epoxy joint

[John Crozier]

Gray,
My next project will use composite horns inserted into precision, re-enforced  slots in the balsa elevator/aileron.
Instructions say "Might want to add some milled fiberglass to the epoxy".
Your response confirms my notion not to do that.
Thanx..croz    (chose your "joints" wisely, even on the mainland))
  ----- 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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