Breaking Force Testing Methods: Standards, Equipment, and Procedures

Breaking force testing is a fundamental mechanical evaluation used to determine the maximum tensile load a material or product can withstand before fracture. This technical guide explains the principles, applicable standards, specimen requirements, equipment configurations, and step-by-step procedures of breaking force test methods across wires, textiles, ropes, films, and elastomers. Whether you are a quality engineer, laboratory technician, or procurement specialist, this article will help you understand how breaking force data is generated, interpreted, and applied in industry.

Technical Overview

Breaking force, also known as ultimate tensile force or rupture force, is defined as the maximum force a specimen sustains during a tensile test carried out to failure. It is expressed in units of force, typically newtons (N), kilonewtons (kN), kilogram-force (kgf), or pound-force (lbf). Unlike tensile strength, which is normalized by the cross-sectional area of the specimen and reported in megapascals (MPa), breaking force is a direct load-based measurement and is therefore the preferred acceptance parameter for products whose geometry is non-uniform or difficult to measure accurately, such as stranded conductors, yarns, ropes, sutures, and flexible packaging seals.

Breaking force testing is performed on a universal testing machine (UTM) equipped with suitable grips and a calibrated load cell. The specimen is clamped between two fixtures, a preload is applied to remove slack, and the moving crosshead extends the specimen at a controlled rate until rupture occurs. During the test, the force–extension curve is recorded, from which breaking force, elongation at break, and other derived parameters are obtained. The method falls under two principal control modes: constant rate of extension (CRE), where the crosshead moves at a fixed speed, and constant rate of loading (CRL), where force increases at a fixed rate. Modern laboratories predominantly use CRE machines because they provide better reproducibility and are required by most current international standards.

Numerous standards organizations publish breaking force test methods tailored to specific product families, including ISO, IEC, ASTM, EN, GB, UL, and JIS. Selecting the correct standard, grip type, gauge length, and test speed is essential, because these parameters directly influence the measured result. Breaking force testing is routinely carried out by accredited third-party laboratories, in-house quality departments, and research institutions to verify product conformity, support incoming inspection, and validate manufacturing processes.

Testing Samples

Breaking force testing applies to an extremely wide range of materials and manufactured products. The following categories represent the most frequently tested sample types, each governed by its own set of standards and fixture requirements.

Metallic Wires and Cable Conductors

Bare and insulated round wires, stranded conductors, and individual wires removed from power or communication cables are tested to confirm conductor integrity. Typical specimens include copper, aluminum, copper-clad aluminum (CCA), tinned copper, and alloy wires with diameters ranging from below 0.1 mm to several millimeters. Relevant methods include IEC 60811-501, ASTM B1 series procedures, ISO 6892-1, ASTM E8/E8M, and GB/T 4909.3 for bare wires.

Yarns and Threads

Spun and filament yarns, sewing threads, and industrial yarns are tested using single-strand or skein methods. Because yarns are delicate and prone to grip damage, capstan or pneumatic yarn grips with wrap-around geometry are used. Governing standards include ASTM D2256, ISO 2062, ASTM D885 for industrial filament yarns, and GB/T 3916.

Woven, Knitted, and Coated Fabrics

Textile fabrics are tested in strip or grab geometries to evaluate the breaking force of the fabric in warp and weft directions. Coated fabrics and coated materials follow ISO 1421 or ASTM D751. Common standards include ISO 13934-1, ISO 13934-2, ASTM D5034, ASTM D5035, and GB/T 3923.1.

Ropes, Webbing, and Slings

Natural and synthetic fiber ropes, lifting slings, safety harness webbing, seat belts, and lashing straps require high-capacity load frames, often with bollard or specialized drum grips to prevent premature failure at the clamp. Reference standards include ISO 2307 for ropes, ASTM D6267 for webbing, and EN 1492-1/EN 1492-2 for textile slings.

Plastic Films and Flexible Packaging

Packaging films, laminates, tear strips, and heat seals are tested to quantify seal strength and film tensile behavior. Very thin specimens require low-force load cells and careful specimen cutting. Applicable standards include ASTM D882, ISO 527-3, ASTM F88 for seal strength, and GB/T 1040.3.

Rubber, Elastomers, and Thermoplastics

Dumbbell-shaped specimens of rubber compounds, thermoplastic materials, and elastomeric components are tested for breaking force, tensile strength, and elongation at break. Standards include ASTM D412, ISO 37, ASTM D638, and ISO 527-1.

Paper, Board, and Nonwovens

Paper strips, filter materials, and nonwoven fabrics are evaluated for breaking force and breaking length using standards such as ISO 1924-2, ASTM D828, ASTM D5035, and ISO 9073-3 for nonwovens.

Medical and Specialty Products

Surgical sutures, dental floss, medical tubing, catheters, and ligatures are tested under pharmacopoeial and industry standards to verify minimum breaking force, which is a critical safety parameter for these devices.

Testing Items

A breaking force test program typically reports the following measurable and derived quantities, depending on the product standard and customer specification:

• Breaking force (maximum force at rupture), reported in N, kN, kgf, or lbf
• Breaking strength in some textile contexts, expressed in units such as N, gf, or N/50 mm strip width
• Elongation at break, expressed as a percentage of the original gauge length
• Breaking tenacity for yarns, calculated as breaking force divided by linear density and reported in cN/tex or g/den
• Tensile strength and yield strength for materials with measurable cross-sections, reported in MPa
• Young's modulus and initial modulus, determined from the slope of the force–extension curve
• Work to break (toughness), represented by the area under the force–extension curve
• Seal strength and seal failure mode for flexible packaging specimens
• Knot breaking force for sutures and threads tested with a knot in the gauge section
• Statistical results including mean value, standard deviation, coefficient of variation, and minimum/maximum values across the specimen set

Testing Methods

While the exact procedure varies by material and standard, breaking force testing follows a common methodological framework consisting of conditioning, specimen preparation, equipment setup, test execution, and data processing. The general workflow below reflects the requirements of the most widely applied CRE-based standards.

Specimen Preparation and Conditioning

Specimens are cut or machined to the dimensions specified by the governing standard, with strict control of gauge length and width. Textile and paper specimens are conditioned in a standard atmosphere, typically 20 ± 2 °C and 65 ± 4 % relative humidity for at least 24 hours, while plastics are conditioned at 23 ± 2 °C and 50 ± 10 % relative humidity. Metal wire specimens are cleaned of insulation or lubricant where required. Frayed, twisted, or damaged specimens are rejected because pre-existing defects cause premature rupture and non-representative results.

Equipment and Calibration

A universal testing machine fitted with a load cell appropriate to the expected force range is used; the load cell capacity should be selected so that the breaking force falls between roughly 15 % and 85 % of capacity for optimal accuracy. Force verification is performed in accordance with ISO 7500-1 or ASTM E4, and extension measurement follows ISO 9513 or ASTM E83. Grip selection is critical: wedge and mechanical wedge grips suit wires and rigid specimens, pneumatic side-action grips suit films and fabrics, capstan or drum grips suit yarns and cords, and bollard grips suit high-force ropes and slings. Contact extensometers or non-contact video extensometers are attached when accurate modulus or elongation measurement is required.

Test Execution Procedure

• Verify machine calibration status and select the correct load cell and grips for the material and expected force level.
• Set the gauge length as specified by the standard, for example 250 mm or 500 mm for yarns, 100 mm or 200 mm for fabrics, 500 mm for ropes, and 50 mm to 200 mm for films and wires.
• Set the test speed according to the standard, for example 20 mm/min for many yarn tests, 100 mm/min for fabric strip tests, or 500 mm/min for rope tests, matching the specified strain rate.
• Mount the specimen centrally and symmetrically in the grips, ensuring the longitudinal axis aligns with the line of force application to avoid bending or torsional loads.
• Apply the specified preload to remove slack without elongating the specimen beyond its elastic response.
• Zero the force and extension readings, then start the test and run the crosshead until the specimen ruptures completely.
• Record the maximum force reached, the extension at break, and the full force–extension curve; note the failure location and mode, discarding specimens that fail inside or slip at the grips when the standard so requires.
• Repeat the test on the number of specimens required by the standard, commonly five to ten valid results per direction or condition.
• If specimen breakage occurs at the grips, retension with a modified clamping method, padding, or an alternative grip type, and document the corrective action.

Data Processing and Reporting

For each valid specimen, the breaking force is read directly as the peak of the force–extension curve. Elongation at break is calculated as the extension at maximum force divided by the initial gauge length, multiplied by 100. Tenacity is calculated by dividing breaking force by linear density. Statistical summaries are prepared according to the standard, usually the arithmetic mean and coefficient of variation. The test report must identify the standard applied, specimen identification, gauge length, test speed, preload, conditioning atmosphere, number of specimens, individual and mean results, failure descriptions, and any deviations from the standard method. Results are typically compared against product specifications, such as the minimum breaking force requirements in the relevant IEC, ASTM, EN, GB, UL, or JIS product standards.

Factors Affecting Accuracy

• Grip slippage or jaw breaks, which artificially lower the measured breaking force
• Incorrect gauge length, since longer gauges generally reduce apparent breaking force in materials with strength variability
• Test speed, because most materials exhibit rate-dependent behavior, with higher speeds raising measured force in polymers and textiles
• Specimen alignment errors that introduce bending stresses and cause premature failure
• Inadequate conditioning of moisture-sensitive materials such as textiles, paper, and nylon
• Load cell selection outside the optimal force range, degrading measurement uncertainty
• Operator variation in cutting and mounting textile specimens

Applications

Breaking force testing supports quality control, product certification, failure analysis, and research across many industries:

• Wire and cable manufacturing, verifying that conductors meet minimum tensile requirements before stranding, insulation extrusion, and shipment
• Textile and apparel production, confirming fabric strength in warp and weft directions and certifying yarn lots
• Lifting equipment and personal protective equipment, where breaking force data underpins safe working load ratings for slings, harnesses, lanyards, and ropes
• Flexible packaging, validating heat seal integrity and film strength for food, pharmaceutical, and industrial packaging
• Automotive and aerospace supply chains, qualifying seat belt webbing, safety cables, tie-downs, and composite reinforcement yarns
• Medical device manufacturing, demonstrating that sutures, ligatures, and tubing meet minimum breaking force limits required for patient safety
• Construction and geosynthetics, evaluating geotextiles, geomembranes, and reinforcement strips used in civil engineering works
• Incoming inspection and supplier audits, where breaking force serves as a rapid acceptance criterion for raw materials
• Failure investigation and litigation support, comparing the residual strength of failed parts against specification values

FAQ

What is the difference between breaking force and tensile strength?

Breaking force is the absolute maximum load, expressed in force units such as N or lbf, sustained by a specimen before rupture. Tensile strength is the breaking force divided by the original cross-sectional area, expressed in stress units such as MPa. Breaking force is preferred for products like wires, yarns, ropes, and webbing where the effective load-bearing area is difficult to define, while tensile strength is standard for machined metal and molded plastic test pieces.

What is the difference between CRE and CRL machines?

A CRE (constant rate of extension) machine separates its grips at a fixed speed, making extension the controlled variable; it is required by most modern ISO and ASTM textile standards. A CRL (constant rate of loading) machine increases force at a constant rate, and results from the two modes are not directly interchangeable for viscoelastic materials.

How many specimens should be tested?

Most standards require a minimum of five valid specimens per material direction or condition, and some require ten. Specimens that slip in the grips or break at the jaw line are typically invalid and must be replaced so that the reported mean reflects at least the specified number of valid mid-gauge failures.

Why did my specimen break at the grip, and is the result valid?

Jaw breaks occur when local stress concentration or grip serration damage weakens the specimen at the clamp. Such results are usually lower than the true material strength and are excluded by most standards. Corrective actions include reducing clamping pressure, adding grip padding or tabs, switching to capstan or wrap-type grips, or adjusting gauge length.

Which standards govern breaking force testing for common products?

Frequently applied methods include ISO 6892-1 and ASTM E8/E8M for metallic materials, IEC 60811-501 for cable conductors, GB/T 4909.3 for bare wires, ASTM D2256 and ISO 2062 for yarns, ISO 13934-1 and ASTM D5034 for fabrics, ISO 2307 for ropes, ASTM D882 and ISO 527-3 for films, ASTM F88 for seal strength, and ASTM D412 and ISO 37 for rubbers.

Do breaking force results depend on test speed and gauge length?

Yes. Polymeric, textile, and rubber materials are strain-rate sensitive, so higher test speeds generally produce higher breaking forces and lower elongation. Longer gauge lengths increase the probability of including weak points, which typically lowers measured breaking force in heterogeneous materials. Standards therefore fix these parameters precisely, and any deviation must be reported.

Can breaking force testing be performed by an external laboratory?

Yes. Accredited third-party laboratories routinely perform breaking force testing under ISO/IEC 17025 management systems, providing traceable calibration, standard-compliant procedures, and test reports suitable for certification, customer acceptance, and regulatory submission.