Root and Face bend tests
Root and face bend tests are another simple low cost method of testing. It gives very simple to understand results and will show any signs of poor fusion or weaknesses such as porosity within the weld. There are numerous variations on this method, we will look at one of the most simplest methods.
Whether the sample piece is bent root up or root down decides whether it is a root or face bend test, with the root on the outside of the bend, in tension that would be a root bend test.
Once a suitable section of the weld is selected, it is prepared for testing. The test piece is then put into a bending jig and force applied to it directly over the welded area. The piece should bend around without cracking. A crack would show a weakness of the weld. A neatly bent strip would show the weld is as strong as the parent metal. This is another testing method that is suited to students learning welding due to its ease and low cost. When used in recorded circumstances, a test procedure would be issued, specifying the details such as radius of the punch used and degrees it needs to be bent to. Another version of these tests is a side bend, as pictured below.
Preparing
The first step is to select a suitable section to test. These are best taken from a start/stop position, central 50mm of a butt joint, and not directly from either end of a larger run, the sides of the strip to be tested need to be fully welded and square. Remove any high spots of the weld before cutting you test strip. It is also a good time to mark the root and face side, either marker pen, or by hard stamping.
Cutting the strip can be done by almost any process, sawing, flame cutting, or abrasive disc. The strip needs to be narrow enough for you to be able to exert the force, around 25mm wide seems to work well for most simple tests.
Cut the strip, and de-burr the edges, taking extra care on long edges, this is an important step, or it may create false results caused by normal bending cracks that occur at sharp edges. The test piece should be allowed to cool naturally between each step, and not quenched, as this can make the heat affected zone (HAZ) brittle.
Bending
Once the test strip is prepared, you can set up your bending jig. A small hydraulic press or fly press is adequate for this purpose. The top punch should have a radius of approximately twice that of the material thickness being tested. The die needs to have sufficient clearance to allow the top punch to pass through it, plus twice the material thickness. For example, for testing a 5mm thick sample, you would use a punch with a radius of 10mm (diameter of 20mm). Then the bottom needs an opening of 30mm.
Align the test strip in the tool, with the weld centralised between the bottom die, then apply pressure to bend it to 90degrees, further if you so wish.
The results
This is fairly self explanatory really, if the weld doesn’t break, or show signs of cracking, its good. The strip should show a nice even radius, if it has a sharp angle change mid way, this would be down to the weld beginning to fail. If the weld does break during testing, you will need to see where it has broken.
If the strip broke into two, with virtually the entire weld still on one side, that would point to undercut, or poor fusion.
If the weld had broken into two, look carefully, you may see signs of a slag inclusion, or porosity, which has weakened the weld in-line with the defect.
The images below show an example of a passed bend test, the root is at the top of the picture, and in tension. The weld has shown no signs of defects in this case. In the event of a failure you would expect to seethe edge of the root peeling away from the parent metal. You will notice the face of the weld has been ground flush, this is to allow the sample to be bent evenly.
Thanks to forum member, Knoba, for the images on this page.