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Posted by Terry Keeley on May 7, 2008, 10:08 am
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Have some 0.030" 6Al4V sheet I was trying to heat-treat and it was warping
so I'm giving up on it. A buddy had some Ti alloy that was much "stiffer"
than what I had, looking at Matweb there are all kinds of different alloys,
some with Mo, Zr, Sn, some already heat-treated etc.
When looking at specs., what's important for "stiffness"? Is it tensile
strength? Are there certain Ti alloys that when treated are stiffer than
others?
TIA
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Posted by Ed Huntress on May 7, 2008, 10:35 am
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> Have some 0.030" 6Al4V sheet I was trying to heat-treat and it was warping
> so I'm giving up on it. A buddy had some Ti alloy that was much "stiffer"
> than what I had, looking at Matweb there are all kinds of different
> alloys, some with Mo, Zr, Sn, some already heat-treated etc.
>
> When looking at specs., what's important for "stiffness"? Is it tensile
> strength? Are there certain Ti alloys that when treated are stiffer than
> others?
>
> TIA
Once again (I think we discussed this before?), the "stiffness" of a
material is the measure of its Modulus of Elasticity, or Young's Modulus.
There is no necessary relationship of stiffness to strength.
All titanium alloys fall into a narrow range of elastic modulus, roughly
105 - 116 GPa. That's about half the stiffness of steel, of almost any grade
(except stainless grades).
Now, excuse me if you know this, but "stiffness" by this measure is the
"springiness" of a material. In other words, this is the value of resistance
if you bend the material and it can still spring back. If it doesn't spring
back, but bends permanently, you're looking at *strength* values, not
*stiffness* values, and that's another kettle of fish.
You'll find the Modulus of Elasticity values for metals on Matweb. You'll
notice that they hardly change from one alloy of titanium to another. In
other words, the sheet your buddy had was no stiffer than the 6A14V that you
had. It may have been slightly thicker, and only a small difference in
thickness leads to a much larger difference in stiffness.
--
Ed Huntress
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Posted by Terry Keeley on May 7, 2008, 12:03 pm
Please log in for more thread options Thanks for the reply Ed, you have helped with this in the past, much
appreciated.
We're using it for turn fins on our model boats, we both have material
that's 0.030" thick, both Ti alloys but neither of us are sure what.
Mine bends fairly easily and holds it's shape, his is hard to bend and
springs back to it's original position easily. When he puts a bend in it he
says he has to "over-bend" it quite a bit, I don't.
The main difference I see in these materials is tensile strength, although
Young's Modulus isn't given for them all, but you say they fall into a
narrow range anyway. Hardness is also similar around Rc 38, so is it the
tensile strength I should be looking at?
Here's matweb specs for annealed 6-4, and heat-treated 6-2-4-2 and 6-2-4-6:
http://www.matweb.com/search/DataSheet.aspx?MatID=14239
http://www.matweb.com/search/DataSheet.aspx?MatID=14799&ckck=1
http://www.matweb.com/search/DataSheet.aspx?MatID=14800
I'm looking for what he's got :)
>
>> Have some 0.030" 6Al4V sheet I was trying to heat-treat and it was
>> warping so I'm giving up on it. A buddy had some Ti alloy that was much
>> "stiffer" than what I had, looking at Matweb there are all kinds of
>> different alloys, some with Mo, Zr, Sn, some already heat-treated etc.
>>
>> When looking at specs., what's important for "stiffness"? Is it tensile
>> strength? Are there certain Ti alloys that when treated are stiffer than
>> others?
>>
>> TIA
>
> Once again (I think we discussed this before?), the "stiffness" of a
> material is the measure of its Modulus of Elasticity, or Young's Modulus.
> There is no necessary relationship of stiffness to strength.
>
> All titanium alloys fall into a narrow range of elastic modulus, roughly
> 105 - 116 GPa. That's about half the stiffness of steel, of almost any
> grade (except stainless grades).
>
> Now, excuse me if you know this, but "stiffness" by this measure is the
> "springiness" of a material. In other words, this is the value of
> resistance if you bend the material and it can still spring back. If it
> doesn't spring back, but bends permanently, you're looking at *strength*
> values, not *stiffness* values, and that's another kettle of fish.
>
> You'll find the Modulus of Elasticity values for metals on Matweb. You'll
> notice that they hardly change from one alloy of titanium to another. In
> other words, the sheet your buddy had was no stiffer than the 6A14V that
> you had. It may have been slightly thicker, and only a small difference in
> thickness leads to a much larger difference in stiffness.
>
> --
> Ed Huntress
>
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Posted by Ed Huntress on May 7, 2008, 12:33 pm
Please log in for more thread options
> Thanks for the reply Ed, you have helped with this in the past, much
> appreciated.
>
> We're using it for turn fins on our model boats, we both have material
> that's 0.030" thick, both Ti alloys but neither of us are sure what.
>
> Mine bends fairly easily and holds it's shape, his is hard to bend and
> springs back to it's original position easily. When he puts a bend in it
> he says he has to "over-bend" it quite a bit, I don't.
OK, that's a yield-strength issue then, not "stiffness."
>
> The main difference I see in these materials is tensile strength, although
> Young's Modulus isn't given for them all, but you say they fall into a
> narrow range anyway. Hardness is also similar around Rc 38, so is it the
> tensile strength I should be looking at?
Yes. Bending strength, the resistance to taking a permanent bend, is a
combination of tensile and compression *yield* strength, but for metals,
their ratio is fairly constant. Tensile strength is not quite in proportion
to yield strength for some alloys, but, if you can't find yield strength,
tensile strength will get you close enough. Matweb does provide you with
tensile yield strength so that's the best figure to use for comparison among
titanium grades.
>
> Here's matweb specs for annealed 6-4, and heat-treated 6-2-4-2 and
> 6-2-4-6:
>
> http://www.matweb.com/search/DataSheet.aspx?MatID=14239
>
> http://www.matweb.com/search/DataSheet.aspx?MatID=14799&ckck=1
>
> http://www.matweb.com/search/DataSheet.aspx?MatID=14800
>
> I'm looking for what he's got :)
The last one should be noticeably more resistant to permanent bending than
the first.
BTW, is there a reason you're using titanium for this job? If weight isn't
an issue, then consider the fact that 410 stainless steel, for example, has
about the same resistance to permanent bending (around 800 - 900 MPa) but
roughly twice the resistance to springing (around 200 GPa versus 110 for
titanium). In other words, it's no stronger, but it should have twice the
resistance to flutter and so on. It's also close to twice as heavy as
titanium, but that can't be a big deal with a 0.030" thick fin.
--
Ed Huntress
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Posted by Terry Keeley on May 7, 2008, 12:46 pm
Please log in for more thread options Thanks again Ed, so high tensile strength is what I'm after.
Actually weight is a huge factor, these are for record attempt boats and any
weight on the right sponson has to be minimized to help counter prop-walk.
I'm running in a class that has a 120+ mph record and my buddy I mentioned
just broke the record in his class running 103+, here's pix if you're
interested:
http://gallery.intlwaters.com/thumbnails.php?album=1002
Here's the fin I'm talking about on my hull:
http://gallery.intlwaters.com/albums/userpics/10068/normal_ScanImage011.jpg
Wonder if one of the magnesium alloys might be better?
>
>> Thanks for the reply Ed, you have helped with this in the past, much
>> appreciated.
>>
>> We're using it for turn fins on our model boats, we both have material
>> that's 0.030" thick, both Ti alloys but neither of us are sure what.
>>
>> Mine bends fairly easily and holds it's shape, his is hard to bend and
>> springs back to it's original position easily. When he puts a bend in it
>> he says he has to "over-bend" it quite a bit, I don't.
>
> OK, that's a yield-strength issue then, not "stiffness."
>
>>
>> The main difference I see in these materials is tensile strength,
>> although Young's Modulus isn't given for them all, but you say they fall
>> into a narrow range anyway. Hardness is also similar around Rc 38, so is
>> it the tensile strength I should be looking at?
>
> Yes. Bending strength, the resistance to taking a permanent bend, is a
> combination of tensile and compression *yield* strength, but for metals,
> their ratio is fairly constant. Tensile strength is not quite in
> proportion to yield strength for some alloys, but, if you can't find yield
> strength, tensile strength will get you close enough. Matweb does provide
> you with tensile yield strength so that's the best figure to use for
> comparison among titanium grades.
>
>>
>> Here's matweb specs for annealed 6-4, and heat-treated 6-2-4-2 and
>> 6-2-4-6:
>>
>> http://www.matweb.com/search/DataSheet.aspx?MatID=14239
>>
>> http://www.matweb.com/search/DataSheet.aspx?MatID=14799&ckck=1
>>
>> http://www.matweb.com/search/DataSheet.aspx?MatID=14800
>>
>> I'm looking for what he's got :)
>
> The last one should be noticeably more resistant to permanent bending than
> the first.
>
> BTW, is there a reason you're using titanium for this job? If weight isn't
> an issue, then consider the fact that 410 stainless steel, for example,
> has about the same resistance to permanent bending (around 800 - 900 MPa)
> but roughly twice the resistance to springing (around 200 GPa versus 110
> for titanium). In other words, it's no stronger, but it should have twice
> the resistance to flutter and so on. It's also close to twice as heavy as
> titanium, but that can't be a big deal with a 0.030" thick fin.
>
> --
> Ed Huntress
>
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> so I'm giving up on it. A buddy had some Ti alloy that was much "stiffer"
> than what I had, looking at Matweb there are all kinds of different
> alloys, some with Mo, Zr, Sn, some already heat-treated etc.
>
> When looking at specs., what's important for "stiffness"? Is it tensile
> strength? Are there certain Ti alloys that when treated are stiffer than
> others?
>
> TIA