Bend-testing,Simple Pass/Fail Procedure.SOLUTIONS with Effective, Practical AdviceBend-testing is a qualitative procedure simple to perform and much informative that, when applied to welded specimens, provides means of evaluation of both the ductility and the soundness of welds, without needing instrumented equipment. This procedure was briefly addressed in our page on Welding Testing and 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, by bending them around a cylinder of specified radius through a given angle so that the tested strips look like the letter U, with the weld at the bend location. The maximum extension due to considerable plastic yield in tension occurs at the outside of the bend, while longitudinal compression occurs on the inside. (Sponsored Links) 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, or if its heat affected zone (HAZ) is hard and brittle. Bend-testing is acceptable if no cracks are found visually 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 the 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 permit 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 moving one of the plates toward the other. Testing is stopped after bending is complete 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. 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) - ANSI/AWS D1.1/D1.1M:2010 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 less thick specimens for qualification testing. Transverse Bend-testing specimens are sensitive to the relative strength of base metal, HAZ and weld. 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. In both cases, if permitted by the codes, one can resort to Longitudinal Bend-testing specimens, (where the weld is perpendicular to the bending axis) which do not fail as those above, because the different zones are strained equally. However they may not be equally severe as the transverse ones. 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 those defects. 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. For further information readers may wish to see the following: Bend-testing Understanding Bend Tests Nick-break test Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
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