Discussion in 'Tube and Fabric' started by wsimpso1, Nov 4, 2019.
Look at that. Nice table design. We must have gotten our data on strengths from different places...
I started with data from https://www.makeitfrom.com/material-properties/Cold-Finished-4130-Cr-Mo-Steel
I've reverse-engineered your materials, I think :
4130 : Sy = 63,000 ; Ss = 36,400 ; E = 30,000 ; D = 0.282
2024 : Sy = 39,000 ; Ss = 22,500 ; E = 10,500 ; D = 0.110
Sorry to nitpick, but re your latest update:
4130 bending moment (1558 in.lb) appears to be at yield, not yield/FOS
2024 1x065 :
weight appears too low (matches 1x035)
I get slightly different tensile (4964 vs 4953) and nearly double the 60" column (402 vs 237) : but that may be a bug in my ss
Latest version, with lots of ally sections and Billski materials props.
When Billski and I get the same results, I'll move on to square sections
I will check.
I'm not meaning to hassle you - I'm just enjoying tinkering with spreadsheets, and learning stuff as I go. The lookup table implementation was an experiment that worked.
I can see how to do most of the square calcs, if it's worth doing, but haven't looked at compression yet.
You guys need to take a break and have a couple of beers. Beer helps everything.
Thank you guys for giving your time and brain power to assist the community of experimental aviation, in the construction of Safer aircraft.
I'm afraid I got carried away with square tubes:
Assuming H = outer size, and h = inner size = (H - 2 * Wall)
A = H^2 - h^2
I = (H^4 - h^4)/12
J = (H^4 - h^4)/6
S = (H^4 - h^4)/(6*H)
k = sqrt(I/A)
y (diagonal) = H/sqrt(2)
Tension = Sy * A
Bending (square) = Sy * I/(H/2)
Bending (diagonal) = Sy * I / y
Torsion = Shear * J / y
I'm looking at buckling now... [Edit] Looks like the buckling formulas are the same as round tube
I need some definitive material specs :
The numbers I've found for 4130 and 2024 online vary from site to site.
Math looks right for square tubes. AdrianS is using the common assumption that we neglect the corner radii. This only very slightly overstates our stiffnesses and strengths, but is commonly done. One issue with square tubes is the straight walls will get into crippling (also called elastic instability) more easily than circular tubes and so an additional check should be performed using the criteria for this published in the classic texts. I will run that in mine too.
Yield and shear strengths published do vary because alloy content, impurity content, and heat treat do vary. If we have statistically valid test data from our supplier and the process that supplier uses for our material, we can use some statistical minumum off that data. One source for steel or aluminum? Ha. I never did that, and we bought steel for many of our part numbers in the millions of pounds per year. Just use the lowest value you come across, ensure your design is secure at that level, and know that most of the time, your material is better than that. The difference in design when selecting stock sizes is usually insignificant to your decision. Even when specifying material thicknesses, any weight reductions you can get are generally tiny and not worth the fuss. Use min strength values for any material and be happy.
Steel density is 0.282-0.283 lb/in^3, E is 30 E6 psi. In the deep dark past, we used to use 29E6, but somewhere in there, the world settled on 30E6.
Aluminum density is 0.100, E is 10.3 E6.
I have included the latest version of the steel and aluminum tables. Thanks to Adrian for noticing my typos. Note that I omitted 1-1/4 x 0.065 aluminum. 1-1/4 x 0.058 does the job on everything and is lighter.
On to square tubes.
It's been freezing and raining here, so I stayed indoors all day.
Did you know that using data tables and local names, you can end up with cell formulas like =Pi/4*(Osize^2-Isize^2)
My round tube calcs agree with Billski to a % or so.
So I did my version of square with the same sizes
That's more than enough for now.
note : THIS SPREADSHEET MAY CONTAIN ERRORS!
[Edit] removed the spreadsheet, because I just thought of a way to massively simplify the data table.
Look at the bad things when you take a round 4130 tube and make a streamline tube from it.
You're not going to sucker me into adding asymmetrical shapes...yet.
Inquiring minds want to know. I can wait. I'm still a half of a mile (804.672 meters) down the road.
This has got to be the first spreadsheet I've some in a looong time that's purely in kings' units.
So I've got to the stage of having drop down selections for material, shape and section, and a colour-coded results chart like Billski's.
I've added filtering for the sections, so you can limit the list to what's available.
I can't do carbon without someone providing more information : I'm a computer guy - manipulating formulas is my thing, not deriving them.
I could do rectangles and ellipses, but I don't really see the point.
Thank you for this information. Very nice of you sir!
I believe the plan is to work out the alum to 4130 then move on to carbon. As others see this there is gonna be a lot of conversation. Maybe too much.
Looking at the results, might sway me toward aluminum. Cherrymax makes a reliable product for attachment.
People are gonna ask about 6061T6.
I have a "drop down" select from a list for materials, so adding another metal is easy - I could probably add titanium too : why not?
I'll see if I can get specs for 6061-T6 (and titanium ) and add them tomorrow.
This has been a bit of an Excel learning exercise, but I've enjoyed it. Some things would be easier in later Excel versions, but I'm keeping it as backwards compatible as possible.
ps[-]The stress/strain theory isn't new to me, but it's the first time I've actually used it.[/-]
Edit : I clean forgot I used this stuff for checking some flywheel designs.
A good comparison here.
Composites are HARD, because both strength and stiffness are different between tension and compression.
I've found plausible data on cf including those values, but the math will be a while yet (or I'll just plagiarize Billski)
I'm off to finish reading the internet.
Ok, Titanium is interesting.
Data from sandvic metals, grade 9 drawn tube :
Yield = 105,000, Density = 0.160, E = 16,500
It's stronger and lighter than steel, but much less stiff. So for long tube compression, we need to go up in diameter.
note: I have just assumed 1/8" sizes are available
The titanium figures got me thinking : we're comparing tubes for equivalent strength.
That's not the same as equivalent stiffness.
I can work out twist at Tlimit for torque : what loading would the bending deflection be?
I was thinking a cantilever of a given length, with an end weight stressed to load limit, then look at end deflection.
Is that a reasonable measure?
ps Billski - I hope you don't mind me hijacking your thread. If you'd rather I pipe down, let me know.
He is working in the background, or raking leaves, carry on!
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