I remember reading about the 747 and they flexed the wings a ridiculous amount like 50' or something.
Yes the wings flex to total vertical, then they snap, like they were made of a hard plastic, not a malleable aluminum, truly an engineering marvel. This is in a static test (bend until it breaks). In the dynamic testing they are flexed less distance, but many many times (for weeks).
A couple notes to tie this conversation back together. Thanks Dan, yours was a good, sensible intro to metals that (I think) people can understand, and congrats you kept it short. I'll try to take it a little farther for those interested.
I think when you have a dynamic load you have to consider fatigue failure as well as regular failure. With a static load, you just figure for regular failure.
Consider a semi flatbed trailer.
Put a load on it an park it.
That's a static load. As the load is set down the frame flexes, then stops. It will hold that load virtually forever.
Now take that same load/trailer and drive it hard over rough roads and you now have dynamic loading.
The frame is now flexing over and over as it is bearing it's load.
If the peak stress exceds the fatigue limit of the trailer, small micro-cracks will form and grow, resulting in eventual failure. This can take a long time. Or if the stress is great enough, it can fail rather quickly.
Some metals don't have a fatigue lmit. Any/all flexing takes life off the part.
Steel has a fatigue limit. Flexing below this limit can go on virtually forever.
Aluminum for example does not have a fatigue limit. It grows cracks whenever its flexed, therefore must be "retired" at some point. A good example is an aluminum airplane, when it's gone thru a certain number of cycles (pressurization upon reaching altitude, then de-pressurization upon landing, for example) it is simply retired, never to fly again. If the plane was made with thicker aluminum, it would last longer. If it was not pressurized with every flight it would last longer. If flown with lesser weight onboard, and never in rough weather, it would last longer. That aluminum must be re-melted to be re-used. There is a reason you never see aluminum springs.
Steel has a fatigue limit. If you flex it
below its limit, it can repeat that virtually 'forever'. An example is the valve springs on your car engine, they are flexed below the fatigue limit of spring steel and can flex (at that loading) forever. An 8 hour day of driving flexes a valve spring 1 million times. They can hold tension forever. But a steel element that is flexed BEYOND its fatigue limit suffers permanent, cumulative 'wounds' and will reduce in strength, and can fail under lesser future loads.
Stress risers can increase the stress at one specific location, causing problems. Lets say you have a trailer hitch "torsion tube" that is designed to flex a little and thus protect all of its mounting points by lessening road impacts (almost like a secondary 'suspension'). It is flexing continuously below the fatigue limit of that specific steel and can do this forever. If you weld something out in the middle where it flexes, you just created a hard point, and the material just adjacent to that hard point has MUCH higher stresses, perhaps beyond the fatigue limit, then cracks can start. If your weld is undercut at this hard point, even worse.
So back again to the Reese trailer hitch, which Teikas Dad has decided NOT to weld on (I think). I suspect TBN members are curious, they want to know, they want to build, create, increase understanding. As I said before, either reduce the load on it by adding structure, or increase the "torsional section" of that member (reducing the twist on it). Here are a couple ways to strengthen it. Method 1 is an OK newbie project (if the welds are good). Method 1 could even be bolted, which may even satisfy Mr Reese.
If it's not possible to add more structure (spare tire in the way etc) and you MUST weld on it then do it like this. These are methods to increase the "torsional section". The two crossed out do not increase the torsional section (not much) because they are not "closed". They increase bending strength but not torsional. You can skip weld this, but they should be good welds. But trailer hitches should always have good welds, right?
Well this thread has run its course I think, so I hope nobody minds my typing a book onto the end of it, but it was a good example.
