Is titanium too brittle for chainplates?

[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?