epoxy joint

[Gray E Fowler]

John,

I do not understand what you mean composite horns/balsa.....but I do know 
a thing about joints ( I once lived in Hawaii hanging out with the 
locals).
Epoxy is the "toughest" unfilled. By this I means in a seam joint the 
primary stress will be flexing, and an unfilled epoxy can handle this the 
best.
Milled fibers are about 1/64" long. In a composite the fibers are far 
stronger than the resin. The job of the resin is to flex a little to enable stress to be transferred from one fiber to another. The problem 
with milled fibers is that as the transfer stress it is only for that 1/64 
of a inch and then it terminates creating a stress riser. The epoxy then 
fractures at the end of that fiber at a stress level much lower than if 
the fiber was not there at all. These type of fibers work great on 
thermoplastics which are much softer and flexable, but not so great on 
thermosets. Milled fibers in an epoxy will reduce elongation, reduce tensile strength, increase the heat distortion temperature, increase hardness and increase compression strength.

Microballoons are not structural, they are used to reduce density. The 
volume is huge and the balloons are weak therefore the mixture is weaker 
in every aspect. The reality is though that a microballoon epoxy is still 
usually strong enough for our applications. Add this to the fact that we 
are always striving for weight reduction and you can determine the proper 
application. 

Filets in general are useless other than for cosmetic reasons. On a 
properly assembled bond joint the filet will never see stress until the 
load capacity of the joint itself is exceed. At that point the joint 
breaks and considering that the joint itself is orders of magnitude 
stronger than the filet, the filet breaks instantly. Filets are dead 
weight, and usually more dead weight than you think it is-but man they 
sure look GOOD when someone inspects the inside of your plane-which in 
this hobby is second only to winning the NATs (sorry-cannot always stop 
the sarcasm). 



Gray Fowler
Principal Chemical Engineer
Composites Engineering




"John Crozier" <sjcrozier at comcast.net>
Sent by: discussion-request at nsrca.org
01/15/2005 09:35 AM
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        Subject:        epoxy joint


Sometime in my crafting life, I have acquired the notion that adding 
milled glass, micro-balloons, etc.,  to epoxy when making a good fitting 
joint, only weakens the joint. (diminishes, or dilutes the bonding 
strength of the epoxy).
In this case it would be composite horns to balsa.  No fillet is needed.
Anybody wanna jump in?

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