The ultimate breaking force test is the decisive destructive verification for round link chains used in mining conveyors, lifting equipment, lashing systems, and marine mooring applications. By loading chain specimens in axial tension until complete fracture, the test confirms that both the breaking force and the breaking elongation satisfy the minimum requirements of standards such as ISO 610, ISO 1704, EN 818, and GB/T 12718. This article explains the test principle, sample requirements, equipment, loading procedure, acceptance criteria, and typical industrial applications.
Technical Overview
Round link chains are welded steel chains whose links are formed from round bar stock, joined by flash butt or electric resistance welding, and subsequently quenched and tempered. Because a single chain failure can drop a suspended load, stop an underground conveyor, or release lashed cargo, these products are treated as safety-critical components and their mechanical properties are strictly controlled by product standards. The ultimate breaking force test, also known as the breaking load test or destructive tensile test, applies a steadily increasing axial tensile load to a short chain specimen until one or more links fracture, and records the maximum force sustained by the specimen.
The test serves two purposes: first, to verify that the actual breaking force of a production batch equals or exceeds the specified minimum breaking force (MBF); second, to confirm that the chain retains sufficient ductility at fracture, expressed as total elongation. Testing is normally performed in accredited third-party laboratories or in manufacturers' certified test houses using large-capacity horizontal or vertical tensile testing machines.
- Primary output: measured ultimate breaking force in kN
- Secondary output: total breaking elongation in percent
- Governing principle: quasi-static axial tension until fracture
- Related verifications: proof force test, dimensional inspection, surface quality check
The most relevant reference standards include:
- ISO 610 – High tensile steel chains (round link) for mining conveyors and ploughs
- ISO 1704 – Welded link anchor chains
- ISO 1834 – Lifting chains, general conditions of acceptance
- ISO 3075 / ISO 3076 – Round steel short link chains for lifting purposes, grade 8
- EN 818-1 / EN 818-2 – Short link chains for lifting purposes, general conditions and sling chains
- EN 12195-3 – Lashing chains for load securing
- GB/T 12718 – High strength round link chains for mining applications
- ASTM A391/A391M – Alloy steel chain slings (grade 100 chain practice)
- ISO 7500-1 – Verification of static uniaxial testing machines
- ISO 9513 – Calibration of extensometers used in uniaxial testing
Testing Samples
High-Strength Mining Round Link Chains
Manufactured in accordance with ISO 610 and GB/T 12718, these heat-treated chains cover nominal bar diameters from approximately 10 mm to 48 mm in strength grades B, C, and D, and drive scraper conveyors, armored face conveyors, and stage loaders in underground coal mines. Tensile specimens are cut from production lengths and typically consist of three to seven or more complete links, with sufficient free links between the grips to ensure a valid fracture location.
Lifting and Hoist Round Link Chains
Short link chains to EN 818-1/-2 or ISO 3075/3076, usually in grade 8, are used in chain slings, hand chain hoists, and electric hoists. For grade 8 chains the minimum breaking force corresponds to a 4:1 ratio to the working load limit. Specimens for the breaking force test are taken from each heat-treatment batch and contain at least the minimum number of links prescribed by the standard, commonly five or more.
Lashing and Securing Chains
Grade 8 and grade 10 lashing chains tested to EN 12195-3 derive their standard lashing capacity from the breaking force. Ultimate breaking force testing confirms that serial production maintains the required safety ratio between breaking force and lashing capacity for road, rail, and marine cargo securing.
Anchor and Offshore Mooring Chains
Welded link anchor chains covered by ISO 1704, often stud-linked and produced in large diameters, demand very high machine capacities. For extremely large chain dimensions, acceptance may alternatively be based on proof force testing combined with material verification, where the standard permits it.
Sample Condition Requirements
- Specimens must be representative of the heat-treatment batch and free of visible surface defects before testing
- Link dimensions, weld flash condition, and straightness shall be verified prior to mounting
- The surface finish is tested in the as-delivered condition (bare, black oxidized, galvanized, or painted) unless the standard requires otherwise
- Specimens shall not previously have been loaded beyond the manufacturing proof force
Testing Items
The core quantities evaluated during an ultimate breaking force experiment are summarized below.
- Ultimate breaking force: the maximum tensile force recorded at fracture, compared with the specified minimum breaking force for the chain size and grade
- Total breaking elongation: the combined elastic and plastic extension of the gauge length at the moment of fracture, compared with the minimum ductility requirement, commonly in the range of roughly 12% to 25% depending on grade and standard
- Force-elongation curve: recorded continuously to document elastic and plastic behavior and to reveal anomalies such as premature yielding or stepwise extension
- Fracture characteristics: the location of fracture (link body, weld zone, or gripping area) and the fracture mode, which provide diagnostic evidence on weld quality and heat treatment
- Pre-test dimensional verification: bar diameter, link pitch, outside width, and weld profile as supporting data for the force evaluation
Some specifications additionally require measurement of the permanent set after the manufacturing proof force has been applied and released. This is a separate, non-destructive verification performed on a different specimen, since a chain used for the breaking force test cannot be returned to service.
Testing Methods
Equipment and Calibration
The test is performed on a hydraulic or electromechanical universal testing machine with sufficient capacity, commonly from 1,000 kN up to 6,000 kN depending on the chain size. The force measuring system shall be calibrated and verified in accordance with ISO 7500-1, typically to class 1 accuracy or better. Elongation may be determined by calibrated extensometers meeting ISO 9513, by corrected crosshead displacement, or by marking the gauge length and measuring it before and after fracture.
Specimen Mounting
The specimen is mounted using purpose-designed grips, shackles, or chain pulleys that engage the end links and transmit pure axial tension without inducing bending. Enough free links must remain between the grips so that fracture occurs in the free portion of the chain rather than at the grip interface; a fracture within the gripping zone is generally treated as an invalid result unless the recorded force already exceeded the specified minimum.
Loading Procedure
- Apply a small seating load to remove slack, then zero the force and elongation measurements
- Increase the load continuously and smoothly, controlling the rate as specified by the product standard, commonly on the order of 20 N/mm² per second of nominal cross-sectional stress or an equivalent constant crosshead speed of a few millimeters per minute
- Continue loading through the elastic range, yield plateau, and strain-hardening range while continuously recording force and extension
- Proceed until complete fracture of one or more links and record the maximum force as the ultimate breaking force
- After fracture, reassemble the specimen, measure the final distance between the original gauge marks, and calculate the total breaking elongation
The test is normally conducted at ambient temperatures between approximately 10 °C and 35 °C unless otherwise specified. Because the elastic energy stored in a heavily loaded chain is released violently at fracture, protective screens, barriers, and restricted-access zones around the machine are mandatory safety precautions.
Acceptance Criteria and Evaluation
- The measured breaking force shall be equal to or greater than the specified minimum breaking force for the nominal diameter and grade
- The total breaking elongation shall meet or exceed the minimum value required by the product standard
- The fracture shall occur outside the gripping zone; fractures through the weld line are formally acceptable but, when repeated, indicate a weak welding process
- Where several specimens are tested per batch, all individual results must satisfy the requirements unless the standard permits statistical evaluation of the mean
Test Report
A complete test report identifies the chain manufacturer, grade, nominal diameter, batch and heat number, the applicable product standard, specimen configuration, gauge length, machine identification and calibration class, ambient temperature, loading rate, the force-elongation curve, the measured breaking force and elongation, a description of the fracture location and appearance, and a clear statement of conformity or non-conformity.
Applications
Ultimate breaking force testing is applied wherever round link chains act as load-bearing safety components:
- Underground coal mining: batch release and type approval of scraper conveyor and armored face conveyor chains to ISO 610 and GB/T 12718
- Lifting technology: incoming inspection and batch testing of hoist chains and chain sling components to EN 818 and ISO 3075/3076
- Load securing: certification of lashing chains for road, rail, and sea transport to EN 12195-3
- Shipbuilding and offshore: qualification of anchor and mooring chain production to ISO 1704
- Bulk material handling: quality control of round steel chains for bucket elevators, en-masse conveyors, and heavy-duty towing duties
- Research and development: comparative evaluation of steel grades, welding parameters, and heat-treatment routes during new chain development
- Failure analysis: re-testing of retained batch samples to support investigation of in-service chain failures
FAQ
What is the difference between proof force and breaking force?
The proof force is a controlled, non-destructive load applied to every chain during manufacture, typically between about 40% and 60% of the specified minimum breaking force, in order to relieve residual welding stresses and expose defects. The breaking force test is destructive: it loads a sample chain until fracture to prove the true ultimate capacity. A chain that has passed its proof test remains fully serviceable, whereas a breaking force specimen is destroyed.
How many links should a test specimen contain?
The required specimen length depends on the product standard and the chain size. Common practice is at least five complete links for lifting chains and three to seven or more links for mining chains, always ensuring enough free links between the grips for a valid fracture. Adapter or filler links at the ends are not counted within the measured gauge length.
Why is minimum elongation required in addition to minimum force?
High breaking force alone is not sufficient, because a brittle chain can be strong yet fail suddenly without warning. The minimum elongation requirement guarantees ductility, ensuring that the steel and the weld zone elongate visibly before failure. Low elongation results frequently reveal incorrect quenching and tempering or weld embrittlement.
What happens if the specimen fractures inside the grip area?
If fracture occurs at or within the grips, the result is normally declared invalid because the gripping stress concentration influenced the failure. The test is then repeated with a new specimen. Where the force already reached the specified minimum before the invalid fracture occurred, many standards permit acceptance of the result.
Can a single breaking test represent an entire production batch?
Sampling plans in the product standards define how many specimens per heat-treatment batch or production length must be tested, often several samples distributed across the batch. Because the test is destructive, it is combined with 100% proof loading and dimensional inspection to achieve full batch assurance.
What factors most commonly cause low breaking force results?
Typical root causes include insufficient weld penetration or incomplete flash removal, improper heat treatment resulting in incorrect hardness, decarburization at the weld zone, surface defects or non-metallic inclusions in the bar stock, and dimensional deviations that reduce the effective load-bearing cross-section below the specified minimum.