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ISO 8491 Metal Tube Bend Test

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ISO 8491 Metallic Tube Full-Section Bend Test

ISO 8491 define a unified bend test method for evaluating the plastic bending ductility of circular full-section metallic tubes, exclusively for tubes with outer diameter D ≤ 65 mm (product standards may narrow this diameter range further). 

Main Industry fields: boilers & pressure vessels, oil & gas pipelines, automotive (exhaust/chassis tubing), heat exchangers, structural steel tubing, power generation (including nuclear).


Test principle: 

A straight, full-section circular tube is bent around a grooved former of specified inside radius r until the bend angle α reaches the value required by the relevant product standard. The tube is tested unfilled, and the goal is to assess its ability to undergo plastic deformation in bending without failure.


Test Equipment required for ISO 8491: 

Steel Pipe Bending Testing Machine

Recommend UnitedTest GGW-60H Steel Pipe Bending Testing Machine a special equipment for plane bending test of metal steel pipe. Conform with ISO 8491 metal tube bend test methods. 

Designed to prevent ovalisation (flattening) of the tube cross-section during bending, which avoids distorted test results caused by section collapse.

Grooved bending form

The groove profile must perfectly match the tube’s outer diameter D. The bottom radius r of the groove is stipulated by product specifications.

Tolerance of groove radius r, groove depth, and groove ovality directly alter test outcomes and must be controlled tightly. 

ISO 8491 Metal Tube Bend Test


Test Specimen Information

Form: A straight segment cut from the original tube; full circular cross-section must be retained (no slitting or cutting into strips).

Length rule: No fixed mandatory length; the specimen just needs sufficient length to be fully clamped and bent on the tube bending machine.

Special rule for welded tubes: Welded tube specimens retain full tube geometry without pre-treatment before testing.


Key Test Parameters and stipulations:

SymbolParameter DefinitionUnit
DTube outer diametermm
LOriginal length of test piecemm
rRadius of the grooved former groove bottommm
αTarget bend angle after full deformationDegree

Tube must not ovalise during bending → machine must support that.

Weld position rule for welded tubes (neutral axis default).

Acceptance criterion defaults to "no visible cracks without magnification" if product standard is silent.


Details test procedure of the ISO 8491 test:

Bending Operation

Test the unfilled hollow tube specimen (no filler inside the tube). 

Operate the tube bending machine to bend the tube, ensuring full surface contact between the tube outer wall and the grooved former across the entire bending arc until the preset bend angle α is achieved.

Special Rule for Welded Tubes

Unless product standards state otherwise, the weld seam must be positioned at 90° relative to the bending plane (aligned with the neutral bending axis) to eliminate biased stress on the weld during deformation.

Result Judgement Criteria

Follow acceptance rules specified in the relevant tube product standard first.

If no product-specific rules exist: The specimen passes the test if no visible cracks can be observed by naked eyes without magnification tools. Any visible surface crack counts as a failure.


Applicable Industry Fields

This standard serves all manufacturing and inspection sectors that produce or use small-diameter metallic circular tubes, including:

Metallurgical tube production (carbon steel, stainless steel, copper, aluminium alloy tubes);

Plumbing and pipeline engineering (fluid conveying thin-walled tubes);

Automotive tubing (hydraulic, brake, fuel delivery small tubes);

HVAC and refrigeration heat exchange tubes;

Instrumentation, aerospace thin precision tubular components;

Construction metal tubular fittings and bent pipe parts.


Related standard: 

EN 10232Metallic Materials - Tube (in Full Section) - Bend Test
IS 2329Method for bend test on metallic tubes (in full section)
NF A03-091Metallic materials. Tube (in full section). Bend test.
ISO 7438General bend test for flat metal strip/sheet specimens.
TCVN 5891Metallic materials. Tube (in full section).Bend test

GOST 3728

Steel Pipes. Bend Test Method
GB/T 244Metallic material—Tube—Bend test method
GB/T 32976

Metallic materials—Tubes—Transverse bending test


Related products and device

ISO 8491 Metal Tube Bend testing machine

Metallic materials wire reverse bend testing machine manly used to measure metal wire material, like steel wire, steel bar, steel wire rope, steel strand, nude electrical wire, copper alloy wire etc., rotate the provision radius jaws ±90 degree repeat the reverse bending test;

Metal tensile testing machine

WDW Series Computer Control Electronic Universal Testing Machine made by UNITEDTEST range from 100N to 600KN load capacity with various models like single columns, table type, door frame type etc., is used to perform tension, compression, flexure/bending, shearing, peeling etc., test for metal and nonmetal specimens.

Related Standard

ISO 7438 Metallic materials Bending test details

ISO 7438 is a standard that specifies a method for determining the ability of metallic materials to undergo plastic deformation in bending. It is used to evaluate the flexural strength and stiffness of a material , The bend test shall be carried out in testing machines equipped with a bending device with two supports and a former; bending device with a V-block and a former; bending device with a clamp. 

ASTM E290 Metal Bend testing of materials ductility

ASTM E290 Standard Test Methods for Bend Testing of Material for Ductility


ASTM E290 is a destructive mechanical test standard that evaluates the ductility and structural soundness of metallic materials (and welds) by subjecting specimens to controlled bending deformationASTM International. It is a widely used quality control and qualification method for metals, alloys, and welded joints across industries.

ISO 7801 Metallic Wire Reverse bend testing

ISO 7801: Metallic materials -- Wire -- Reverse bend test


Specifies the method for determining the ability of wire of diameter or thickness 0,3 to 10 mm inclusive to undergo plastic deformation during reverse bending. The range of diameters of thicknesses for which ISO 7801 is applicable may be more exactly specified in the relevant product standard. The test consists of repeated bending, through 90 in opposite directions, of a test piece held at one end, each bend being over a cylindrical support of a specified radius.

FAQs for ISO 8491 Metallic Tube Full-Section Bend Test

Q1: What is the core purpose of ISO 8491 bend test?

A1: This standard specifies a full-section bending test method for circular metallic tubes with outer diameter ≤65 mm. It measures the plastic deformation capacity of intact tubular products under bending load, to judge whether the tube can withstand cold bending without cracking during processing or service.


Q2: Why is the ISO 8491 bend test important for metallic tube materials?

A2:It verifies tube ductility: Bending is a common forming process for pipes; the test directly checks if the tube can deform plastically without surface cracks.

Exposes hidden defects: Poor metallurgy, brittle microstructure, faulty welds or uneven wall thickness will produce visible cracks during bending, which tensile tests may not detect.

Guarantees service safety: Tubes for automotive brake lines, refrigeration, hydraulic pipelines bear repeated bending stress. Low bend ductility causes crack leakage or rupture, triggering equipment failure and safety risks.

Standardized quality inspection: It acts as a universal acceptance criterion for tube batches, enabling consistent quality control between manufacturers, suppliers and end users worldwide.

Simulates actual production conditions: The full-tube test retains the original hollow cross-section, which better replicates real bending processing compared with strip bend tests.


Q3: What tubes does ISO 8491 apply to? Are there diameter limits?

A3: It applies to full-section circular cross-section metallic tubes with outer diameter D ≤ 65 mm. Relevant product standards may further narrow the applicable diameter range. The standard is only for intact whole tubes, not strip samples cut from tubes.


Q4: What standard should I use if I test transverse strip samples cut from tubes instead of full tube sections?

A4: ISO 7438 (Metallic materials – Bend test). As stated in the note of Clause 1 of ISO 8491, transverse tube strips must follow ISO 7438 to test bending performance of flat strip specimens. ISO 8491 only covers full hollow tube bending.


Q5: Are there fixed length requirements for ISO 8491 test specimens?

A5: No fixed specified length. The specimen only needs to be a straight tube segment long enough to complete clamping and bending on the tube-bending machine. Any length meeting the machine operation requirement is acceptable.


Q6. What is the difference between ISO 8491 and ISO 7438?

A6: ISO 8491 is for full-section tubes. You bend the actual pipe/tube.

ISO 7438 is for metallic materials in general, specifically referring to flat specimens or transverse strips cut froma tube. If your product standard calls for a "flattening test" or a "strip bend," you use ISO 7438; if it calls for a "tube bend," you use ISO 8491.


Q7: Do I need to fill the hollow tube before bending per ISO 8491?

A7: No. Clause 6.2 clearly requires testing unfilled hollow tube specimens. No sand, mandrel or other filling materials are allowed inside the tube during the bend test.


Q8: What special rule applies to welded tubes during ISO 8491 testing?

A8: Unless specified otherwise in product standards, the weld seam must be placed at 90° to the bending plane (aligned with the neutral bending axis). This avoids concentrated tensile/compressive stress on the weld and ensures unbiased evaluation of weld ductility.


Q9: What factors will affect ISO 8491 bend test results?

A9: The tolerance of the former groove radius r, groove depth and groove ovality of the bending tool all directly influence test outcomes. Improper tool dimensions will cause inconsistent deformation and unreliable pass/fail judgements.

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