epoxy joint

[John Crozier]

thanx...croz
  ----- Original Message ----- 
  From: Gray E Fowler 
  To: discussion at nsrca.org 
  Sent: Monday, January 17, 2005 4:08 PM
  Subject: Re: epoxy joint



  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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