[SS-VehDev] Re: composite short stack/instrumentation section

[Colin Hutchison]

G'day all,

    Sorry for the delay.  I've run some number on a range of lay-ups.  I 
guess it comes down to Margin Of Safety and how close you would like to get 
to the limit.  I've attached a screen grab of the Excel spreadsheet I've 
been using.  The ply count runs from 15 to 32 plies, however, the braided 
tube was modelled using two separate uni directional plies.  Therefore, for 
example, lay-up for ply count 29 is actually 5 layers of uni-directional 
and 12 layers of braid, totalling 17 layers.  The braid angle was also 
modelled at +-60deg instead of the optimum of +-55deg.  This makes the 
calcs a whole lot easier.  I'll attach the complete lay-ups at a later 
stage.  Each ply (as seen in the calcs) is a 200gsm layer of uni.  So the 
braid would be equivalent to 400gsm.

    There are a few factors that affect all the calcs.  One of them being 
the breaking strain of the Carbon.  I've done the calcs for 4000us and 
6000us.  Typically Boeing use a value of around 4000us.  This allows for 
there own safety factors, one of them being holes drilled in the 
surface.  Another is the Young's Modulus of Carbon.  I've used a slightly 
reduced value from typical pre-preg so this should be ok, but it comes back 
to my point that the supplier and quality of the carbon does matter. Also 
the quality of the epoxy will affect the final product.

    If you want to go to bare minimum the you could go as low as ply count 
17 (1 layers of uni and 8 layers of braid).  Allowing for 6000us then the 
minimum MOS is 0.1219. (Btw, the MOS is calculated for each ply in the 
laminate and then the minimum value is the MOS stated).  The casing would 
come out at around 12.1lbs.  If this were to be the lay-up then I would 
probably recommend that the ends be beefed up a little by adding more 
plies.  Basically because you are going to be drilling holes in it.

    I'll also just not here that the holes should be at least 3 times the 
diameter of the hole to the edge of any composite.

    If you would like a little more safety, then ply count 22 (2 layers of 
uni and 10 layers of braid) gives a MOS of 0.1637 at 4000us and weights 
15.7lbs.

    If you can afford the few extra pounds (and I know your pushing it) 
then I'd probably be going for something like the 29 ply count.  This gives 
a good MOS (nearly 1) and at the end of the day will only weight 20.7lbs 
(41.4lbs for both stacks).  If you were to swap from metal to composite 
then you could probably afford the weight and therefore little more safety.

    Each additional ply is approx 0.7lbs (so 1.4lbs for the braid).

    So I guess what I need to know now is, how close are you willing to cut 
it?  I've also done the calcs for 1500psi and not 1100psi, so the minimum 
MOS will probably be enough.  If you like I can run the numbers for 1100psi 
with a required MOS.

    Probably not the right spot to ask here, but what is the final grain 
structure?  Will it have a cylindrical core or star shape etc?

    I still haven't found out about the filament winding, but I will 
hopefully soon.

>I have added up to four layers at a time to a composite tube without bubbling
>or wrinkling.  Under what circumstances do you find this a problem?

Laying up more layers than that and vac-bagging.  Basically the plies have 
excess length and can't go anywhere and so they end up buckling and causing 
wrinkles.


>For the amount of fiber we'll need for this project we should consider a
>different source.  I think that Roman may have good suggestions along
>these lines.  If you (Colin) or any other team member has a thought please
>pipe up!
>
>Once we have a weave proposal everyone can start keeping their eyes out
>for availability of those grades/weaves at their local haunts.

I can get the stuff but unfortunately I have to pay for it.


>I assume that we want to use this sparingly due to price.

Not really.  Carbon is basically sold by weight so it shouldn't matter what 
form it's in (unless you get down to fine weaves or really heavy 
weaves).  Uni-directional should be the same price if not a little lower 
that woven cloth.


>The braided tube works well, I worry about being able to find
>the 'right size'.  I have had to stretch the tube I get quite a bit to 
>match the
>size of project I am working on.  It still works well, you just don't get 
>'optimum'
>performance as you noted.

I know you can get it made up to specific sizes but then you are stuck 
buying a fairly large quantity.  I'll figure out what size the braid needs 
to be to fit a 10in tube and sit at 55-60deg.


>Just adding a few more layers makes sense, but since this is an international
>project we may have to ship the part cross-country for testing.

You could probably do the tests yourself.  Basically if you have the ring 
that has been cut off the end of the casing, all you have to do is sit it 
upright and put a load on it.  When you squeeze the ring the sides will 
deflect outwards a given amount.  If you can measure the load and the 
displacement of the sides then I can work out the actual material 
properties.  So there should be no need to send it cross country.  I'd also 
need to know the dimensions of the ring.


Colin. 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Casing Ply Calcs.jpg
Type: image/jpeg
Size: 294082 bytes
Desc: not available
Url : http://www.exrocketry.net/pipermail/ss-vehdev/attachments/20051014/5135d51f/CasingPlyCalcs-0001.jpg