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Brian Lindahl
(lindahlb) - MLife

Locale: Colorado Rockies
Pot Support Windscreen: Why Ti Foil over SS Shim Stock? on 03/28/2013 12:46:53 MDT Print View

I'm not a materials/mechanical engineer, but I've been looking at some numbers, and for a pot-support windscreen (i.e. Caldera Cone), I'm curious why Ti Foil is being used over Stainless Steel Shim Stock.

From what I gather, the property of importance for pot support is 'modulos of elasticity':
http://www.cineventions.com/steel_aluminum.html

From what I've read, it appears the Ti foil commonly used here (TiGoat/Suluk46) is '15-3-3-3':
http://www.suppliersonline.com/propertypages/15-3-3-3.asp

The density is 0.172
The modulos of elasticity is 13.5

302 Stainless Steel, easily available from Amazon:
http://www.suppliersonline.com/propertypages/302.asp

The density is 0.287
The modulos of elasticity is 28

13.5 * 0.287 / 0.172 = 22.526, which is less than 28

Meaning, for a given weight, 302 stainless steel shim stock is more supportive than 15-3-3-3 Ti Foil. Or, for a given support level, 302 stainless steel weighs less than 15-3-3-3 Ti Foil. When you add in the significantly less cost of 302 stainless steel, it seems like it'd be a no brainer? Or am I doing something wrong here?

Does this also make sense for a cookpot, itself? I mean, Ti pots must be popular for a reason, so maybe different physics/mechanics are coming into play for cookpots.

As an aside, from what I've read, these numbers don't differ much amongst the varrying alloys (etc.) of each.

Note that according to the same website, aluminum generally does not fair any better (better to have a thin windscreen than thick, IMO):
http://www.cineventions.com/steel_aluminum.html

Edited by lindahlb on 03/28/2013 13:12:41 MDT.

Jon Fong
(jonfong) - F - M

Locale: FLAT CAT GEAR
Re: Pot Support Windscreen: Why Ti Foil over SS Shim Stock? on 03/28/2013 14:09:15 MDT Print View

I think that you will find that the Modulus of Elasticity will have minimal impact on the windscreen design. The reason is that windscreen materials are so thin that the amount of stress in the part will be very low. What will make a difference in forming a windscreen will be the shape of the stress-strain curve. A material that is brittle (a straight stress-strain curve that is linear with an abrupt end), will be pretty nice to punch. That being said, it may be impossible to bend without cracking. A material that has a lot of ductility will have a lot of “area under the curve” (the stress-strain curve will be linear and then drift to the right). That material will be easier to deform without breaking, but may be too soft to hold a good shape (aluminum oven foil).
To take advantage of using steel, you are probably talking about material that is very thin (<0.005”). Let’s say that you want to use a 0.003” steel, at those dimensions the steel tends to be 1) very expensive 2) very hard 3) many times very brittle and 4) difficult to work with. Best wishes - Jon

James Cahill
(DMATB) - M

Locale: SOCAL
What Jon said on 03/28/2013 14:35:04 MDT Print View

In addition to what Jon said, which in addition to being scientific is also practically applicable, here's some more science:

I like to use MatWeb.com to find information for work/school stuff (not that there's anything wrong with suppliersonline.com) and some data pulled for those two alloys are as follows:

Ti 15-3-3-3
Ti 15-3-3-3

SS 302
SS 302


Comparing the yield strength (when it plastically deforms) of the two you can see that the stainless will yield much earlier and, in addition, has a lower UTS. Even though the SS has a higher elastic modulus, stiffer isn't always better (circa Jon's explanation).

Again the more important thing in this case is how easy it is to work with the material and how expensive it is.

Brian Lindahl
(lindahlb) - MLife

Locale: Colorado Rockies
Re: What Jon said on 03/28/2013 15:30:19 MDT Print View

Thanks for the info, gurus. How does elastic modulus and yield strength differ? To a layman, like me, plastically deforming (yield strength) and stiffer (elastic modulus) sounds identical, but apparently it's not? How are those two different?

> Again the more important thing in this case is how easy
> it is to work with the material and how expensive it is.

I don't know why I didn't post this link, earlier (oops!), but:
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=41557

It looks like .002" steel is pretty cheap and easy to work with, based on real life experiences when used for windscreens. No indication on how it works as a pot support, but I hope to find out soon.

Jon Fong
(jonfong) - F - M

Locale: FLAT CAT GEAR
Re: Re: What Jon said on 03/28/2013 15:57:28 MDT Print View

The Modulus of Elasticity is a measure of the “stiffness” of the material in the linear range. Steel is ~3 times as “stiff” as aluminum; Titanium is ~50% stiffer than aluminum. The Modulus of Elasticity is given as a rate stress / inches per inch. Yield strength is a measure of the maximum stress a material can be exposed to BEFORE it deforms or yields. If you bend a material and it springs back to its original shape, you have not yet exceeded the yield strength.
For example, let’s look at aluminum. 1000 series (oven foil) and 6061-T6 (tent poles) have the same Modulus. 1000 series has a low yield strength and deforms easily while and 6061-T6 has a high yield strength.
There is a lot more to it and if you need to learn more, you should probably read up on material properties and gain a good understanding of stress strain curves and how to use the chart to design a windscreen. I am not trying to be curt but, there is a learning curve to material science.
I agree with your point thought that a $3 piece of 0.002” steel looks like a good option. I myself would prefer stainless steel over carbon steel. Keep in mind that when you are dealing with super thin materials the surface area of the edge is low and the chances of “paper cuts” are higher. 0.002” steel could leave a deep and nasty cut.


Jon

Brian Lindahl
(lindahlb) - MLife

Locale: Colorado Rockies
Re: Re: Re: What Jon said on 03/28/2013 16:03:19 MDT Print View

Thanks Jon, that was an excellent synopsis and that makes sense. It looks like yield strength of a material varies greatly based on the it's production. Price definiately goes up with the higher yield strength tempers! Spring temper looks to be MANY MANY times more expensive. Probably a lot more difficult to work with as well (higher hardness).

The 302 stainless steel I found on Amazon was ~just as cheap ($15 for 50'x6") and is full hard and claiming a high yield strength, so it could be adequate - I guess we'll find out. The normal yield strength of full hard 302 looks very close to 15-3-3-3 titanium from everything I can find (~140). Good point on the paper cut warning, too!

Edited by lindahlb on 03/28/2013 16:32:17 MDT.

steven franchuk
(Surf) - M
better sellection at mcmaster. com on 03/28/2013 18:11:14 MDT Print View

If you want to try stainless steal mcmaster has a much better sellection than amazon.com

http://www.mcmaster.com/#stainless-steel-foil/=m2x1wq

Brad Walker
(brawa)

Locale: SoCal
Small correction on 03/28/2013 18:35:22 MDT Print View

Small correction: TiGoat sells commercially pure grade 2 (CP2) Ti foil. Not sure about Suluk, but it looks like he uses grade 5 (Ti-6Al-4V) for some things. 15-3-3-3 Ti will may become brittle when exposed to heat and crack, though it is stiffer than higher purity alloys. (See this thread for some cracking: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=44302)

Kevin Beeden
(captain_paranoia) - F

Locale: UK
Why Ti? on 03/28/2013 18:49:14 MDT Print View

Without doing the numbers, but based on practical experience, the physical properties of the metal are pretty well irrelevant for making something like a caldera come (within reason...)

Cut out a template from a piece of standard photocopier paper, and it will just about support a pan of water. I wouldn't recommend actually putting a burner in it, though... The comic shape imparts considerable strength; I've used 0.002" Ti foil to make a clone, and it worked fine. The stiffness did start to have an impact, in that the side walls were quite easy to press in, despite the comic shape.

The main reasons I might use Ti are heat resistance and lightness. Thin Al foil can easily be softened or melted by even an alcohol flame.

SS shim stock will work as well as Ti foil, if marginally heavier.

Franco Darioli
(Franco) - M

Locale: Melbourne
Pot Support Windscreen: Why Ti Foil over SS Shim Stock? on 03/28/2013 19:44:12 MDT Print View

"comic shape '
nothing funny about the cone or did you have this :


cone of silence

in mind ?

Kevin Beeden
(captain_paranoia) - F

Locale: UK
Comic...? on 03/29/2013 02:44:49 MDT Print View

Hmmm... Are those typos me or the iPad's 'clever' spell checker...? Conic

Looks like it's me...