Is titanium too brittle for chainplates?

[Taniwha]

Hi all,

I've been looking at titanium to replace our 46ft Peterson yacht's chainplates, and have noted the positive attitude to it as a material by Brion and many others.

However, in reply to a posting I made on our owners' forum requesting any 1st hand experience, I got the following response from one owner:

"Beware of titanium. Before retiring I was an helicopter pilot and one the
helicopters I flew had a rotor head made of titanium. You can imagine the
tremendous forces applied to this piece of titanium by heavy blades turning at
360RPM, centrifugal forces around 10 tons for each blade.
This rotor head was very light (due to the use of titanium) BUT, and this is a
major drawback of titanium this metal CANNOT receive any blow, in which case it
would develop internal and invisible cracks, unknown until the piece breaks...
To avoid this we had this titanium rotor head coated by a ~1/4 of an inch thick
soft moulding looking like rubber, this "rubber" coating being intended to
absorb any possible blow by a tool or any foreign object.
I believe that all equipments on the deck of a sailboat will receive one day or
another a blow from a solid piece of equipment (tool, anchor,...) and in these
conditions I'll never trust titanium as material for chainplates."

I'd be interested in any comments from knowledgeable people on the above comment. (Brion?)

I've trawled the web, and of course titanium manufacturers expound the positives, and various other chat sites claim doom & gloom (generally not about sailing uses), but I don't feel that either of these sources are totally trustworthy.

Hopefully here you guys are mostly just interested in what works, without a commercial bias, so your inputs would be very welcome.

Thanks,
Paul.

[Brion Toss]

Hello,
Part of the reason, as I understand it, that helicopters use titanium, in places, is to save weight. But materials selection invariably involve (or should involve) any number of factors, including things like strength, cost, fatigue-resistance, corrosion-resistance, machinability, etc. In this case we have an extremely highly-loaded component, where strength and fatigue-resistance are paramount. I do not know what the appropriate grade of titanium would be for this job, but it would likely be several times stronger than a comparable piece of the strongest steel equivalent, among other virtues. And if, for some reason, you made the component out of steel (which used to be the only choice, and is still an option), you can be darn sure that hitting that component would be a bad idea; titanium is hardly unique in harboring "internal and invisible cracks."
So it seems to me that this helicopter pilot -- note, not an engineer, not a metallurgist, not even a machinist -- has extrapolated an important precaution into an absurd absolute: no impact loads on titanium. If this were remotely true, we probably wouldn't see a product like this: http://www.northerntool.com/shop/tools/product: http://www.northerntool.com/shop/too...7785_200357785.
Titanium is new to most of us, and we are still learning how to use it, and to integrate it into our palette of choices. Obviously, we can make mistakes with it, just as with other newish products like Spectra. But objections, however qualified the objector might seem to be, need to be backed up with meaningful information.
Fair leads,
Brion Toss

[Stumble]

Paul,

I sell titanium for a living, in fact I sell a lot of titanium chainplates. The reality is that like with most materials titanium has trade offs that must be engineered to get the best out of it. However because it is different than many metals there are a few realities that get blown way out of proportion. And frankly a helicopter pilot is not likelythebest source of information.

To be clear there are a number of types of brittleness, and like there are a lot of engineering types of strength.

The first one, and where Ti gets is reputation for brittleness is expressed as a percentage. It is measured by the yield/tensile strength. For Grade 5 titanium Ti rates at 92%. Meaning the yield strength is a huge percentage of its tensile strength. 316 stainless for instance is 39%, and 6061 aluminium is 44%.

So by this measure titanium is very brittle. However when you look at the numbers.... Grade 5 titanium has a yield strength of 128,000psi and a tensile of 138,000. The 6061 aluminium has a yield of 8,000psi and a tensile of 18,000. Note that the difference between these numbers is the same. 10,000psi.

Another type of brittleness refers to how for a metal will deform before it breaks. In this measure, and one normally considered more important for chainplates, titanium far surpasses other metals, which is why the best springs are made from titanium. Just think of the memory shape sunglasses. You can literly tie them into knots, and they will spring back to shape. It is this ability to return to bend and return to shape that allows titanium to absorb shock loads that would crack other materials.


That being said there are some fair concerns with titanium usage.

The first is do you design based on the old materials tensile or yield strength?
Second, if you reduce thickness to far because the material allows for it, have you left enough thickness for the pins?

Certainly as a salesman my opinions maybe suspect, so if you have any specific questions I will be happy to back up opinion with engineering references.

[Taniwha]

Hi Brion & Stumble,

Thanks for your replies: you both make very valid points.

Stumble, sorry , I certainly didn't mean to question any individual's integrity, but many industry players cannot be relied on for objective comments (have a look at some of the threads on Cruisers Forum regarding LiFePO4 batteries to see that). So, thanks for posting some figures and an interpretation of what that means in a practical sense.

In our case, because we'll be retrofitting and the plates will be encapsulated, I'll be looking to replace with identically sized plates, which should mean that they'll be over-engineered compared to the SS ones. Thus we won't face any issues regarding under-engineering as per Stumble's comments.

I'll be looking at this over the next month and may come back with some more questions.

Again, thanks for your comments: they've certainly validated what my original position was.

Cheers,
Paul.

[Stumble]

Paul,

Not a problem, I don't take it personally. I understand the issue with dealing with salesmen. There are two common grades used for chainplates, really for most everything except for tubing (which is generally grade 9). Grade 2 has a significantly higher yield strength than 316, or 304 stainless, but a lower tensile strength. Grade 5 is massively stronger in both respects. Conversely though Grade 2 is appreciably cheaper than Grade 5, so where possible the preference is to go with 2 just because of the price difference.

...........Yield...........Tensile
304.....31,000psi.....73,000psi
316.....35,000psi.....80,000psi
G2......40,000psi.....50,000psi
G5....128,000psi...138,000psi

Now most structures are designed around yield strength, since it is at that point that the structure starts to distort. But there is a built in safety margin when dealing with steel alloys because there is a huge delta between the yield and tensile strength. Titanium on the other hand has a higher yeild strength, but the delta is lower.

There are also some additional issues involved that should be taken into account. They may not change a design, but should be taken into account by a good engineer.

1) Most stainless is designed with a given amount of corrosion allowance. How much will depend on the application, and the concern over durability (cruiser vs America's Cup for instance). Titanium however is non-corrosive in the marine environment, so there is no need for a corrosion allowance

2) titanium has a very good fatigue curve compared to stainless. The fatigue knee is actually higher for Titanium, so this may be another place where material, and thus cost could be reduced.


Of course titanium is a relatively expensive metal by volume, so there is a natural desire to reduce the amount of metal, and thus the cost of the finished part. So if a chainplate goes from 100 cubic inches of 316, to 50 of G5 titanium, the cost for the parts can actually be relatively close, in some case the titanium can even be cheaper. However if you just replace size for size, and don't take advantage of titanium's better structual properties you really are going to overpay, because of the extra metal you are using. It may be cheaper than hiring an engineer to redesign the part however.

The other issue a lot of people look at is life cycle costs. If for instance you assume that a 316 chainplate will need to be removed and inspected at 10 years, and likely replaced between 10-20 years. Versus a titanium one that may never need replacing, whatever extra cost the titanium cost at the front end will likely be saved down the road.

[billknny]

C954 Bronze has similar numbers to stainless:

Tensile 85,000
Yield 32,000

It's cheaper than Ti, (3/8" x 2" x 26" flat for $56);

Lasts indefinitely in the marine environment (even encapsulated);

And is a lot easier to work with then Ti (or stainless for that matter). I can cut, drill and shape it at home.

Other than cosmetics (it's not shiny)-- why not use it?