, ASTM D5034, and ISO 13934-1.
Ropes, Cords, Straps, and Fishing Lines
Fiber ropes, safety lanyards, woven straps, and monofilament lines are tested for minimum breaking load in accordance with ISO 2307 and related product standards.
Plastic Films, Bags, and Packaging Materials
Plastic films, tear tapes, sealing bands, and packaging straps are evaluated for breaking force and breaking strength using ASTM D882 and related film tensile methods.
Plastic and Metallic Material Specimens
Dumbbell-shaped specimens from injection-molded or machined materials are tested for ultimate tensile strength and break elongation per ISO 527, ASTM D638, ISO 6892-1, and ASTM E8/E8M.
Testing Items
A complete breaking force test plan typically covers the following measurable parameters:
- Breaking force (maximum tensile load at rupture), expressed in N or kN
- 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
- Force at yield, where applicable for ductile materials
- Failure mode and fracture location description
- Pull-out force for crimped terminals, splices, and terminations
- Minimum breaking load verification against specification limits
Testing Methods
Equipment Requirements
The core instrument is a universal testing machine (UTM) with a suitable load cell capacity, typically selected so that the expected breaking force falls between 20% and 80% of the load cell range for optimal accuracy. Equipment verification should follow ISO 7500-1 or ASTM E4. Depending on the specimen, specialized grips are required:
- Wedge or mechanical grips for rigid metal and plastic specimens
- Pneumatic grips with rubber-faced jaws for films, yarns, and textiles
- Capstan or wrap-type grips for high-strength cords, ropes, and fine wires
- Custom fixtures for terminal pull tests per harness specifications
Specimen Preparation and Conditioning
Specimens must be free of visible damage, kinks, or corrosion and should be cut to the length specified by the applicable standard, including sufficient extra length for gripping. Conditioning is usually performed at 23 ± 2 °C and 50 ± 10% relative humidity for a defined period (commonly 24 hours) unless the product standard states otherwise. The number of specimens is defined by the standard; a minimum of five valid results is common practice.
Test Procedure
The general procedure for a breaking force test is as follows:
- Verify machine calibration status and select an appropriate load cell and grips
- Measure and record specimen dimensions: diameter, width, thickness, and gauge length
- Calculate the cross-sectional area and, where required, convert force to stress
- Set the test speed specified by the standard, typically a constant separation rate of the crosshead
- Mount the specimen axially in the grips, ensuring alignment to avoid bending or slippage
- Apply preload if specified, then run the test to complete rupture
- Record the maximum force, elongation at break, and failure mode
- Repeat for all specimens and calculate the arithmetic mean, standard deviation, and coefficient of variation
- Discard and replace results affected by jaw slip or jaw breakage, where the standard permits
Data Evaluation and Acceptance Criteria
Results are compared against the minimum breaking force specified in the product standard or customer drawing. A specimen passes if the breaking force is equal to or greater than the specified minimum. For statistical acceptance, the mean value must meet the limit and the scatter should fall within the tolerance band defined by the specification. Any fracture occurring within the grip jaws that produces an anomalously low value should be investigated and retested according to the rules of the standard.
Common Test Parameters by Reference Standards
- ISO 6892-1 / ASTM E8: metallic materials, gauge lengths of 50 mm or 5D, method A or B speed control
- ISO 527-1/-2 / ASTM D638: plastics, dumbbell specimens type 1A/1B, speeds from 1 to 500 mm/min
- ISO 2062 / ASTM D2256: yarns, gauge length 250 mm or 500 mm, preload based on linear density
- ISO 13934-1 / ASTM D5034: fabric strip method, gauge length 100 mm or 200 mm
- IEC 60811-501: conductor tensile strength and elongation for electric cables
- IEC 60352-2: minimum pull-out force for crimped connections, graded by conductor size
- ISO 2307: breaking force of ropes with defined specimen length and pre-tension cycles
Applications
Breaking force testing is applied across many industries to verify mechanical reliability, process control, and regulatory compliance:
- Wire and cable manufacturing: incoming conductor inspection and finished cable mechanical qualification
- Automotive and rail: wire harness crimp quality control and periodic process validation
- Textile and apparel: yarn and fabric strength certification for performance garments and industrial fabrics
- Packaging: film, strap, and bag strength verification for load security during transport
- Medical devices: suture and catheter tensile integrity per recognized material standards
- Lifting and safety equipment: rope, sling, and lanyard minimum breaking load verification
- Electronics and appliances: terminal pull tests for soldered and crimped assemblies
- Research and development: material selection, aging studies, and comparison of supplier batches
FAQ
What is the difference between breaking force and tensile strength?
Breaking force is the maximum load in newtons sustained by the specimen before fracture, while tensile strength is the breaking force divided by the original cross-sectional area, expressed in MPa. Tensile strength allows comparison between specimens of different sizes, whereas breaking force is the direct acceptance parameter for wires, terminals, ropes, and similar products.
How many specimens should be tested?
Most standards require at least five valid specimens per sample lot to calculate a statistically meaningful average. Product-specific standards or customer specifications may require additional specimens, particularly when a minimum individual value must be met for every specimen.
What causes invalid or low breaking force results?
Common causes include jaw slippage, fracture occurring inside the grips, specimen misalignment, incorrect gauge length, non-conditioned specimens, worn grip faces, and excessive or insufficient test speed. Each invalid result should be documented and the test repeated with a fresh specimen in accordance with the standard.
Which test speed should be selected?
The test speed is defined by the applicable standard and must not be chosen arbitrarily, because breaking force and elongation results are strain-rate dependent. When a range is permitted, the speed should be documented in the test plan and kept constant for all comparative testing.
Can breaking force tests be performed by external laboratories?
Yes. Many manufacturers submit samples to accredited third-party laboratories for independent verification. The test plan should specify the standard, specimen quantity, conditioning requirements, and reporting format so that results from external laboratories are directly comparable with in-house data.
How often should the testing machine be calibrated?
Load verification per ISO 7500-1 or ASTM E4 is typically performed annually, or more frequently under heavy usage. Grip geometry, extensometer calibration, and crosshead speed verification should also be included in the equipment maintenance schedule defined in the test plan.
