Design to Prevent Aluminum Trailer Fatigue Failure

Design is the beginning of every project – true for both the good things and the bad.  When something breaks, we often say “That is a bad design.”  The key to prevent aluminum trailer failure starts with understanding stress, and hidden factors that quietly add up to a big problem.

In the previous article, we saw a catastrophic aluminum trailer tongue break – dramatic, expensive, and startling.  This follow-up looks at the other side of the story.  How can good design, smart fabrication, and a few strategic reinforcements stop fatigue failures before they start?

Aluminum trailers are light and strong, but they can also be unforgiving.  Small details in design and fabrication (well in advance) can turn a good trip into a day to forget.  In this article we will explore some of the engineering.  We will talk about fabrication in aluminum, beam placement, welding, and material choices – using a practical side to design for strength, where to weld, and what you can do on an existing trailer.  The goal is helping prevent aluminum trailer failure.

Aluminum Fatigue and Failure?

Why are we talking about Aluminum Trailer Fatigue?  Don’t all metals fatigue?

Yes.  Every material has its own characteristics – often called material properties.  Some are soft, some hard (rubber versus glass, for instance).  Some are light, while others are heavy (air versus lead).  Indeed, some bend easily (like paper), while others would rather break than bend (like stone).

For a trailer, we often compare Steel versus Aluminum.  Yes, Aluminum is lighter, but it does not resist fatigue as well.  A simple way to illustrate:  Take two similar pieces of metal, one steel, one aluminum – perhaps 1″ wide by 1/8″ thick.  Bend both once – no problem.  Now bend them back, and do it a few times.  The tighter the bend, the more intense this example.  Aluminum almost always breaks well before the steel.  (Of course, it depends on the alloy and other details.)

The point is, aluminum and steel behave differently.  Steel is generally more forgiving under repeating stress.  The bending example is exaggerated for clarity, and we hope the beams in your trailer never actually bend, no matter the material!

Yes, all metals fatigue, but they manifest it differently.

What is Fatigue?

Simply put, fatigue is slow micro damage of repeating stresses, over and over.  Each stress is below the level to cause a bend, yet the material still “feels” it at the micro level.  Over many cycles, tiny internal changes accumulate until the metal behaves as if it were brittle, then cracks, and finally breaks.

Fatigue is a function of stress cycles – loads that repeat – like going on a bumpy road.  The higher the stress, the fewer cycles it takes before cracks appear.  That is why small design improvements have big life impacts.

There is much more to it, of course, but that is the essence.  If you want more, please see our other articles about aluminum trailers or this article on Wikipedia.

Again, aluminum is not bad.  We have said it many times.  It means we must design for it, differently.  The design must consider the lower endurance limit of aluminum, and interaction with welding – the reasons cracks form even under normal loading.

Now, coming back to trailers – this is why we are discussing ways to prevent aluminum trailer failure.  The same principles apply to steel, but on a smaller scale – because steel behaves different.  The science is technical, yet our goal here is to apply it practically.

What Is Stress Concentration?

One more engineering concept to know:  Stress Concentration.
This refers to areas where stress is locally higher than in the surrounding material.

A simple example uses a balloon.

When you press your finger into the side of a balloon, stress in that small area rises.  The material stretches more right where the pressure is applied.  If your fingernail is not sharp, the balloon usually handles this localized extra stress fine.

(Hopefully like the picture.)

Now replace your finger with a pin.  You don’t have to push it very far before the balloon pops.  The tiny point concentrates stress so intensely that the balloon fails almost instantly.

That is stress concentration.  This same principle causes cracks to form in sharp corners, near welds, or near bolt holes.

Of course, we are not poking the metal, yet each trailer frame has areas where stress is locally higher just due to the geometry.

In our trailer frame example, the point of stress concentration is where the surface of highest stress face meets the weld.  Label A below.  Of course, we are not poking it, but the trailer function puts that area in tension, and the weld interrupts the nice “flow” of forces in the beam material, making it a point stress concentration.

Now we will see how design choices make a difference.  We will illustrate with the example of the aluminum trailer tongue break in the previous article.  The content below will make more sense if you read that article first.

Examining The Failed Aluminum Trailer

Let’s start with the trailer that failed.  Before redesigning or reinforcing, it helps to see the problem clearly.  The image below shows the original aluminum trailer frame layout, as best I can deduce.  For clarity, I have kept only the most relevant parts.

Since the tongue beams broke from the bottom, we will look below.  Next is a closer look at the beam intersection at the point of the break.  (Before failure, from the underside of tongue beam.)  The orange areas represent welds in the affected zone.

The Purple surface is the high-stress portion of the beam.  The highest stress point is A, which is the first location where the tongue beam connects to the deck frame.  (Also the location of the tongue beam failure.  See the previous article for photos.)

We have 3 things colliding at A.  First, this area naturally experiences higher stress because of the tongue load.  Second, the beams intersect, which means transferring loads, and stress.  Then, third, welding.  None are bad alone, but in combination, there is a challenge.

Since the example trailer was in use for a while, we know the normal stresses are below the material yield point, likely well below.  However, as in the previous article, welding changes things.  See our other article about how welding alters the material in the heat-affected zone.  It is true of both steel and aluminum, though more pronounced in aluminum.

To be successful with a trailer frame like this, we must reduce stress, or increase strength, or hopefully both.  How do we do it without introducing a compromise in some other area?

Here are some design variations to prevent weak points, and thus prevent aluminum trailer failure.

Engineering Design Rules to Prevent Fatigue

In concept, these are the most important factors in a design to mitigate the effects of fatigue.

1. Choose correct beam size and wall thickness.  (For example, changing material thickness from 1/8″ to 3/16″ as suggested in the previous article).
2. Use thicker material in areas where stresses are higher.
3. Avoid abrupt geometry transitions in high-stress areas.  (This minimizes places for stress to concentrate.)
4. Keep welds away from high-stress faces whenever possible.  (There are tricks to do this as you can see in the example of welding spring brackets.)  Almost always, we say “Do not weld across stressed beams.”  That is true for tension and compression.
5. Add gussets or other stiffening members to spread the load.
6. Use safety factors appropriate for the both the material and the environment.  (Aluminum in a loaded vibration environment.)

That is the theory.  Now, how do we apply it in a practical way to prevent aluminum trailer failure?

Side Note To Design In Aluminum

The concepts below for design to prevent aluminum trailer failure are specific to this example, and only to the tongue.  In other articles we mention the need for special design in aluminum trailers, and this is one.  There are many.  Proper application depends on geometry, trailer goals, and use.

Please use this info as a consideration, not as full aluminum trailer design.

Prevent Aluminum Trailer Tongue Failure – Change 1

The simplest design change is the most obvious.  Change the material thickness from 1/8″ to 3/16″ for the tongue beams as suggested in the previous article.  The intersecting beams do not need to change, since they carry less load.

I get it.  There is an attraction for keeping trailers light.  Yet, lightweight can go too far, and the cost of “too light” is very high.  A broken trailer ruins much more than one trip.  A small amount of material – only a few pounds – can make all the difference in both strength and fatigue life.

Ah, but thickness alone is only a shortcut if we stop there.  Yes, a thicker beam will improve strength, even fatigue resistance, but why leave the stress concentrations and weld issues unchanged?  A thicker beam by itself may or may not prevent a failure.  It might simply delay it a few years.

2.  Move The Welds To Reduce Stress Concentration

How do we change the stress concentration?  Here is a very simple example.  The tongue beam moves vertically, slightly to a position 1/2″ to 3/4″ from the plane of the other beam.

Note the vertical offset, below the V-Nose beam at B.  In this case the V-Nose beam has a notch at C.  This small vertical movement places the welds away from the high stress surface (purple), which also shifts the stress concentration point off that surface.

The offset does not come free, however.  We must manage the top side of the beam as well.  Here are 3 ways:

• Notch the crossing beam as in the image above at C.  While this may seem easy, it puts a weld across the top face of the tongue beam.  Sure, crack propagation is far less likely for a beam area in compression (like the top of the tongue tube).  Yet, this approach introduces a new risk, and the solution can become its own new problem.  See Rule 4 above.
• For a better approach, put stress-distribution bars D over the tongue tube to fill the void.  Use material (green) that is generally the same thickness as the tube.  Stitch-weld these distribution bars along the edges only.  This method strengthens the area immediately above the beam intersection, and distributes tongue stress to make the beam stronger.
  
• Use a taller tongue beam.  The failed trailer uses 4″ tall beams, so a similar design would need a 5″ tall beam (the next size up).  This works, but is likely overkill.

3.  Prevent Aluminum Trailer Failure Using Gussets

In the previous article, we mention gussets to tie the tongue beams to the nose.  The concept is illustrated below with 2 examples.

• First, the blade gusset in Orange, with the label E.  Use material that is much thicker than the tube walls.  Make the gussets longer for greater effect, but not too much because excess length will cause twist in the beams.  Do not come to the top or bottom surfaces of the beams.Shown with no offset at C.  Also, do the calculations or FEA to know how it will work for your situation.
  
• A second method uses a block-style gusset made from a short section of tube cut to fit.  In the image below, it appears in a yellow color, in two views, with the label H.  It is shown with the beams having a vertical offset C as mentioned above, but it also works without the offset.
  

For both gusset styles, the purpose is to tie the tongue beams to the trailer frame nose members.  The gussets carry only a portion of the tongue load, effectively shortening the tongue lever arm.  They do not extend to the top or bottom faces of the beams, to avoid welding on those high-stress surfaces.

See also the image at the top of the page.  This block style gusset also works nicely in combination with other fixes.

Improving an Existing Trailer (Retrofit Strategies)

The discussion above is great for a new design, but what about a trailer you already own?  If you have one similar to the trailer that failed, what can you do to reinforce the tongue and prevent aluminum trailer failure?

Excellent questions!

First, before anything, inspect the existing trailer beams carefully for hairline cracks.  Do this with the trailer hitched and the tongue under load.  Cracks are easier to see when the beam is in tension.

If cracks exist, stop-drill at the ends of each crack, then weld to repair it.  Have this performed by a qualified aluminum fabricator.

Improvements To Prevent Aluminum Trailer Failure

Here are a couple of practical fixes for owners who already have this style of aluminum trailer frame.  However, it is not as simple as it may first appear.  The existing beams must be properly prepared, and often, it can interfere with existing enclosure shrouding.  That takes some planning to work around.

Please have a skilled aluminum welder do the welding.  Although it may look straightforward, these ideas are not a garage hack job.

1. The first concept is the block gusset above with the H label.  When properly installed, the gussets will drastically reduce stress at the concentration point A.  In a retrofit, you will not have the vertical beam offset as in the image above.
2. The second concept introduces a sister beam under the tongue.

This image shows a sister beam concept to prevent aluminum trailer beam failure.  See the introductory image at the top of the page for another perspective.

The sister beam here is small C-Channel, ideal for the cross-section material distribution.  Stitch-weld it, K, along the intersection lines (both sides).  However, a sister beam can introduce new stress concentrations near its ends because of the abrupt geometry transition in the high-stress beam.  (See the design rules above.)  That is why the sister should be long enough that the ends terminate in lower-stress regions of the tongue beam.

The beam stress is mostly in shear near its ends, while the primary loads are bending through the middle.  For more about how this works, see this article on beam loading.)  As much as possible, we want to maintain a smooth beam cross-section through the areas of highest bending stress.

Design Verification and Inspection

Once any retrofit is complete, inspect it periodically to make sure everything continues to perform as intended.  Cold welds in aluminum are common, so watch for separation or small cracks around the welds, etc..  The easiest way to spot cracks is to look for dark lines.  Dirt settles into cracks, so after wiping the surface clean, take notice of any dark lines.  Inspect these carefully to see if they are scratches, or cracks.

As you gain experience with the retrofit, inspection intervals can expand.  However, with an aluminum trailer, it is good to inspect at least once a year.

Finally, as we say for all trailers, don’t overload it.  Keep proper weight distribution in the trailer.  If we want to prevent aluminum beam failure, it stands to reason – we should avoid excess loading.  That also includes excessively bumpy rides.

Hopefully you find value in this and the previous article.  While aluminum serves as the primary example for this article, the same principles and rules also apply to other materials and equipment.  To prevent trailer material fatigue, and eventual failure, whether in aluminum or others, design with fatigue resistance in mind.

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