epoxy joint

[Gray E Fowler]

John,

On something like a lite ply former, I usually put a filet like amount on 
the former to ensure complete wet out and complete gap fill. The result is 
an absolute minimal filet-cuz the tip of my finger is curved. This is just 
to make sure the entire joint is complete, because you really cannot QC 
the joint-not to add a filet. 
Also a gob of really strong epoxy WILL increase bond strength IF the epoxy 
is stronger than the substrate such as lite ply. But this is a 
bass-ackwards application and once again dead weight.



Gray Fowler
Principal Chemical Engineer
Composites Engineering




"John Ferrell" <johnferrell at earthlink.net>
Sent by: discussion-request at nsrca.org
01/17/2005 02:01 PM
Please respond to discussion

 
        To:     <discussion at nsrca.org>
        cc: 
        Subject:        Re: epoxy joint


"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, "
 
Awesome! and embarrassing!
I cannot even guess how many formers & stringers I have carefully 
encumbered with beautiful, heavy worthless fillets...Sometimes adding bits 
of string to the mix in imitation of pylon racers...
 
But you just made future building easier...
 
John Ferrell    
http://DixieNC.US
----- Original Message ----- 
From: Gray E 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 



"John Crozier" <sjcrozier at comcast.net> 
Sent by: discussion-request at nsrca.org 
01/15/2005 09:35 AM 
Please respond to discussion 
        
        To:        "discussion nsrca" <discussion at nsrca.org> 
        cc:         
        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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