epoxy joint

[Rcmaster199 at aol.com]

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  Fowler_ (mailto:gfowler at raytheon.com)  
To: _discussion at nsrca.org_ (mailto: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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