epoxy joint

[rcaerobob at cox.net]

Ya know, all this talk of "filet"s is making me HUNGRY.  Haven't read much today on "fillets", and one you eat, one you don't....Gray?

<BG>

Bob
> 
> From: Gray E Fowler <gfowler at raytheon.com>
> Date: 2005/01/17 Mon PM 04:08:50 EST
> To: discussion at nsrca.org
> 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? 
> 
> 
> 
> 

Bob Pastorello, El Reno, OK, USA
rcaerobob at cox.net
www.rcaerobats.net

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