Titanium vs other metals

[Stumble]

Originally Posted by ChrisSR

Hi Stumble,

I have always understood that Ti was too brittle for most boat fittings. Yield and failure points being too close together.(i.e. No give just a ping) We used to use hot-dip galvanized steel then stainless steel came in. It was said to be so much stronger. We had failures if the metal could move at all due to its low fatigue tolerance. So by the time you beefed it up, you had not saved any weight, only introduced corrosion with your aluminium. The way I understand it Ti has even less tolerance to fatigue, and will corrode your al. even faster.(higher electrical potential difference)
I am interested in using Ti but need to learn how to without courting failures.
Cheers,
ChrisSR

Chris,

Some if what you are asking sound alike engineering advice, and I honestly can't provide that. I don't have the background. That being said, there are a few things I feel I can clear up...

Yes the delta between yield and failure are relatively cose compared to many metals, but it is deceptive due to the very high numbers relative to other metals. Take Grade 5 Ti, and 6061 aluminium for instance. There is less than a 10% difference between the tensile and yield strength in the Ti. As compared to 6061 where the yield is roughly half of the tensile. This is a concern...

But the yield strength Of the 6061 is roughly 8,000psi with a tensile strength of 18,000psi. So there is a 10,000psi difference. Compare this to Grade 5 with a yield strength of 128,000psi and a tensile strength of 138,000. So we have the same 10,000psi delta, the numbers are just massively in titaniums favor.


An additional issue is that titanium is a very flexible metal. I don't know how to account for this to compare it to other structual metals (not an engineer), but one of the reasons titanium is used in bone replacements is because it is very flexible. This allows it to be very desirable in applications where there are high cyclical shock loads (like springs), because it will flex, reducing the applied force.


Like steel , and unlike aluminium, titanium has a logarithmic increase in cycle life as long as loads are kept below a given point. This tends to occur somewhere around 10^7 cycles. Which depending on the alloy occurs at between 40 and 60 % of tensile strength. In short, it has similar fatigue properties to steel, but it can be alloy specific.

Finally because titanium is the most noble of the structual metals, it will always be the cathode in a galvanic cell. So yes it will corrode aluminium faster than 316 would for instance. This however can be used as an advantage.

First if you are using carbon fiber in your construction, because it is even more noble than titanium but titanium is more noble than anything else, the titanium part will corrode much slower than other metal options. Second, if you have a part that is absolutely mission critical (say chain plates or keel bolts), they are self protecting in the event of electrolysis. Of course as with any galvanic potential, care should be taken to minimize dissimilar metal contact, and bond potential galvanic cells to a sacrificial anode.

References
http://www.alliedtitanium.com/techni..._of_Metals.pdf
http://www.azom.com/article.aspx?Art...=1341#_Fatigue

[Stumble]

I did some checking, and to the best of my knowledge we do not carry titanium wire (I assumed 1x19), but might be able to get it if there is real interest. It would be a very specialty part at this point, and I hate to try and price it unless someone is seriously considering it. If so, let me know and I will track it down with our mills.

[Stumble]

After some more research I found out that to day titanium wire is generally considered a bad idea. The thread on thread movement causes the strands to gall against one another, and leads to very short service lifes, albeit with a great strength to ratio while it is in good shape.

It sounds like a good idea, but it just doesn't work.


The next idea is to see if it could work on rod rigging as a replacement. It might take some experimenting if anyone wants to give it a go. but titanium wire seems to be out, at least until some of the new ceramic coatings get more advanced, and act to buffer the strand on strand movement.

That being said, titanium turnbuckles do quite well, so you could go all titanium except for the wire itself. Which would require a quick trip to Colligo Marine for their Dynex rigging, plus titanium fittings.

[upchurchmr]

Stumble,

You could check on a process called Tiodize. A coating for Ti to reduce/ eliminate galling. We used it on an aircraft, the only place it is done is here in Texas (I'm pretty sure).

Marc

[Stumble]

Originally Posted by upchurchmr

Stumble,

You could check on a process called Tiodize. A coating for Ti to reduce/ eliminate galling. We used it on an aircraft, the only place it is done is here in Texas (I'm pretty sure).

Marc

I looked into it. It is a type of anodizing, and Aparently does a pretty good job cutting down on galling. The problem with using it in wire is that there is just to much surface area. If the coating fails anywhere the entire structure is compromised. And in wire usage there is so much thread to thread movement it just doesn't work.

Aparently the US military spent huge amounts of cash testing to see how to get titanium wire to work, and gave up (or stopped publishing) in the 80's sometime. Since the Tiodoze process started in 66, I am pretty sure it was tested, and passed over. At least for wire.

It is a great process for other things however.