A breaking force testing machine, also known as a tensile tester or universal testing machine, is an essential instrument used to measure the maximum force required to rupture materials such as wires, cables, films, textiles, and metal products. Proper operation of this equipment ensures accurate, repeatable, and standards-compliant test results in quality control laboratories. This guide covers the working principles, sample types, test items, step-by-step operating procedures, industry applications, and frequently asked questions for breaking force testing machine operation.
Technical Overview
A breaking force testing machine is a force-measurement instrument that applies a controlled tensile load to a specimen until it fractures, recording the maximum force, elongation, and complete force-displacement curve throughout the test. The machine typically consists of a rigid load frame, a servo motor or stepper motor drive system, a precision load cell, movable and fixed grips or fixtures, a crosshead displacement measuring system, and a control unit with data acquisition software.
The working principle is straightforward: the drive system moves the crosshead at a constant, programmable speed, pulling the clamped specimen in tension. The load cell converts the applied force into an electrical signal, which is processed by the controller to generate real-time force-versus-elongation data. Modern machines offer force capacities ranging from a few hundred newtons for delicate filaments up to several hundred kilonewtons for structural materials, with measurement accuracy typically within ±0.5% of the indicated value.
Operation and test methods are governed by international and national standards, including ISO 527 and ASTM D638 for plastics, ISO 6892-1 and ASTM E8 for metallic materials, ASTM D882 for thin plastic films, ISO 2062 and ASTM D2256 for yarns, and GB/T 1040 and GB/T 228 series standards. Compliance with these standards requires correct grip selection, appropriate test speed, valid specimen preparation, and regular calibration traceable to accredited metrology institutions.
Testing Samples
Metal Wires and Cables
Conductor wires, stranded cables, steel wires, welding wires, and overhead conductors are tested to determine their breaking force and tensile strength. Specimens are usually prepared with a specified gauge length and free length to prevent stress concentration at the grips. Wedge-action mechanical grips are commonly used for hard metal samples, while specialized capstan or winding drums may be required for flexible stranded conductors to avoid premature strand breakage at the clamping points.
Plastic Films and Packaging Materials
Flexible films, laminates, shrink films, bags, tear strips, and easy-open lids are tested for breaking force, seal strength, and elongation at break. Thin films require pneumatic grips with rubber-faced jaws to distribute clamping pressure evenly and prevent tearing or slippage. Specimens are die-cut into standard dumbbell or strip shapes according to ASTM D882 or ISO 527-3, and edge quality must be free of nicks or notches that would cause premature failure.
Textiles, Yarns, and Webbing
Single yarns, threads, woven fabrics, nonwovens, seat belts, slings, and webbings are tested to evaluate breaking force and elongation. Yarn tests follow the CRE (constant rate of extension) principle specified in ISO 2062, using either capstan grips for linear materials or jaw grips with protective liners for fabric strips. Conditioning of textile specimens at standard temperature and humidity before testing is essential for reproducible results.
Rubber, Elastomers, and Seals
Rubber strips, O-ring cord, hoses, seals, and elastic bands are tested for tensile strength, elongation at break, and stress at a given strain. Because elastomers extend considerably before rupture, the machine must offer sufficient crosshead travel, and non-contact extensometers are recommended for accurate strain measurement at high elongation.
Adhesives, Tapes, and Fasteners
Pressure-sensitive tapes, labels, hook-and-loop fasteners, crimp terminals, and connectors are tested for peel force, pull-off force, and crimp tensile force. Dedicated fixtures such as 90-degree or 180-degree peel fixtures, plate clamps, and connector holders are installed to match the geometry of each product.
Testing Items
Depending on the specimen type and applicable standard, a breaking force testing machine can determine the following parameters:
- Breaking force (maximum force at rupture), expressed in newtons or kilonewtons
- Tensile strength (maximum force divided by original cross-sectional area), expressed in megapascals
- Elongation at break, expressed as a percentage of the original gauge length
- Yield strength and yield point elongation for ductile metals and plastics
- Force at specified elongation, commonly used for elastic materials
- Seal and seam strength for packaging and joined specimens
- Peel force and T-peel strength for adhesive joints and laminates
- Knot breaking force for fishing lines, sutures, and yarns
- Complete force-displacement or stress-strain curve with modulus calculation
Testing Methods
Pre-Test Preparation
Confirm that the laboratory environment meets the standard conditions, typically 23 ± 2 °C and 50 ± 10% relative humidity for plastics and textiles, and that specimens have been conditioned for the required period. Verify that the machine calibration certificate is valid, inspect grips and fixtures for wear or damage, and ensure the emergency stop function is operational.
Machine Startup and Warm-Up
Switch on the main power, then the controller and display unit. Allow the load cell and electronics to warm up for at least 10 to 15 minutes so that readings stabilize. Perform a zero-balance and tare operation, and select the correct force range so that the anticipated breaking force falls between 20% and 80% of the range for optimal accuracy.
Specimen Mounting
Select grips appropriate to the material, install them securely, and set the initial gauge length according to the standard. Clamp the specimen symmetrically in the upper and lower grips with equal tightening force to eliminate bending stress. For pneumatic grips, set the air pressure as specified; for wedge grips, ensure the specimen is seated fully and pre-loaded slightly so that slippage cannot occur during the test.
Parameter Setting
In the control software or on the control panel, input the specimen dimensions (width, thickness, or diameter), gauge length, test speed, and the parameters to be calculated. Test speed must match the standard; for example, thin films are often tested at speeds of 100 to 500 mm/min, metals at low speeds of a few millimeters per minute, and yarns at 20 to 500 mm/min depending on the specification. Set break detection criteria so the machine stops automatically when force drops below a defined percentage of the peak value.
Test Execution
Clear all personnel from the moving parts of the machine, then start the test. The crosshead extends the specimen at the set speed while the system records force and displacement continuously. Observe the specimen for slippage or grip-end failure; if fracture occurs within the grip jaws, the result is typically invalid and the test must be repeated on a new specimen. When the specimen ruptures, the machine stops automatically and displays the breaking force and calculated results.
Data Recording and Repeats
Record the test results, and test a sufficient number of specimens as required by the standard, commonly five for films and metals. Calculate the arithmetic mean, and if required, the standard deviation and coefficient of variation. Report individual values, mean values, specimen dimensions, test speed, grip type, and environmental conditions in the final test report.
Shutdown and Maintenance
Return the crosshead to its home position, release any remaining clamping pressure, and remove specimens and debris. Switch off the software, controller, and main power in the correct order. Clean the grips, lubricate lead screws or guide columns per the maintenance schedule, and cover the machine when not in use. Perform periodic verification with certified reference weights or force standards at least once a year, or more frequently under heavy use.
Applications
Breaking force testing machines are widely used across industries where tensile integrity determines product safety and performance:
- Wire and cable manufacturing, for verifying conductor breaking force and stranding quality before shipment
- Flexible and rigid packaging, for validating film tensile strength, seal integrity, and easy-open performance
- Textile and garment production, for yarn and fabric strength control and lot acceptance
- Medical device manufacturing, for suture, catheter, and ligature tensile validation to ISO and ASTM medical standards
- Automotive and aerospace supply chains, for seat belt webbing, wiring harnesses, and fastener pull-out verification
- Metal processing and fastener production, for incoming material inspection and finished product conformity testing
- Electronics assembly, for crimp terminal pull force and connector retention force checks
- Accredited third-party laboratories, for independent conformity assessment and certification support
FAQ
How often should a breaking force testing machine be calibrated?
Annual calibration by an accredited metrology body is the general requirement, and many quality systems such as ISO/IEC 17025 call for interim verification between calibrations. Daily zero checks and periodic verification with certified weights help detect drift and maintain confidence in results.
Why does the specimen slip in the grips during testing?
Slippage usually results from insufficient clamping force, worn or contaminated grip faces, or incorrect grip selection for the material. Solutions include increasing pneumatic pressure, cleaning or replacing jaw inserts, using serrated faces for hard materials or rubber-lined faces for films, and verifying that the specimen is inserted to the full grip depth.
What should be done if the specimen breaks at the grip edge?
Grip-end fracture invalidates the result because stress concentration at the jaw distorts the measured breaking force. Remedies include padding the specimen at the grips, reducing clamping pressure, using capstan or windlass grips for wire and yarn, slightly loosening wedge grips, or adding protective tabs to film specimens.
How is the correct test speed determined?
The test speed is dictated by the applicable standard and the material behavior. Brittle materials and metals are tested at low speeds, while flexible films and elastomers require higher speeds. Using the wrong speed changes the measured breaking force because polymers and metals are strain-rate sensitive, so strict adherence to the standard is essential.
What safety precautions must operators observe?
Operators should wear safety glasses to protect against specimen fragments, keep hands and loose clothing away from the moving crosshead, never stand directly in line with highly stressed specimens such as hard metals, and always engage the emergency stop if any abnormal behavior occurs. Only trained and authorized personnel should operate the equipment.
Can one machine test both very weak and very strong materials?
Yes, provided the machine supports interchangeable load cells of different capacities. Selecting a load cell whose capacity matches the expected breaking force, ideally loading it between 20% and 80% of its range, preserves measurement accuracy across a wide spectrum of materials.
