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Up Topic Welding Industry / General Welding Discussion / % Elongation check using Bend test
- - By waqasmalik (**) Date 03-22-2020 15:13
Hello all. Hope this email finds you well.

While its been quite a time that i have been involved in welding and i know the significance of bend test that it is used to demonstrate weld joint soundness and ductility of the weld which is usually given by % elongation. But i dont know if the purpose is not welder qualification, why not to skip the bend test?

For procedure qualification, tensile testing will be carried out to find out ultimate tensile strength, yield strength and % elonagation as well. Why not this % elongation check is sufficient? Is any there any corelation between the % elongation values checked by tensile testing or bend testing? Can they be corelated? Which method more accurately depicts the % elongation of welded joints ?

Thank you in advance.

Any help shall be highly appreciated.

Best Regards

Waqas
Parent - - By 803056 (*****) Date 03-22-2020 17:09
Bend testing is performed to verify the weld soundness, i.e., free of unacceptable discontinuities. Most bend tests strain the specimen with the idea that any discontinuity of critical size will initiate a crack/open indication when it is strained beyond a certain value.

The bend radius is a function of the base metal thickness and elongation of the base metal when testing to AWS, ASME, or military welding standard. API uses a standard bend radius regardless of the thickness or ductility of the base metal, so to say an API qualification test using a bend test is equivalent to an AWS or ASME or military requirements is far from the case.

Whether the testing is part of the testing regiment associated with the qualification of a WPS or to evaluate the welder's skill, the function is the same. The goal is to access the weld's soundness.

In the case of qualifying a WPS, several welding standards require VT of the completed weld, volumetric examination (RT or UT), reduced section tensile testing to verify the mechanical properties are produced, bend testing for soundness, and possibly CVN and/or hardness testing. All the testing is to verify the WPS is capable of producing a weld with the mechanical properties needed per the code or per project requirements.

The welder performance testing is to verify the welder has the skills needed to produce a sound weld if the welder follows a qualified WPS. If the welders does not follow the WPS, for instance high voltage is used, higher amperage is used, higher heat input is used, etc., there is no assurance the resulting weld meets the mechanical properties produced when the WPS was qualified. The test is considered to have failed if the welder does not work within the ranges listed by the approved WPS.

There are exceptions to the testing regiment listed above. For example, if one is qualifying a WPS for welding cast iron, only the reduced tensile test is required. The ductility of cast iron is nil, thus an attempt to perform a bend test is futile.

Best regards - Al
Parent - - By waqasmalik (**) Date 03-22-2020 18:16
Thanx AL for commenting.

Actually i was reading a research article published in a well reputed journal in which author laser welded 2 mm thick 6061-T6 with 4043 filler rod. He didnt report the % elongation results after tensile testing infact he presented a formula to quantify the ductility using bent test. Formula is as under

e= T/ 2A + T

e=% elongation

T= Specimen thickness

A= Radius of curvature on inside of bent specimen.

Sample passed the root bend test with value of 8.13% but failed face bend with cracks appearing HAZ. SEM images of fractured samples are available.
Parent - - By waqasmalik (**) Date 03-22-2020 18:31
Meanwhile,i consulted ASM Handbook volume 8 on Mechanical testing and found this.

"Bend tests for ductility differ fundamentally from other mechanical tests in that most mechanical tests are designed to give a quantitative result and have an objective endpoint. In contrast, bending ductility tests give a pass/fail result with a subjective endpoint; the test operator judges whether a surface has undergone cracking.The bending ductility test developed as a shop-floor material inspection test because of its pass/fail qualities and the simplicity and low cost of the required tooling. As a consequence, the development of bending ductility test methods and apparatuses has been carried out by users rather than by mechanical-test equipment manufacturers".

After reading above, i am of the view that bend test cannot quantify the ductility. Its results is pass/fail. I have emailed the author of research article to share the origin of this formula as he has not refernced it. I will share as soon as he replies.

Every one stay safe and protect yourself and others.
Parent - - By 803056 (*****) Date 03-22-2020 18:53
The formula cited is the one used by ASME and AWS to determine the bend radius.

Al
Parent - - By waqasmalik (**) Date 03-23-2020 05:24
Thanx Al. Great as always.

It means the author rearranged it to calculate % elongation by keeping the thickness and bent redius fixed.
Parent - - By 803056 (*****) Date 03-23-2020 14:13
That calculation only indicates the elongation met or exceeded the value calculated assuming the weld was "sound". It isn't a calculation of the actual elongation. For that, there's no substitution for the reduced section tensile test.

Al
Parent - By waqasmalik (**) Date 03-25-2020 03:25
Got it. Tank you sir..
Up Topic Welding Industry / General Welding Discussion / % Elongation check using Bend test

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