epoxy joint

[Cameron Smith]

I want to hear about the Hawaiian Locals!!!
 
-----Original Message-----
From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]
On Behalf Of Rcmaster199 at aol.com
Sent: Monday, January 17, 2005 1:25 PM
To: discussion at nsrca.org
Subject: Re: epoxy joint
 
Interesting, Gray. Would you also discuss some details about chopped
filler (say 1/16" to 1/2" chop length) and continuous strand filler?
Also how material stiffness and impact resistance are affected?
 
MattK
 
 
In a message dated 1/17/2005 1:18:24 PM Eastern Standard Time,
amirneshati at earthlink.net writes:
----- Original Message ----- 
From: Gray E <mailto:gfowler at raytheon.com>  Fowler 
To: discussion at nsrca.org 
Sent: Monday, January 17, 2005 9:57 AM
Subject: Re: epoxy joint
 

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 
 
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