The repeated bending test, also known as the reverse bend test, is a standardized mechanical procedure used to evaluate the ductility, formability, and crack resistance of metallic wires, bars, strips, and flexible assemblies under alternating plastic deformation. By bending a specimen alternately in opposite directions around a specified radius until cracking or fracture occurs, the test reveals surface defects and material quality issues that static tensile testing cannot detect. This guide explains the test principle, applicable standards, sample types, test items, procedures, and typical industrial applications.
Technical Overview
The repeated bending test, also called the reverse bend test or flexural endurance test, is a destructive mechanical test that assesses the ability of a material to withstand alternating plastic deformation without cracking or fracturing. During the test, a specimen is clamped at one end and bent alternately to the left and right, typically through an angle of 90 degrees, around a cylindrical roller or mandrel of specified radius. The operation is repeated until visible cracks appear or the specimen fractures, and the number of bending cycles sustained is recorded as the primary result.
Because each reversal strain-hardens the material and concentrates deformation at the surface of the bend zone, the test is highly sensitive to surface defects, inclusions, segregation, decarburization, and insufficient ductility. It is therefore widely used as a routine quality control tool in wire drawing, spring and fastener manufacturing, reinforcing steel production, and cable manufacturing. Principal standards include ISO 7801 for the reverse bend test of metallic wire, its Chinese equivalent GB/T 238, the European adoption EN ISO 7801, ASTM E290 and ASTM A370 for bend testing of metallic products, ISO 6935-2 and GB/T 1499.2 for reinforcing bars, IEC 60227-2 and IEC 60245-2 for cable flexing tests, and UL 2556 for wire and cable test methods.
- Test type: destructive test based on alternating plastic deformation, in the quasi-static to low-cycle deformation regime
- Typical bend angle: 90 degrees per stroke in alternating directions
- Key result: number of reverse bends sustained before cracking or complete fracture
- Complementary to tensile, torsion, wrapping, and flattening tests in a full mechanical test program
Testing Samples
Sample types commonly submitted to accredited third-party laboratories for repeated bending evaluation include the following categories.
Metallic Wire
Carbon steel wire, alloy steel wire, spring wire, welding wire, wire for wire ropes, and nonferrous wires such as copper, aluminum, and their alloys are the most frequent specimens. The applicable diameter range under ISO 7801 and GB/T 238 is typically 0.3 mm to 10 mm, although individual product specifications may extend or restrict this range.
Steel Reinforcement Bars
Hot-rolled ribbed bars and plain bars for reinforced concrete are subjected to bend and reverse bend tests to verify ductility and bending performance, as required by ISO 6935-2, EN 10080 related schemes, GB/T 1499.2, and ASTM A370.
Metallic Strip, Sheet, and Foil
Narrow strip used for springs, saw blades, fasteners, and electrical contacts is tested to evaluate formability, edge quality, and rolling direction effects. Thin foils may be evaluated by adapted flexing procedures in which bending radius and applied tension are carefully controlled.
Electrical Wires, Cables, and Flexible Cords
Insulated conductors, flexible cords, and cable samples are evaluated by back-and-forth flexing tests in accordance with IEC 60227-2, IEC 60245-2, and UL 2556 to simulate service movement, verifying conductor integrity, insulation condition, and continuity after repeated flexing.
Coated and Plated Components
Galvanized, tinned, or enameled wire is tested to check coating continuity and adhesion, because repeated bending accelerates coating cracking, flaking, and exposure of the substrate.
Testing Items
Depending on the product standard and the purpose of the evaluation, the following items are typically measured or assessed during a repeated bending test program.
- Number of reverse bends sustained up to the first visible crack
- Number of reverse bends sustained up to complete fracture
- Presence of surface cracks, fissures, tears, or delamination after a specified number of cycles
- Coating integrity, including cracking, peeling, or flaking of metallic and organic coatings
- Influence of bend radius, roller diameter, and clamping force on bending performance
- Bend performance after heat treatment, artificial aging, or environmental conditioning
- Low-temperature or elevated-temperature bend resistance where specified by the product standard
Testing Methods
Apparatus
A typical reverse bend machine consists of a rigid frame, two parallel rollers of specified radius, a clamping device with a rotating bending arm, and a cycle counter. The specimen is passed between the rollers and clamped so that its free end can be bent alternately in both directions around one of the rollers. The roller diameter, roller parallelism, and clamping force are set according to the relevant standard and product specification.
Specimen Preparation
Specimens are cut to the length specified by the standard, with smooth and burr-free ends to avoid premature failure at artificial stress raisers. If straightening is necessary, it must be performed by a method that does not alter the properties of the material, and the procedure should be noted in the test report. Tests are normally conducted at ambient temperature between 10 and 35 degrees Celsius unless otherwise specified.
Test Procedure
- Mount the specimen between the rollers and clamp it securely, applying any tension force required by the standard.
- Bend the free end of the specimen through 90 degrees in one direction at a uniform, controlled rate.
- Return the arm and bend the specimen through 90 degrees in the opposite direction; one complete alternating cycle is counted as one reverse bend.
- Repeat the operation continuously until visible cracking, fracture, or the specified number of cycles is reached.
- Examine the bent zone visually, normally without magnification unless the applicable standard permits or requires it.
Evaluation and Acceptance Criteria
Acceptance is generally defined by the product specification, for example the requirement to withstand a specified number of reverse bends without cracks visible to the unaided eye. The test report should state the number of bends to crack or fracture, the roller radius, the test temperature, the bending rate, and a description of the failure mode. For reinforcing bars, a common scheme requires an initial bend followed by straightening and a reverse bend after artificial aging to confirm adequate ductility.
Applications
Repeated bending tests support quality control, product qualification, and failure analysis across a wide range of industries wherever ductility and resistance to alternating deformation are essential.
- Wire drawing and wire rope industry: routine release testing of wire ductility and surface quality at each drawing stage
- Construction materials: acceptance testing of reinforcing bars and prestressing wire before delivery to construction sites
- Spring and fastener manufacturing: verification of coiling and forming capability of spring wire and strip
- Cable and cord industry: durability assessment of flexible cords, power cables, and robotic cable assemblies under simulated movement
- Automotive and electronics: flexing endurance of wiring harnesses, connectors, contacts, and flexible interconnects
- Material research and failure analysis: screening for inclusions, decarburization, hydrogen embrittlement, and coating defects, and comparison of heat treatment conditions
FAQ
What is the purpose of the repeated bending test?
The test determines whether a material can tolerate alternating plastic deformation without cracking, which directly reflects its ductility, surface quality, and suitability for subsequent forming operations or flexing service. It is a fast and economical screening method for detecting material defects that tensile testing alone may not reveal.
Which standards govern repeated bending tests?
The core method for metallic wire is specified in ISO 7801, GB/T 238, and EN ISO 7801. Related bend test methods are covered by ASTM E290, ASTM A370, and JIS Z 2248, while reinforcing steel requirements appear in ISO 6935-2, EN 10080, and GB/T 1499.2. Cable flexing tests are specified in IEC 60227-2, IEC 60245-2, and UL 2556.
How does repeated bending differ from high-cycle fatigue testing?
Repeated bending imposes plastic deformation in every cycle and is completed within a small number of reversals, whereas high-cycle fatigue testing applies elastic stresses over thousands to millions of cycles to derive an endurance curve. Repeated bending is therefore a ductility and quality screening test rather than a means of determining a fatigue strength value.
What factors can influence the test result?
Bend radius, bending rate, roller parallelism, clamping force, specimen straightening method, surface condition, and test temperature all affect the number of bends sustained. Consistent apparatus calibration and strict adherence to the specified parameters are essential for reproducible results.
Can the test be performed at low temperature?
Yes. When a product standard requires low-temperature bend performance, the specimen and apparatus are conditioned at the specified temperature before testing, and the test is completed within a defined time after removal from the conditioning environment so that the material remains at the target temperature.
How should results be reported?
A complete report includes sample identification, dimensions, roller radius, test temperature, bending rate, the number of reverse bends to crack or fracture, the failure mode, and a statement of conformity with the applicable acceptance criteria.