7075-T6 Alum vs. Titanium Stakes

Hi Josh,

I am not sure I agree with you. Grade 5 titanium has a higher bend radius then 7075-T6 aluminum.

Lawson:

Your strength comparison calculations are not using the right engineering criteria for design. Your engineer friend is using axial stress as the criteria, but you must also perform a buckling analysis, which is the failure mode that a tent stake is most likely to experience (it will bend long before it will break). In buckling, the elastic modulus is a critical factor. I've performed a buckling analysis using the diameters you've given, and the aluminum stake can resist a buckling force (like from a hammer pounding on it) 53% higher than the titanium stake. Also, as another person mentioned, the weight difference is marginal for the two designs (less than 1%).

That being said, my personal preference is for Ti stakes because they withstand the impact of hammer blows better. Aluminum stakes get all distorted where they are hammered, and this can lead to the top of the stake breaking off while hammering due to a phenomenon known as stress concentration.

I'd be happy to email you a pdf of my buckling analysis if you'd like. If so, PM me.

Regards,
Scott

it is tent stakes were talking about here ???

7075-T6 i believe is used in climbing equipment … and i dont believe any of the forces use for staking even nasty heavy weight vampires would ever exceed that for climbing …

IMO its all academic, itll all be plenty strong for staking purposes

I tested the CF stakes I had made, for initial strength by driving them into a hard gravel driveway along with a easton stake, and a ground hog. At the point the aluminum stakes started to bend, the carbon fiber stakes were still fine. I did this in several locations. I personally, feel that if the stake will perform to where a high quality aluminum stake will deform that is sufficient. I'm sure you could shatter it at some point if you really wanted to, but this point would be above the point where other high quality stakes would have already deformed. For the record, The Vargo ti stakes will deform about the same, a steel nail stake will deform at some point as well, as will a durapeg (not a popular stake around here).

"7075-T6 i believe is used in climbing equipment … and i dont believe any of the forces use for staking even nasty heavy weight vampires would ever exceed that for climbing …"

this isn't really saying much, because force by itself is meaningless. It's stress that matters and when critical stresses of a given material are exceeded. this is the combined result of force, shape, and material properties.

I could support the weight of a building on a pad of paper. so given your example, paper is capable of supporting loads much higher than what a tent stake will see. but how well is that paper tent stake going to hold up.

people use aluminum all the time … i cant really see stakes pulling being more "forceful" than taking a lead fall on a biner or small alum chock

now someone said whacking a stake n hard gravel with a rock can bend it (alum and titanium) … absolutely … but then i suggest that if the ground is that hard and you find big enough rocks to whack it, you anchor with the rocks instead … unless yr carrying a hammer anyways =P

i would say that a lot of these fancy materials save a marginal amount of weight … and while it is theoretically stronger, the reality is that it wont be enough to make all that much of a difference

but we do get to use those fancy words that sound so cool, and i feel so much better paying those exclusive prices ;)

hmmmmm

MSR ground hogs are usually around 2.00 a piece.I wouldn't consider those exclusive. Most of the stakes, except ti nails etc are in the same ballpark. Not sure where stakes get exclusive prices , unless you usually use plastic walmart stakes.

ground hogs are alum …

http://www.rei.com/category/4500663

the prices for alum are quite a bit cheaper than those fancy titanium ones … which i believe was the point of this thread ;)

Hi Scott,

Thanks for running a buckling analysis on this.. I will say though, I am a bit confused. It appears like you said the aluminum has 53% more buckling resistance then the titanium, but the titanium handles buckling stress better due to its resistance to stress concentration. So besides stress concentration would the aluminum stakes actually be stronger or do a better job of resisting buckling like I first proposed? If so this is great news.. According to my calculations a 7" long .156" diameter 7075-T6 Aluminum hook stake would weight 6.76 grams while a 7" long .125" diameter 6-4 Titanium hook stake would weigh 7.23 grams. That's a 7% weight reduction for something thats stronger, weighs less and costs half as much.

Thanks again,
Lawson

Lawson

You dont have to agree with me, I can tell you exactly where it will fail, no numbers, no computers. I'm just saying, I'd avoid putting a hook in them. Years ago when I built aluminum bike frames, I used to bet our "engineer" where the frames would fail, I always won. Got any money?

Kevin

Those carbon stakes are brilliant!

"I am a bit confused. It appears like you said the aluminum has 53% more buckling resistance then the titanium, but the titanium handles buckling stress better due to its resistance to stress concentration. So besides stress concentration would the aluminum stakes actually be stronger or do a better job of resisting buckling like I first proposed?"

The buckling and the stress concentration are two different issues. The aluminum will not buckle until a load is applied that is 53% higher than the load at which the titanium will buckle. So, yes, the aluminum is significantly stronger when it comes to buckling. The stress concentration applies when looking at the hook end of the stake. When you hammer aluminum, it deforms more than titanium due to the ductility. As most UL backpackers use rocks to hammer their stakes, the deformation is very jagged. These jagged marks concentrate stress in the material. In other words, it is possible for the hook to snap off due to the stress concentration in these jagged marks. So, in this respect, the titanium is more durable.

"According to my calculations a 7" long .156" diameter 7075-T6 Aluminum hook stake would weight 6.76 grams while a 7" long .125" diameter 6-4 Titanium hook stake would weigh 7.23 grams. That's a 7% weight reduction for something thats stronger, weighs less and costs half as much. "

I get 6.20g for the aluminum and 6.24g for the titanium – virtually no difference. That's based on diameters of 1.56 in for aluminum and 1.25 for titanium and densities of .102 lb/cu. in. for aluminum and .160 lb/cu. in. for titanium.

Regards,
Scott

The carbon ones look a lot like yours don't they ?

Fact is though, they work. Look, in a WFR class I took the instructor said, all bleeding stops ..eventually, well all stakes break at some point , eventually. May as well go with the lightest, best holding ones you can and know there will be some attrition.

FWIW, I was originally trying to work out some other stakes with Lawson, but alas , they fell through.

Kevin

Thanks for the reply Scott. The hook part takes an extra inch of rod. So they are actually 8" long before bending. I probably entered the density of the aluminum wrong and is the reason I keep getting 7% but either way the aluminum is lighter (1%) and stronger (53%) so this is good news. Maybe instead of a replacement, the aluminum stakes can be a cheaper alternative to the titanium ones. Now the tricky part is finding the stuff… Thanks again :)

Yes, putting a hook into the end of the wire will either crack the wire or severely weaken it, and that applies to most any metal and most any alloy (copper, Nitinol, etc excepted).

Also, a hook at the top end makes the stake bend when being hammered into the soil. The load tends to hit off-axis. This can lead to buckling and failure.

So why use a hook AT ALL?

Why not press-fit a small Aluminium cap onto the main wire stake instead. You can hand-press on or hammer on the larger area, it will hold the guy rope onto the stake well enough, and you can use it to extract the stake as well. Being totally straight there is neither weakening due to bending, nor buckling when hammering (gently).

Hum … must make up a few myself and test the idea.

Cheers

Roger, isn't that the way that a standard Easton tubular aluminum stake is made?

Or are you trying to re-invent the wheel?

–B.G.–

You seem to be trying to design the lightest tent stakes and yet no one has mentioned the stress state that the design is to overcome.

Presumably the shaft is in axial compression during insertion into the ground.
Then it would be under a bending stress during use. This is a little complex because the stress distribution is going to be a function of the soil mechanics and more so because the actual stress state will depend on the ability of the stake to cut through the dirt – i.e. shape and size dependent. This is where length may become important as the stake progressively bends and cuts through. Shapes that are very resistant to soil deformation may end up generating a stress concentration at the air to soil interface.
The final stress state is where you remove the stake which is simply axial tension.

If you want to do this I would recommend come kind of FEM where the constitutive properties of the materials could be found although analyzing the exact mechanical conditions of use and soil are going to demand extensive testing – wait! – 3 years work over a tent stake? I wouldn't bother but for those of you that want to, go for it!

By the way T6 refers to 'peak aged' and is a process of aging the Al after quenching. 2000, 6000 and 7000 series are designed to be 'worked' (formed) and then aged ('tempered' if you like). i.e. they are all plastically formed into shape. If done hot they will accommodate large plastic strain without cracking – after all we have all flown around on materials like this without consequence.

What kind of Ti are you talking about? CP? 3-2.5? or 6-4?, perhaps a beta alloy? What shape? Tubular?

If you want to pursue this further, I wound recommend Mike Ashby's book on materials design: ttp://www.amazon.com/Materials-Selection-Mechanical-Design-Fourth/dp/1856176630/ref=sr_1_2?s=books&ie=UTF8&qid=1309014321&sr=1-2

Hi Bob

> isn't that the way that a standard Easton tubular aluminum stake is made?
Well, sort of, except that the Easton stake uses a tube while what we are talking about here is a much thinner wire. So the head I am talking about would be quite a bit smaller.

Very little new under the sun.

Cheers

I'm just going to throw a little opinion unrelated to the random technical data here, in that I'm of the opinion that the stake market, especially for nail/hook type stakes is pretty well covered.

I'm personally of the belief that you were on to something much more significant with the corkscrew Ti's you were developing Lawson, since that would fill a void needed with a certain soil type that's often quite troubling IMHO.

I also have to agree that I think CF if a better option if trying to lighten the traditional nail type stake, and can be acquired quite inexpensively in tube form.

I've never met an Aluminum stake, regardless of the calculations, that survived more than a couple of trips the way I abuse them, with the exception of the ground-hog type stakes (which admittedly are much better in Al Y form, vs the Ti V type), which get their strength from their geometry, and even those, I constantly bend and destroy.

When I use the thin Ti shepherds hooks, which I never *rely* on, the modulus factor seems to be highly important to their "durability", considering they're only useful for highly compacted ground. Even those are often bent and damaged.

I'm not going to wade into the technical aspect of this, just offering my useless 2c.

edit: word change, too many "opinion" uses in too short a space. ;)

Javan

Actual practicality over theoretical acadamia…..heresy I say. I demand more abstract, slathered in conjecture. At least for the entertainment value.

I thought the corkscrew stakes were a really neat idea too. Maybe in reality they didn't work so good (tough to screw in?) but if there is a possibility down this avenue then you should explore it.

I have used the REI 7075 stakes – in fact they are my favorites. I don't like the skinny Ti stakes for the the types of soil conditions I usually encounter, and the REI stakes are fatter enough to do the deed, and can handle being pounded in with a rock if the soils is harder.