All that is stated to explain that the trailer is a separate stressed element. This allows the tongue to be evaluated as a simple lever. I will try to setup a quick spreadsheet for the PSI/length calculations.
In short there is a doubler at the front edge lever point that extends ~14" on each rail. There is also another cross brace that is not yet added because of some tongue box fab that I am waiting on.
Debraizhot wrote:I believe the thrust of your questions cover two points:
a: Cantilevered beam vs simply supported beam with a concentrated load
b: Stress cycle resistance before cracking
For the sake of all that may read this I may cover some old ground, I know from the work I have seen on your trailer you already know most of this.
For question a: I believe either method can be used. The reason I selected the cantilever is that all bend stresses are comprehended in reference to that single element.
If I used the beam method I would need to address the actual joint itself. That is probably a better approach, but I tend to default to simpler methods for complex problems.
After looking at the Lisa program, I see your reasoning.
For question b: Yes I had the same concern. The big issue with trailers and other road structures cracking is the use of welding. Take a look at this video from the 1 minute mark on.
https://www.youtube.com/watch?v=7_q8dXhewh4
Wow, you wonder what he hauled on that trailer. I did note that he never mentioned the tongue though.
What you see is the cracks all form in the HAZ (heat affected zone) and are the result of welding. This is why so many Aluminum trailer manufactures are moving to VHB tapes and other products even over rivets. As for the problem of cycle stress fractures that can occur even if welding is avoided-I would make two points. First much of how this is avoided is the result of heat treating and artificial aging. It may even be fair to state that the T6 as a treatment is the result of the needs of aircraft to resit many cycles without cracking.
I don't weld across the top of the steel rails for that reason & can see why, with aluminum, using rivets, bolts or clamps, as you have, would be a better choice than any welding at all.
For most aircraft the design limits are somewhere between 25,000 and 80,000 cycles. In fact there are commercial aircraft for sale today that have more that 75,000 cycles. I would suggest those numbers are beyond any stress that recreational vehicles would endure. But there is the issue of dynamic loads and you are correct the spreadsheet models only statics. So a simple method would be to take the maximum G-loads from road stress and design for that level. Except that a typical pothole exposes most vehicles to acceleration of 50-80Gs. What happens in real life is that those stresses occur through the tires/axles/suspension and are filtered/dampened before they get to the other structures. In addition the time period of the impact, although a very high in load is very short in time. All of these things tend to reduce the actual impact to the rest of the structure. A good example is a rifle. Consider that when you fire a large caliber hunting rifle the force that is applied to the shooter and the force at the muzzle are the same. The only reason the shooter is not harmed is that the force is spread through 8-10 sq inches of contact area and the force is dampened by the mass of the rifle itself. If the rifle is a semi-auto, the bolt and gas/springs further dampen this load over time. So a load that is clearly lethal becomes very safe.
Still a conundrum for me. After reading the DTD handbook page, however, it occurred to me the it would be interesting & useful to attach sensors to the tongue in order to measure G-forces, deflection & cycles/time graphed out over different road conditions.
I believe though what you are getting at is this (is there a simple way to KNOW a particular structure is light and safe for all/most conditions?). Yes there is, but it requires higher order math and is not something that can be implemented easily in a spreadsheet. I am sure is it possible to integrate some dynamics in spreadsheets. I have seen some that have used fourier transforms and mohr's circle in spreadsheets. But most would just use some FEA(finite element analysis) SW. If you have an interest in this area take a look at Lisa or VisualFEA. Both are free for limited structures.
http://lisafea.com/
http://www.visualfea.com/download.htm Over my head!
In reference to the practical design elements of strong design take a look at the "Handbook for Damage Tolerant Design". This is a great resource you may find helpful.
http://www.afgrow.net/applications/dtdhandbook/sections/page9_2_1.aspx
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