What is the first thing you think when looking at this image? “Oh My !! That’s Awful.” – was my thought. According to the driver, “thankfully it didn’t happen when I was on the highway ten minutes prior.” This catastrophic tongue break is both tongue beams of an aluminum frame trailer. But How? And Why?
A website visitor sent these photos — and I keep thinking about how horrible it would feel. Suddenly, the front of your trailer drops, and digs into the road surface. Oh my! What would race through my mind after a sudden crunch and lurch to hard drag? Thankfully, only the trailer and roadway are hurt.
This is not a simple break. The tongue “snapped”. Both sides are a clean break, starting at the bottom side of the tube, and ripping up the sides. Finally, bending at the top.
As an Engineering Consultant, one of the things I do is Forensic Analysis. That’s the study of failures, to help understand what happens. For this aluminum trailer frame break, I see 3 important things highlighted with the arrows in the close-up photo below.
- First is the PINK arrow. I do not think it is a coincidence that the tongue break starts at the same place the weld ends. On both sides of the trailer.
- Second, the ORANGE arrow. While it is a little hard to see, the material grain structure is quite different at the ORANGE arrow, than at the BLUE arrow. It is also dark. Dirt may darken this area in the crunch, but it looks to me like this dirt is more embedded. Dark material is typical of being there a while. Also, from what I can see of the material grain structure, it is smoother, and therefore more typical of something that happens slowly.
- Third, the BLUE arrow. As we might expect, the final rip of the material probably happened very fast. The shinny grain structure with the rough striations is typical of a rapid material tear.
Aluminum Trailer Break Timeline
The owner says, “my less than 2yr aluminum trailer broke.” So, that is the start of our timeline.
This failure looks to me like a fatigue crack initiated at the weld interface, which slowly propagated both up the inside, and across the tube bottom. The darker area indicates where the crack has been growing for a while. The color getting lighter across the bottom face indicates more recent crack expansion. I suggest that it started a while ago, probably months, then propagated slowly at first, then faster until the bottom outside corner. At the corner, maybe a little before or a little after, the catastrophic and sudden rip up the outside occurred.
It is hard to tell from the images what is happening with the other wall of the tube. The crack propagation may have gone up that side slowly from the weld, but it is hard to say without closer inspection.
This image shows one side of the trailer tongue. The other side is similar, so it is plausible that as a crack on one side progressed, it added stress on the other side creating a similar situation. While it is hard to tell which side started first, it is really not important. This trailer design has both sides sharing the tongue load duties.
Once a crack starts, it does not take so much load in the trailer for it to continue. It is a self fulfilling disaster, because the tip of the crack is a stress riser making extra high stress. This makes the crack grow, and the more it grows, the more stress there is at the tip, until it fails.
Why Did The Aluminum Trailer Break?
Please note: I have not seen the actual trailer, nor do I understand all that the trailer went through prior to the break. I have only a snapshot in time for the aftermath of the event. However, this is not unlike other failures of aluminum trailer frames, so I will speak to the break from that perspective.
As with most failures, it is rarely just one cause. Almost always, there is a series of events which add up to a final catastrophe. So, let’s look at some things which are likely contributors to a beam fracture like this. We will start with a list, then tie it together below.
Typical Material Failure Modes
A failure that will “fracture” a trailer beam is more typical of “fatigue” compared to bending like in this other tongue failure article. It is a difference in sudden overload versus long time stress. It is also a difference in how various materials react when the design is not sufficient for the task.
Along with these photos, the trailer owner also sent images of the cargo inside. Or, should I say the nearly empty trailer. There are cubbies on either side for tools, but it is obvious the trailer was not even close to capacity. So why did it break? And why at that moment? – Because the trailer was not heavily loaded.
When comparing Aluminum with Steel two big differences are ‘ductility’ and ‘fatigue resistance’. (Think of ‘ductility’ as “bendability”. Think of ‘fatigue resistance’ as the ability to withstand repeat loads and resist cracks.) Steel of the type we use for trailer frames, prefers to bend, and generally resists the creation of cracks. Aluminum will also bend, but it is more prone to cracks, especially fatigue cracks, which will cause a break.
Both materials are affected by heat from welding, as in this weld caused failure in steel, but aluminum is more susceptible. That does not mean everything welded will break, but location of the welds is important. Fatigue resistance is a difference in material failure modes.
In our previous article about welding aluminum trailers, we point out that aluminum trailer design requires more skill, more gusseting, and often different material sizes. Weld joints require more support with aluminum, which is a result of the material differences in the discussion above.
Secondly, areas around the welds must be carefully thought through, because we don’t want heat distress in the materials in areas of high stress.
With that in mind, here are 3 things to note about this trailer:
- We see the weld along the bottom edge of the beam, but the weld cannot wrap vertical and continue up because of the acute angle. (Very hard to get a welder in that sharp angle.) That means the highest stress will be right at the point of the angle – right where the weld ends.
- In nearly every trailer design, one of the weakest places is where the tongue meets the trailer frame – as pointed out in this article about calculating loads.
- Cracks start in tension members, not usually in compression places.
Knowing the things above, if the design has a weld on the tension side at the highest stress point, there should be compensation. Either by changing the design, or by making sure the beams are strong enough. ( I truly believe 2 well placed gussets would totally change the behavior of this trailer.)
May inject a personal bias? The cost and weight of a little more material, in the right places, is insignificant compared to the cost, inconvenience, and potential liability of a failure.
Fabrication / Skills Failure
We can’t really judge the welding skill for this trailer. It looks good, and it has held up thus far. However, the fabrication knowledge for aluminum is certainly lacking.
Is it the responsibility of the designer? or of the fabricator? to know where and how to support the structure for welding aluminum? To know if it is safe?
This trailer is made in a factory, so the fabricator is probably following directions. I’m not OK with that. If he has sufficient welding skill and certification to work there, then he should also know what he is doing is tempting fate. He should bring that to the attention of the designer. (And, maybe he did. We don’t know.)
I say “Designer” and hesitate to say “Engineer” for most small trailer manufacturers. I have been hired by some, because they don’t have an engineer on staff, then fired because I show that the design is not sufficient. It is, unfortunately, prevalent in that industry, but am I wrong? Look at the photos above, then decide.
Without knowing a lot more about the design intent, I can’t properly judge the material thickness choice. However, I will say, the 2″ x 4 ” x 1/8″ thick wall as tongue beams on a 7000 lb. trailer — aluminum or steel — seem a little light to me. That’s easy to say now, but obviously the safety factor was not sufficient. (FYI, you need a higher safety factor for an Aluminum trailer to avoid fatigue failures like this tongue break.)
Loading and Use
For a trailer, the rubber meets the road in actual use. We have already established that the trailer was not significantly loaded at the time of failure. However, we don’t know the history. We can assume that the trailer was not frequently overloaded (maybe never), because the tongue did not fail in an Overload manner. Fatigue takes time to propagate, and it does not require high loads to do it. If the elements are all there, it will just happen over time.
We have good evidence that crack propagation in the aluminum trailer frame took a long time to break the beam. The dark, smoother portion of fracture surface tells us that. So, in the end, we don’t know if overload or abuse was some kind of initiator, but there is no evidence to suggest it.
I believe, if overload were present sufficient to initiate the crack, it would also have been sufficient to grow the crack quickly. The failure would happen very soon after — maybe minutes.
Certainly loading and use are important for investigating a failure, but information available does not point to that as a strong contributing factor.
Put It All Together
Now we have seen some of the trailer, discussed the trailer, and we have talked about some of the possible contributing factors. Do we know enough to tie them together?
Rather than trying to assign the exact “WHY” to this aluminum trailer break, let’s talk generalities. Here is a list of what I think of the timeline.
- Design. A strong trailer begins with good, sound engineering design. As with the notes above, missing the small details can make a big difference.
- Material Choice. Aluminum is definitely a factor in the trailer frame break above. It is NOT the “Cause”, but it is a factor. Choice of material size in design is more accurately the chief contributing factor, IMHO. (This can come from not doing the analysis – seat-of-the-pants guessing – or not properly applying safety factor for aluminum and expected use cases.)
- Fabrication Skill & Technique. This is partly a design issue (above), and partly an implementation issue. Look at the trailer in this article about aluminum trailers, then compare beam sizes, and the added extras on the frame. They are both 7K trailers, but there is a big difference in fabrication technique.
What would I do different?
- Design. As in the notes above, I think a couple well placed gussets would make a huge difference. I would support the beam bottom such that the acute angle weld was not on the most stressed point. That might be a taller tongue beam, or a gusset over the area, or a complete design change. Either way, I would ALSO increase the wall thickness from 1/8″ to 3/16″ because that is so easy, and really quite cheap. (And, it only adds a couple pounds to the trailer.) Finally, a shaped cross gusset tying the centers of the 2 beams to the nose would do wonders for distributing load. (Oh, I would surely do the design using proper CAD analysis tools to remove “hot spots” and validate the design.)
- Material Choice. In general I don’t have an issue with Aluminum trailers. However, Aluminum requires an added level of skill and experience, both in the design, and in fabrication. So, if you don’t have both the knowledge AND the skill, then choose steel. From a personal point of view, I prefer steel for all the above reasons. While there are down-sides, we know how to conquer them.
- Fabrication Skill & Technique. I believe engineering and fabrication are hand-in-hand. Engineering defines fabrication, then fabricators must inform engineering. Skill for fabricating with aluminum is not a choice, it’s essential. For me, even with experience, I will not weld an aluminum trailer. I find someone with extensive, proven skill. It is not as easy as many people think. And, unfortunately, true skill in fabricating fatiguing aluminum structures is not so common.
Aluminum is cool, for sure, and there are some great advantages. Just make sure when you buy or build with aluminum, the design and fabrication supports the material choice properly. For design in general, as we optimize for light weight, we must also have much more accurate information and analysis to assure a safety.
Obviously this manufacturer did not take the time to really think it through, and now their customer must deal with the fall-out. That is NOT fair. It might be legal, but it is not right. I hope they will compensate him with much more than just a repair — because the problem started, I believe, at the most fundamental parts of the design.