Quote:
Originally Posted by old greg
Thanks for the shout out. 
It's "Ah-Crop-Uh-Vich" btw.
Anyway, your test isn't really applicable to fatigue life for a couple of reasons. The big one is that fatigue strength @ half of a cycle is always going to be the same as ultimate tensile strength. The issue you are actually worrying about is that there are little bits of titanium oxide/nitride etc. in the filler and HAZ that are acting as miniature stress-risers in the grain structure of the weld. When you load that material in tension, teeny tiny little cracks start to form. When you load it in tension over and over again those cracks start to grow and, over time, cause the metal to weaken. That's something you can't simulate just by squishing stuff in a vise. The other issue is the way you are loading the material. Bending it along the tube wall isn't really representative of the loads an exhaust will see in service.
The way to test it, imho, would be to have a piece of 1" solid bar, bored out at one end to match your tubing and welded to a ~12" length of the stuff. Clamp the solid end in a vise and have an electric motor with an eccentric weight at the end of the tube to provide a fully reversing load. Then run the motor and see how long it takes for the weld to break. Repeat the test at least once with a different load... maybe just cut away the broken stuff and HAZ, the reweld and test at a shorter length. Load vs. Fatigue Life should be a straight line on a log-log plot.
Failing that, the best thing you can do to prevent fatigue failure is use a flex section. Basically, give the tubing room to move so that you minimize stresses due to expansion and contraction. |
The weight used would be proportional to the intended weight of the exhaust? The loading that it's subject to is basically itself moving around from inertial and vibration, right?
We're also hoping that slip-fitting it will reduce some of the expansion issues.
Quote:
Originally Posted by Homemade WRX
without watching the whole video, did he ever comment on what grade he was using? I'm guessing Gr5...really common and does well with heat.
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That's part of the problem. It's mystery titanium. ('He' is me, btw...) I'm not sure if it's Commercially Pure or 6 Al 4V (Gr5?) or what. Apparently it was scrap from a company that does fancy heat exchangers. On Burns' website it sounds like CP2 is what they use. I'm still trying to sort out all the different terminology for the grades, as it doesn't seem to be as universal as aluminum or SAE steel yet. Some are the 'Grade' as you mention, some refer to the alloy, 6-4 for example, aircraft stuff has Alpha, Beta, and whatever the C stands for, with a minimum yield number, C100 for example.
Also for Serialk1llr, aluminum exhaust stuff on Burns' site:
http://www.burnsstainless.com/yieldstrength.aspx