I love seeing failures. Not that anyone wants things to fail, of course, we always try to make things that last. However, when a failure pops up, I love to look it over and understand the “how” and the “why”. It’s an excellent opportunity for learning, and I love to learn. In this case it’s a failure that highlights welding strength and what happens in the periphery. Let’s see what we can learn.
This example is one worth sharing because it’s a real life illustration of the theory we talk about.
This part started life as a mounted trailer drawbar receiver. This is a typical 2″ receiver to accept a trailer hitch drawbar. It’s just like the ones on the back of a car, but with a mounting plate. The plate (once flat) welds to the receiver, then bolts on.
While we don’t know the exact dates, it seems to have functioned perfect for something like 15 years. Then the duty changed a little. Now, as you can see in the images, at some point, something went horribly wrong.
It’s like looking at a crime scene and trying to pull together the facts of what went down. We see broken and twisted bits of what was once a single flat piece of steel. We see witness marks of the welding (many years ago), and witness marks from the bolts that held it. That’s the start.
Looking a little closer we also see that some of the “tearing” or fracturing of the material happened fast, and some happened slow. How do we know? In metallurgy a fast break in steel leaves a granular look, often dull in appearance, but clean. It looks kind of like very fine sand.
On the other hand, a slow break (like fatigue failure) usually has some shiny areas because the inner portions of the crack fret against each other making the surfaces a little more shiny and a little less grainy. It may also look a little dirty since it takes some time.
I’m not an expert, so I won’t elaborate. It’s fascinating nonetheless to examine the members and see where the cracks probably started, and to see how they likely progressed. Super interesting.
For this sample, it appears one crack started near the end of one weld. Progression was likely past the weld to the outside edge of the plate. Once the initial full fracture happened, the rest probably went pretty fast — likely continuing fatigue near the welds on the other side too. With one side mostly broken, a lot more stress on the other side made the break happen faster. Even with the added stress, the failure appears to start near the edges of the welds.
This mangled metal took some serious force as it tore. Impressive.
Welding Changes Material Strength
We have discussed welding results in previous articles like this one about welding brackets on a trailer frame and this one about building with aluminum. It’s one thing to talk about how welding affects strength of the base material, and quite another to see the outcome after stress.
You might look at this failure example and say “yes, but the unwelded areas also broke”. And, you are right.
You might also look and say “the weld didn’t break”. Again, you are right.
While these are both good observations, it appears the fracture started in the fringes by the welds. As we note in the previous articles, the primary area of concern is the material right near the weld. Not the weld itself.
Truly, a weld increases thickness of material locally, so it is actually stronger. Unfortunately, the area right around the weld also has the extreme heat, but it does not get the benefit of added material for strength. Also, stress is induced in the material at a “boundary” of where the “hot” and “cool” And, we see that in this failure sample. It’s not that the material is weak, just that welding as locally changed the strength properties making it more susceptible to fatigue, cracks and breaking.
In every experience, it’s good to think about what we can learn. Here is a sample that felt good, and for 15 years it performed well. The conditions changed a little and that caused (or at least finished) the failure. In this case, nothing of consequence was damaged, but we don’t want a failure again. So, what can we learn?
- For high stress elements, think about welds that are in a line and the possibility of propagating cracks along the weld lines.
- We like good weld penetration as that makes it strong. On the other hand, the heat stains here indicate the weld heat was perhaps a little too high. When we have high through heat like this, we need to think about potential welding strength changes.
- Stitch welds are intended to spread heat distress so the material strength is not as affected by the welding. Hard to say here how much that helped, yet judging from the marks, it did help.
- 15 years is a long time in service. Goes to show that even when it works for a while does not mean it’s invincible.
- I think the base plate material was a little too thin for the application overall. This is a case of being right on the edge of good enough / strong enough.
- The small change in service was a little more than the bracket could hold. It was designed for one application, which it did fine, but the change was too much. We need to watch how we make changes, then make sure the parts are up for it.
Professionally, just as well as in DIY, we always do the best job possible — as I’m sure you do also. Keep your eyes out for things to learn so you can always improve.
Solution – Welding Again For Strength
The example here illustrates the efficacy of a concepts we teach. Welding does change the material, so moving the welds from the direct stress points is a big way to help. Another way is to use a buffer (thicker) material directly by the weld, and to support the weld area. Look again at our recommendations with welding brackets on a trailer frame. You’ll see the same fundamentals with this solution.
How about a few photos of the revised receiver plate? Now, the main welds are no longer in line (on the same piece of metal), and the loads are through thicker material. The receiver welds are now at the ends as well as on the sides. Most of the welds are now on the thicker members, and they share loads all the way from the receiver to the bolts. The plate material will still cover as needed, and it combines the loading, but it is not the main weld member any longer.
In 15 years we’ll see if this works or not, but I personally think it’s a much stronger design. Could I have just used a thicker piece of material for the plate? Sure. That would also work, though it would be heavier. I have to admit, however, I didn’t have a thicker piece that wide, and he needed the fix right away. This idea works as a good way to optimize the design a little too. Having the welds on separate pieces certainly eliminates the possibility of propagating a crack all along as before.
Hopefully there is something here to take away as we highlight an example of strength changes in the material from welding.