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."
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.
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.
|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.|
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