Titanium? a Q for the metalurically inclined...

You see it mentioned all over the place, Suzuki (and others) use Titanium
valves in some of their motors, there is even....holy crap, muffler parts...
made out of it...

I was doing some googling, you can find this property of elemental Titanium
around..."Titanium burns in air and is the only element that burns in
nitrogen" Holy crap, sounds like a great thing to make valves out of to me,
especially exhaust valves ;o)

We actually use Titanium fasteners at work for certain things, they are
quite pricey, eg an M6 x long enough SHCS is about $6 ea. (I don't know
specifically what the alloy is )

Someone brought in a chunk of this stuff once at work for an R&D effort,
again I don't know specifically what the alloy was, it looked and felt like
Aluminum, but the guys tried to machine it , it was kinda like...maybe the
leaf spring out of your car.

So, what's the real scoop on Titanium? Is anything "titanium" you buy for
your bike actually really "titanium" ??

D.

 

What is the ignition temperature? Lots of parts of your bike burn in air.

Sure. Plenty of parts are Titanium, though I couldn't tell you what
alloys are used. Honda's RC30 used Titanium con rods. You find it used
in places where weight is a factor: valves, con rods, sometimes valve
springs. In chasing ultimate lightening within the limits of certain
racing classes, one might replace stock fasteners with Titanium. This
sort of action is at the lower end of the 'bang-for-buck' scale, but if
it's a legal advantage and you've got the money and have already done
everything else...

 

That's interesting. I worked in the test lab of a company that made
stainless steel and titanium pneumatic ducting for aerospace. We had a
well-equipped shop to make test fixtures...

I thought that the reason for using heliarc to weld titanium was to
stop the weld from oxiding immediately. I was practicing welding scraps
of titanium together at work. It welds very easily, flows like butter,
almost no filler rod was needed...

But another guy in the shop told me that titanium would spit back at
you if your helium ran out while you were welding it. He mentioned
another guy getting hit in the nose by a flying bit of titanium when
that happened...

 

ignition temp in air is 2200F. Now check your engine's temps plus
technically it's not in air.

 

The real reason you HAVE to thoroughly bathe both sides of the weld in
inert gas is that just a few molecules of oxygen or nitrogen will
embrittle the weld... at the very least cause a noticeable
discoloration (bluish-gray). If you welded titanium without inert
gas, the results would be truly scary in terms of strength (or lack
thereof). The jig that's used to weld my frames plumbs gas into the
inside of the frame tubes, in addition to the gas from the welder that
bathes the area being welded.

Mark Hickey

 

In the 1960s, BSA made a titanium-framed competition crosser (or maybe
enduro).

It was a nightmare because the frame kept breaking and had to be welded
up in a special sealed-atmospehere tank or something.

I can't find much about it on Google, but I do remember an article in
the UK press which reckoned it cost (in modern values, and the article
was written in the mid-to-late 1970s) the equivalent of a quarter of a
million pounds.

However, BSA simply lost the cost in its accounting procedures. A
craftsman might spend hours machining some component from solid billet,
and the cost was "zero"....

 

AMA outlawed Ti frames because of it and titanium axles as well. Their
tech inspectors used a magnet to make sure the axles were still steel in
the early superbike days. Bad thing about Titanium is that it breaks
from inside out. Steel and aluminum crack at the surface first so you
can magneflux or zyglo check to check for pending failure. Titanium
frames could pass Zyglo one second and catastrophically fail the next.
Since the race organizers couldn't figure out how to check them safely
they outlawed them. Now it's used in lots of pieces like Forumula 1 cars
suspension. I haven't checked in a while but they must have developed
some sort of Xray or sound test or just trust the process more. Ever see
a Titanium press? It's like powder naturally but when they squeeze it
hard enough, it can form blocks of strong metal. The splash that comes
out of the mold vents is like metal sea foam. It's actually not very
good in frames anyway because it has a very low Young's modulus,
basically no spring back, so a bike frame feels dead. I was less than
impressed with the Titanium fasteners since it seems like they have
tensile strength but not shear. I tried allen bolts and the hex hole
twisted out at a much lower torque than steel so I couldn't take
advantage of the strength. Aluminum was much more practical in the low
load bearing work that the Ti's worked in.

 

<Snip>

Thanks for the info. Most interesting.

 

Titanium doesn't fail in any different manner than steel or aluminum
in terms of where it fails - there's nothing I can think of that would
cause an axle to fail from the inside out except for a void in the
material. The reason you don't want to swap a titanium axle for a
steel one is that while titanium is stronger per pound than steel,
it's not going to be as strong on a volume basis. So if you design a
new titanium axle that's larger it can be lighter and stronger than a
steel axle - but simply replacing a steel part (which presumably has
been designed to just barely handle the stress it's likely to see
rather than having been radically over-designed), you're asking for
trouble. Replace it with an aluminum part of the same size and you're
committing suicide.

They're certainly not worried about axles "failing from the inside
out". Like any other material, titanium can be used to form parts up
to nearly any task, hence its use in fighter aircraft.

It all depends on the alloy. High-strength alloys like 6Al/4V have
low YM, while CP (Commercially Pure) ti has much higher YM. The
3Al/2.5V (3% aluminum, 2.5% Vanadium) alloy I use in my bicycle frames
has a YM in between.

As for "spring back"... if a frame doesn't "spring back" every time
it's flexed, you're going to have a very short ride or race. The same
mythology about the "frame feeling dead" exists in the bicycle world,
where at least the other factor affecting the way a bike feels are a
lot smaller (1", 120psi tires and no suspension). Throw inches of
suspension travel, huge (by comparison) low pressure (by comparison)
tires and a thick, wide padded saddle and I'll guarantee you couldn't
tell a titanium frame from a steel one if they were designed for the
same mission statement (the ti frame would have slighly larger OD
tubes).

If you're replacing steel fasteners with ti fasteners, you're LOWERING
the tensile and shear strength of the bolt, not raising it (unless
you're replacing some very low-strength steel hardware of course). If
an aluminum bolt will survive in the application, a titanium bolt will
be overkill, and a steel bolt even more overkill.

Mark "I like ti" Hickey

 

Maybe Titanium doesn't have a fatique limit, or a really low one.
I think this is a concern with Aluminum vrs steel for parts subject to
cyclic loading/vibration.

Isn't the stress the result of the forces/moments acting on a part and its
sectional properties? If so, replacing a part with an dimensionally
identical one of a different material should produce the same stresses. The
question might be if the new material can cope with these stresses and the
conditions its under.

Does this make sense?

D.

 

pete wrote:

The above remark means that you don't know the difference between
"stress" and "strain", and probably don't know about the "modulus of
elasticity" or other ecclesiastical concepts ;-)

And, I refuse to further discuss the theological aspects of mechanical
engineering in an open forum where infidels may be lurking. Who knows,
the ghost of Hoyt McKagen may not be requiescating in pacem ;-)

 

No, you're correct. For engineering purposes, stress is solely dependent
on loads and geometry.

Deflections, strain, etc are related to the material, however.

-Sean

 

That's true.

Exactly. Theoretically any aluminum structure that's flexed a bit
will eventually fail. The only real issue is whether it will fail
before or after you retire the part/bike/frame.

The other question was admirably answered by others...

Mark Hickey

 

Thanks Krusty, I think. I was getting ready to post a long winded reply and
the noticed the smilelys. )

People either know this or they don't. The YM, or E, or whatever...is like a
spring constant for the material. It's an atomic property. You can't change
it (much) by heat treatment, lightly alloying ect, It's use is to determine
deflections and stuff. It doesn't appear in common stress equations because
it's not relevent.

I got all hot under the collar over a discussion on another NG where they
thought changing the preload of a spring effected its "rate". The spring
rate is a property of the material, it's "e",and it's dimensions, and has
nothing to do with the preload. Whatsoever. Trustme ).

D.

 

Actually, Pete is correct assuming the amount of deformation is not a
concern and you do not exceed the yield stress of the material. (I have
not been following this thread)
fa = P/A (axial stress)
fb = Mc/I or M/S (bending stress)
No need to know the modulus of elasticity, only section properties.

The modulus of elasticity will allow you to compute the deformation
under the computed stresses.

 

Parts do bend, and stretch, and twist. The mechanical engineer has to
cover all the possibilities in his design, plus make the part light
enough to satisfy performance requirements or meet cost constraints.
Can't sell the customer a part that is too strong, he'll never buy
another one, the company's profits will suffer...

Somebody once said that any fool could design a part that weighs five
pounds to do a job, but it takes an aeronautical engineer to design a
part that weighs 2 pounds and does the same job without a disastrous
failure...

Some parts can tolerate a lot of bending, and that's a good thing, when
you're talking about the wings and fuselage of a jet airliner flying
through turbulent air. It's fairly easy to calculate how much the wings
of the plane in your design study can flap up and down and still return
to their original position. The alternative would be a wing that was so
stiff it tore off at the wing root...

Motorcycle axles have a different requirement. They need to exhibit a
certain rigidity. They need to be stiff. The designer wants the wheel
to rotate in a certain plane, it's supposed to remain parallel to the
fork tubes and the wheel is supposed to rotate perpendicular the axle.
At least, so far as the rider can detect, to make the bike's handling
predictable...

And the transient accelerations working on the axle will usually be
bending it *a very tiny amount* in the middle at whatever frequency the
front wheel is striking the pavement irregularities while travelling at
high speeds across rough roads. Who knows how much the axle is bending?
How many bending cycles will it take, before it breaks? The rider can't
tell until the bike steers "funny" and spits him onto the ground...

There was a story told once about some open-wheeled car racers who
weren't having any problems with the soft steel bolts that fastened
their wheels to the hubs. But they found a supply of high strength
aircraft fasteners built to very high military standards for the
government. The price was right, the bolts were almost free at surplus
prices. So the racers removed the cheap soft steel bolts and replaced
them with the high strength bolts...

The high strength bolts began breaking regularly. They had the
strength, but they could withstand many deflection cycles, like the
softer steel bolts. They were strong, but they weren't tough enough to
take the punishment. They weren't such a good deal after all...

Take the expen$ive titanium axle bought by a wealthy privateer just
because he could get one from HRC in hopeful attempt to give his Honda
just one more little "edge" over the low budget guy. Do you think he's
going to accept limited service life, and toss the expen$ive axle into
the trash can after one or two seasons? Nah! It cost him too much...

So the sanctioning body has to change the rule book to keep him from
killing himself when the axle breaks from an unsuspected flaw...

Different high tech materials have other characteristics. Like the
carbon fiber brake disks that would fail disastrously. They had to be
outlawed. Heck, they worked on Formula I cars, so the motorcycle racers
had to try them. The Formula I teams would use a set of carbon fiber
brakes for a short time and then scrap them. A factory team racing a
bike with carbon fiber disks might sell them to a privateer, he might
use them a short time and then sell them to some sucker who didn't know
about the problem with voids in the epoxy and delamination that causes
sudden failure, like when that Airbus lost its carbon fiber vertical
stabilizer over New Jersey, killing everybody on board...

Well, that was blamed on "pilot error", but the vertical stabilizer did
break off...

What about light weight, high strength carbon fiber wheels that will
delaminate suddenly, and shatter? Are they still legal?

Recently I was looking at the cover of Cycle World. It shows a custom
motorcycle with an exotic carbon fiber girder front suspension. I
wonder how the owner of such a motorcycle is going to react when the
engineers tell him, "Oh, by the way, your girder has a limited service
life, it may fail disastrously if you park your motorcycle in the
sunlight too often..."

 

We know that titanium is lighter than steel. When engineers start
specifying titanium parts to replace steel parts because of "fashion"
concerns, that should be a red flag to intelligent riders. We can't do
anything to help the truly moronic riders that are driven to buy the
most fashionable motorcycles just because they saw some Italian guy win
a road race on a titanium part-equipped bike though...

used for fasteners.

Sure, it's strong, but is it TOUGH enough to withstand repeated strain
cycles? That's the problem, not tensile strength...

OK, so it's light and cosmetically appealing in a situation where if it
breaks it doesn't cause an accident, it just falls off the motorbike...

Spelling Nazi...

I spent a night in a bed and breakfast in Limerick once. Caught a
dreadful cold. One of the worst i ever had...