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Bending to test Conformance
Bend-testing is a much informative qualitative procedure simple to perform.
When applied to welded specimens, it evaluates both the ductility and soundness of welds, without instrumented equipment.
This procedure was briefly addressed in our page on Welding Testing.
It is also mentioned in an Article published in section 7 of issue 13 of Practical Welding Letter for September 2004. Click on PWL#013 to see it.
Bend tests are applied to metal strips including a weld, to test the material at the bend location.
Bending of the specimen is done around a cylinder of specified radius through a given angle.
Once bent, the tested strips look like the letter U.
The maximum extension occurs at the outside of the bend, due to considerable plastic yield in tension, while longitudinal compression occurs on the inside.
The bend tensile deformation is sufficient to cause cracking and rupture of the specimen if the weld contains major flaws like porosity or lack of fusion.
The same happens if its heat affected zone (HAZ) is hard and brittle.
Bend-testing is acceptable if no cracks are visible on the convex surface of specimens.
On the contrary, if the fracture follows a path of internal discontinuities opening up cracks, porosity or incomplete fusion, the lack of soundness is obvious.
A cracked or fractured specimen documents failure.
In reporting Bend-testing for weld qualification, it is important to specify location of the specimen on the weld test panel, and direction of weld with reference to the bend radius.
Bend-testing is therefore a procedure capable to separate quality weld specimens from defective ones.
The confidence in the capability of this test to perform positive selection is such that it is prescribed by codes that establish qualification requirements for procedures or welders.
When preparing plates for weld Bend-testing, one has to cut them so that the side of welding be perpendicular to the rolling direction.
This is because rolled plates may be weaker and fail if bent around an axis of bending parallel to the rolling direction.
Bend-testing can be free or guided.
In free bending, after an initial partial deformation to establish bending progress in the wanted direction, the specimen is put in upright position between two plates of a press.
To keep the specimen from falling, its ends are introduced in slots of the plates.
Then the specimen is bent by the pressure exerted when the press is operated by reducing the space between the plates.
Testing is stopped after bending is complete (the specimen sides become parallel) but there is no control of the actual radius.
Most welding qualification codes require guided Bend-testing. This is usually performed in a fixture designed for this purpose, the specimen is freely supported on two rolls at the required distance from one another.
A plunger of the prescribed radius is made to press the specimens in the space between the rolls, until the two sides of the specimen turn freely up and become parallel.
At that point the plunger is lifted and the bent specimen removed for observation.
Alternatively, the specimen, with one end clamped in a fixture, is wrapped around a cylinder of the correct radius, by turning a lever with a free turning roll pushing the specimen face until the final shape is reached. (See the Video further down).
As an example here are the requirements of ANSI/AWS D1.1 - Structural Welding Code - Steel:
Material with Yield Strength up to 345 MPa (50 ksi) -
Bend Radius 19 mm (0.75")
Material with Yield Strength 345 - 620 MPa (50-90 ksi) -
Bend Radius 25 mm (1")
Material with Yield Strength over 620 MPa (90 ksi) -
Bend Radius 32 mm (1.25")
Structural Welding Code - Steel
American Welding Society
Guided bend specimens may be longitudinal or transverse to the weld axis.
Transverse Bend-testing specimens have the weld parallel to the axis of bending. They may be root bend, side bend or face bend depending on the portion in tension during the test.
Root and face welds have generally the reinforcements machined flush with the specimen surface before Bend-testing.
To get specimens of specified thickness, they may be sliced from the weld test panel being submitted to Bend-testing.
Side bend tests strain the whole section of the weld.
Therefore they may reveal internal flaws not readily found by either root or face bend tests. They are usually required for thickness between 10 and 19 mm (0.375 and 0.750").
If the thickness of the welded specimens exceeds 38 mm (1.5"), they can be split in multiple thinner specimens for qualification testing.
Transverse Bend-testing specimens are sensitive to the relative strength of base metal, HAZ and weld.
Therefore the bending radius of the different zones may not match exactly the plunger bend radius.
In that case, if the radius is smaller, specimens may fail because the ductility is locally exceeded and not because of weld defects.
The same reasoning applies for welds between dissimilar metals.
To evaluate such welds, if permitted by the codes, longitudinal bend tests can be used because weld metal, heat affected zone and weld metal are strained equally and simultaneously.
However they may not be equally severe as the transverse ones.
Wraparound Bend-testing fixtures though, may minimize the effect of bending to different radii.
Besides for plates, bend tests can be performed also for pipes on slightly curved surfaces.
Nick-break test is a variant intended to test the soundness of a weld.
From a groove welded test panel, a section of say 50 mm (2") width is removed.
Two saw cuts are machined before bending, for a depth of about 6 mm (1/4"), at both the width sides of the specimen section, along the whole depth of the weld.
The presence of internal defects will rupture the weld, exposing them.
These tests are performed on representative weld test qualification specimens.
It is highly recommended to every welding shop to perform some sort of Bend-testing, even if not formally required by Codes, to verify production quality and to improve one's manual welding skill.
The following documents prescribe the testing techniques:
Standard Methods for Mechanical Testing of Welds (US Customary Units)
American Welding Society / 02-May-2007 / 154 pages
ANSI/AWS B4.0M:2000 (R2010)
Standard Methods for Mechanical Testing of Welds (Metric Customary Units)
American Welding Society / 25-Jul-2000 / 115 pages
Standard Test Method for Guided Bend Test for Ductility of Welds
ASTM International / 01-Sep-2008 / 4 pages
Standard Test Methods for Bend Testing of Material for Ductility
ASTM International / 01-Apr-2009 / 10 pages
An Article on Filler Metal 312 failing transverse side bend tests was published (4) in Issue 150 of Practical Welding Letter for February 2016.
Click on PWL#150.
An Article on Overview of modern Materials Testing Systems was published (7) in Issue 153 of Practical Welding Letter for May 2016.
Click on PWL#153.
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For further information readers may wish to see the following:
Understanding Bend Tests
|Watch the following Video on
Welding GMAW (MIG) Certification bend test
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