Reliability Engineering Snapshot TM

Illustrated Case Studies in the Industrial World of Failure Analysis, Non Destructive Evaluation, and Predictive Maintenance

 

Material Properties

Case Study No. 91:

Every day, somewhere in a plant this size, there is a weld giving way. At least 50% of all those welds that fail are caused by a lack of fusion. The problem is always caused by improper surface preparation or wrong amps. For those people wanting to minimize the chances of this happening, there is a relatively inexpensive method of testing welders in-house by using what is referred to as the weld "bend test."Guided Bend Test On Weld - Traverse Bend

The test doesn't have to be done in-house. There are plenty of testing labs that perform many weld test functions, one of which is the bend test. However, it can be done in-house if desired. The jig that is used to perform the bend test can be constructed. The construction drawings are contained in the AWS D 1.1 specifications.

The picture to the left shows a 3/4" thick weld bend specimen. The weld was bent traverse to the direction of the weld, in a "U" shape. The picture shows the outside of that "U" shape. Notice how clearly, and clean, the fusion line appears. Also notice how there aren't any tears at the fusion line or anywhere throughout the cross section of the weld. This is very important because it means that 1) there is complete fusion, and 2) the weld is ductile. If the weld were brittle because of the many weld passes laid down in the course of welding, or if there were a lack of fusion, the weld would tear.

Weld Bend Test - Face

The specimens can also be bent along the weld axis, either with the root or the face of the weld on the outside of the bend. The picture to the left shows a "root bend" test. The root of the weld is located between the white arrows in the picture. Notice how there aren't any tears. The root pass is the most critical weld pass of them all. If it tears, everything else follows suit.

When welding something very critical that doesn't have a "pre-qualified" weld procedure, it is invaluable to perform the bend tests described above. It is also prudent to perform tensile tests on the weld. A tensile test is shown in the picture below. Outside testing labs have to do this test. This test was done to qualify a welding procedure on a large vessel (see the August 2000 articles for MACHINE DESIGN and MATERIAL PROPERTIES). If the weld is good it should fail at or above its ultimate tensile strength. If it fails below its ultimate tensile strength then something is wrong with the weld procedure and further investigation is warranted.

Tensile Test - Weld Specimen
The two tensile specimens shown above failed above their ultimate tensile strength. They failed in the heat affected zone of the weld, meaning the weld itself was stronger. Notice how the bottom specimen shows very clearly a "necking down" at the fracture zone. This means the material is ductile and was not adversely effected by the weld procedure.

 

All Pictures and Text Copyright © 2001 - 2016 Contact Mr. Adler Adler Engineering LLC of Wyoming USA

Great care has been taken in the compilation of this article. However, no warranty, expressed or implied, including without limitation, warranties of merchantability or fitness for a particular purpose, are given in connection with this article or any article archived on this website. Although this information is believed to be accurate by the author, the author cannot guarantee favorable results will be obtained from the use of this article alone. This article is intended for use by persons at their sole discretion and risk. Since the conditions of product or material use are outside of the author's control, the author assumes no liability or obligation in connection with any use of this information. The author is not liable for special, indirect or consequential damages resulting from the use of this material.

No part of this article or any article archived in this website, or any part thereof, may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, without the prior written permission of the copyright holder R. H. Adler. Nothing contained in this article or any article archived in this website shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters of patent, copyright, or trademark, and nothing contained in this article or any article archived in this website, shall be construed as a defense against any alleged infringement of letters of patent, copyright, or trademark, or as a defense against any liability for such infringement.