Unlike steel, aluminum does not have a lower stress level (other than zero stress) where fatigue is not a factor.
To say it another way, aluminum is always going to fail by fatigue if it is subjected to any cyclic loading no matter how low the stress is.
Even if the original welds were perfect (Right!), you can expect fatigue to cause cracking. Even once the frame is repaired, it will crack elsewhere. The aluminum frame has a finite fatigue life after which the battle against cracks is on going, never to be won.
Fish plates, cover plates, call them what you may, will only transfer to load to another area that is near it's endurance limit and the cracks will initiate in the new area where the stresses are highest.
I'm not saying you are wasting time, effort, and money, just be aware that aluminum is going to crack at some point in time once the fatigue limit has been reached.
The other responses regarding knowing the alloy, proper selection of filler metal, etc. are all true. However, don't overlook the importance of proper cleaning and joint preparation. Most of the problems I've seen with aluminum go back to cleaning and preparation.
Another item to consider, finishing of the welds. Avoid notches like the plague. Grind any CJP groove welds flush. Concave fillet welds perform better than convex fillets, etc. Any porosity detected should be excavated and rewelded. Any welds ground to improve the surface contour should be lightly tapped with a wire brush. The tapping will reveal the porosity as slight rounded depressions. You have to look close to see the, but its effort well spent. You can "pop" the depressions with a sharp scribe if they at due to porosity (and they will be!). Intersecting welds (x, y, and z axis) can result in stresses that easily exceed the tensile strength of the weld. Intersecting welds are prime location to examine closely for cracks.
Good luck - Al
Thank you all for the great responses. In the book "Welding Fabrication and Repair" by Frank Marlow there is a good chapter on repairing borken truck frames. It describes all of the reinforcing methods that have been described on this thread. It suggests bolting a plate tight against the web of the C-channel and welding stiffeners along the flanges. As long as the bolt holes aren't too large and they are drilled in the neutral axis of the member, it shouldn't weaken the channel too much, right? I am trying to figure out how to do as little welding as possible. This trailer was not designed by an engineer, and it appears to have been welded by someone who mostly knew what they were doing. While I agree with the comment about the trailer being underdesigned for the loads it is experiencing, the crack is on the TOP flange, not the bottom one, which is where I would expect it to be. I work for a whitewater rafting company where this trailer is driven on sketchy West Virginia roads with me and my fellow raft guides riding in the back of a stake bed truck which is pulling the trailer. I don't want to be in the truck if this trailer fails on the road, and buying a new one is out of the question, so I am trying to convince my owner to let me fix it. And I only charge him $20/hr!
The middle of the web [neutral axis] in theory is loaded only in shear IF the load is a force parallel to the web. That will be the safest place to drill if You are using bolted repairs. I am unfamiliar with the book You have, but You are most likely better off folowing that advice than disregarding it. Might the crack have started from side loading, like if someone jacknifed it backing up? If it is built from 6061-T651, a common extrusion alloy and heat treatment, it is unlikely to bend [permanantly] much before failure.
If this material is the tapered flange type C channel, it is likely 6061 T6 alloy.
Hi Al!
first of all , No Ofense there Devo but, sometimes we overlook the fact that this gentleman named "Devo" may not be privvy to all of the nomenclature we tend to unknowingly share with him in this thread.
So my question to Devo would be: Do you understand all of the acronyms, terms, that are being used in this thread wiht the ultimate goal of passing along suggestions that will help you repair your trailer???
Respectfully,
Henry
I appreciate your concern Henry. While I am relative novice when it comes to welding (6 months rebuilding coal mining equipment and another year of repair work/odd jobs), I have taken a couple of courses from Lincoln Electric and I have acquired a decent library from which I have gained a lot of good info. Just for fun sometimes I like to try and read the research supplements in the welding journal, but usually they just make my eyes cross. So far, everything posted on this thread has been making sense to me, but I am never afraid to admit when I am clueless. I have found that once I admit my ignorance, a great wealth of knowledge will come my way. I even used trigonometry on a recent welding project! Never thought that would happen. So thanks for the advice and keep it coming.