Breaking Force Testing: Principles, Methods, Standards, and Applications

Breaking force testing is a fundamental mechanical evaluation that measures the maximum tensile load a material, component, or joint can withstand before fracture or separation occurs. This article explains the technical principles, sample types, measured parameters, recognized international standards, and industry applications of breaking force testing. Whether you are validating crimped electrical connections, textile yarns, plastic films, or medical tubing, understanding this test method is essential for quality control and regulatory compliance.

Technical Overview

Breaking force testing, also referred to as tensile break force testing or pull-off force testing, is a destructive mechanical test designed to determine the maximum tensile load a specimen can sustain before it fractures, ruptures, or separates. During the test, a specimen is clamped between two grips and stretched at a controlled rate until failure occurs. The peak force recorded at the moment of failure is defined as the breaking force, typically expressed in newtons (N), kilograms-force (kgf), or pounds-force (lbf).

Definition and Working Principle

The test operates on the principle of uniaxial tension. A universal testing machine applies a steadily increasing tensile load along the axis of the specimen while a calibrated load cell continuously records force and displacement data. When the applied load exceeds the cohesive or structural strength of the specimen, failure occurs at the weakest point. Depending on the material and the applicable standard, failure may be defined as complete fracture, rupture, separation of a joint, or pull-out of a conductor from a terminal.

Why Breaking Force Testing Matters

Breaking force data provides direct evidence of whether a product or material meets the minimum mechanical strength requirements specified in design documents, purchase specifications, or regulatory standards. It is widely used for incoming material inspection, manufacturing process validation, batch release testing, failure analysis, and product certification by accredited third-party laboratories. In safety-critical applications such as electrical wiring, lifting components, and medical devices, adequate breaking force is not merely a quality indicator but a mandatory compliance requirement.

Testing Samples

Breaking force testing is applicable to a broad range of materials and assembled components. Typical sample categories include the following:

Metallic Materials

Metal wire, steel wire rope, cable conductors, fasteners, rivets, welded joints, and cold-headed components are tested to verify tensile integrity. Metallic specimens often follow standardized dumbbell or straight lengths, and the breaking force result may be converted into tensile strength by dividing the maximum force by the original cross-sectional area.

Plastics and Polymer Materials

Injection-molded plastics, extruded profiles, films, sheets, and polymer compounds are tested using dumbbell-shaped specimens. For thin and flexible packaging films, strip or ribbon specimens are pulled to rupture to determine the force and elongation behavior of the material.

Rubbers and Elastomers

Vulcanized rubber and thermoplastic elastomer products, including seals, hoses, conveyor belts, and cable insulation and sheathing compounds, are tested using ring, dumbbell, or straight specimens to evaluate tensile strength and elongation at break.

Textiles, Yarns, and Fibers

Spun and filament yarns, industrial threads, sewing threads, webbing, straps, and woven or knitted fabrics are tested to determine breaking force, breaking tenacity, and elongation. Capstan or drum-type grips are commonly used to prevent slippage and jaw damage of delicate filament specimens.

Electrical Connectors and Crimped Joints

Crimped terminals, splices, solderless connections, compression connectors, and conductor-terminating assemblies are subjected to axial pull-off force testing to confirm that the mechanical retention between conductor and connector meets the minimum force specified by the relevant electrical standard.

Medical Devices

Intravascular catheters, tubing sets, sutures, adhesive bonds, and device subassemblies are tested to verify bond strength and tensile durability, ensuring that components do not separate under clinically foreseeable tensile loads.

Packaging Materials

Flexible packaging films, heat seals, lid stocks, and laminates are tested to measure seal strength and material tensile break force, supporting package integrity validation and shelf-life studies.

Testing Items

Depending on the specimen type and the governing standard, breaking force testing may include the following measured or calculated parameters:

• Maximum breaking force: the peak tensile load sustained by the specimen at failure.
• Tensile strength: the maximum force divided by the original cross-sectional area of the specimen, expressed in MPa or psi.
• Elongation at break: the percentage increase in gauge length at the moment of rupture.
• Breaking tenacity: the breaking force expressed relative to the linear density of yarn or fiber, typically in cN/tex or g/den.
• Yield point and yield strength: the stress at which the material begins to deform plastically, for metallic and polymeric specimens.
• Pull-off or pull-out force: the axial force required to separate a crimped, clamped, or bonded joint.
• Seal strength: the force required to separate a flexible package seal under tensile loading.
• Failure mode classification: fracture within the gauge length, jaw break, conductor pull-out, or slippage, each of which affects the validity of the result.
• Statistical evaluation: mean value, standard deviation, and coefficient of variation across a specified number of valid specimens.

Testing Methods

Breaking force tests are performed using universal testing machines equipped with appropriate load cells, grips, and data acquisition systems. The general methodology and representative standards are summarized below.

Test Equipment

A typical test setup consists of a rigid frame, a movable crosshead, a precision load cell calibrated to ISO 7500-1 or equivalent accuracy class, and interchangeable grips such as wedge grips, pneumatic grips, capstan grips, or custom fixtures. An extensometer conforming to ISO 9513 may be attached when accurate strain measurement is required. Environmental chambers allow testing at elevated or sub-ambient temperatures when specified by the product standard.

Specimen Preparation and Conditioning

Specimens are cut, machined, or molded according to the dimensions prescribed by the applicable standard. Before testing, polymeric and textile specimens are conditioned in a standard atmosphere, commonly 23 degrees Celsius and 50 percent relative humidity in accordance with ISO 291, unless otherwise specified. Specimen width, thickness, or linear density is measured at multiple points to ensure accurate calculation of strength values.

Test Procedure

The specimen is mounted in the grips with a defined gauge length and a small preload is applied to remove slack. The crosshead then extends at a constant rate specified by the standard, and force-displacement data are recorded continuously until the specimen fails. Key procedural considerations include the following:

• Maintaining axial alignment to avoid bending stresses that produce artificially low results.
• Selecting a crosshead speed appropriate to the material, since strain rate significantly influences measured breaking force.
• Using grip liners, capstan wraps, or extended gauge lengths to prevent jaw breaks and slippage.
• Rejecting specimens that fail inside the jaws unless the standard explicitly permits such failure modes.
• Testing the number of replicates required by the standard and reporting statistical results.

Expression of Results

The breaking force is reported as the maximum force recorded during the test. Where required, tensile strength is calculated by dividing the breaking force by the original minimum cross-sectional area, and elongation at break is calculated as the ratio of extension at failure to the initial gauge length, expressed as a percentage. For crimped connections, the measured pull-off force is compared directly against the minimum force table or formula specified by the electrical standard.

Representative Standards

• ISO 6892-1 and ASTM E8/E8M: tensile testing of metallic materials at room temperature.
• ISO 527-1 and ISO 527-2, together with ASTM D638: tensile properties of plastics.
• ASTM D882: tensile properties of thin plastic sheeting and films.
• ISO 37 and ASTM D412: tensile stress-strain properties of vulcanized and thermoplastic rubbers.
• IEC 60811-501: tensile strength and elongation tests on cable insulation and sheathing compounds.
• ISO 2062 and ASTM D2256: determination of breaking force and elongation of yarns.
• ISO 13934-1 and ASTM D5034: tensile properties of textile fabrics by the strip method.
• UL 486A-486B: pull-out force requirements for wire connectors and soldering lugs.
• IEC 60352-1: pull-out force requirements for solderless crimped connections.
• GB/T 4909 series: test methods for bare conductors, including tensile and pull-out evaluation.
• JIS Z 2241: method of tensile test for metallic materials.
• ISO 3108: determination of breaking force for steel wire ropes.
• ASTM F88: seal strength measurement for flexible barrier materials.

Applications

Breaking force testing supports design verification, production control, and certification across many industries:

• Wire and cable manufacturing: verification of conductor tensile performance, insulation and sheath strength, and crimp termination integrity before product release.
• Automotive and transportation: validation of wiring harness terminals, safety belt webbing, plastic clips, and structural fasteners.
• Medical devices: confirmation of catheter bond strength, tubing connector retention, and suture tensile performance for regulatory submissions.
• Packaging: seal strength and film tensile verification for food, pharmaceutical, and sterile barrier packaging systems.
• Textiles and technical fibers: quality grading of yarns, sewing threads, geotextiles, and industrial webbing.
• Electronics and electrical assembly: process control of crimping and pressing operations through routine pull-off force audits.
• Construction and lifting equipment: tensile verification of wire ropes, anchors, ties, and pre-stressing components.

FAQ

What is the difference between breaking force and tensile strength?

Breaking force is the absolute maximum tensile load, expressed in units of force such as newtons, at which the specimen fails. Tensile strength is a normalized engineering stress obtained by dividing the breaking force by the original cross-sectional area of the specimen, expressed in MPa or psi. Breaking force is the primary reported value for yarns, crimped joints, and finished assemblies, while tensile strength is commonly reported for metals, plastics, and rubbers.

What equipment is used for breaking force testing?

A universal testing machine fitted with a calibrated load cell and appropriate grips is the standard equipment. Load cell capacity should be matched to the expected breaking force, ideally so that the failure load falls within the calibrated range with sufficient resolution. Specialized grips such as capstan or pneumatic types are selected according to the specimen geometry and material.

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

Jaw breaks occur when stress concentration or grip damage causes premature failure at the grip line rather than within the gauge length. They typically produce artificially low results and are treated as invalid unless the applicable standard accepts them. Remedies include using cushioned grip faces, adjusting grip pressure, wrapping the specimen ends, increasing gauge length, or switching to capstan-type grips.

How many specimens should be tested?

The required number of specimens depends on the governing standard and the statistical confidence needed. Material standards commonly specify five or ten valid replicates, while production control and crimp pull-off audits may define sample sizes based on lot size and sampling plans. Results are usually reported as individual values together with the mean and coefficient of variation.

Can breaking force testing be performed at high or low temperatures?

Yes. Environmental chambers integrated with the testing machine allow testing at elevated temperatures, sub-zero temperatures, or controlled humidity. This is particularly relevant for cable compounds, automotive components, and polymers whose mechanical behavior changes significantly with temperature. The test conditions must follow the temperature and conditioning requirements stated in the applicable product standard.

How is a crimped connection judged as passing or failing?

For crimped terminals and solderless connections, the measured pull-off force must equal or exceed the minimum force value specified by standards such as UL 486A-486B or IEC 60352-1 for the given conductor size. The failure mode, whether conductor breakage, pull-out, or slippage, is also recorded and evaluated. A connection generally passes only when the measured force meets the minimum requirement and the failure mode is acceptable under the standard.