<br><font size=2 face="sans-serif">John,</font>
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<br><font size=2 face="sans-serif">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. </font>
<br><font size=2 face="sans-serif">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.<br>
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Gray Fowler<br>
Principal Chemical Engineer<br>
Composites Engineering</font>
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<td><font size=1 face="sans-serif"><b>"John Ferrell" <johnferrell@earthlink.net></b></font>
<br><font size=1 face="sans-serif">Sent by: discussion-request@nsrca.org</font>
<p><font size=1 face="sans-serif">01/17/2005 02:01 PM</font>
<br><font size=1 face="sans-serif">Please respond to discussion</font>
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<br><font size=1 face="sans-serif"> To: <discussion@nsrca.org></font>
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<br><font size=1 face="sans-serif"> Subject: Re: epoxy joint</font></table>
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<br><font size=2 face="Arial">"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, "</font>
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<br><font size=2 face="Arial">Awesome! and embarrassing!</font>
<br><font size=2 face="Arial">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...</font>
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<br><font size=2 face="Arial">But you just made future building easier...</font>
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<br><font size=2 face="Arial">John Ferrell </font><font size=2 color=blue face="Arial"><u><br>
</u></font><a href=http://dixienc.us/><font size=2 color=blue face="Arial"><u>http://DixieNC.US</u></font></a>
<br><font size=3>----- Original Message ----- </font>
<br><font size=3><b>From:</b> </font><a href=mailto:gfowler@raytheon.com><font size=3 color=blue><u>Gray E Fowler</u></font></a><font size=3> </font>
<br><font size=3><b>To:</b> </font><a href=mailto:discussion@nsrca.org><font size=3 color=blue><u>discussion@nsrca.org</u></font></a><font size=3> </font>
<br><font size=3><b>Sent:</b> Monday, January 17, 2005 9:57 AM</font>
<br><font size=3><b>Subject:</b> Re: epoxy joint</font>
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John,</font><font size=3> <br>
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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).</font><font size=3> </font><font size=2 face="sans-serif"><br>
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.</font><font size=3> </font><font size=2 face="sans-serif"><br>
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 <b>little</b> 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<b> reduce</b> elongation, <b>reduce</b> tensile strength, <b>increase</b> the heat distortion temperature, <b>increase</b> hardness and <b>increase</b> compression strength.</font><font size=3> <br>
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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. </font><font size=3><br>
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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). <br>
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Gray Fowler<br>
Principal Chemical Engineer<br>
Composites Engineering</font><font size=3> <br>
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<td width=44%><font size=1 face="sans-serif"><b>"John Crozier" <sjcrozier@comcast.net></b></font><font size=3> </font><font size=1 face="sans-serif"><br>
Sent by: discussion-request@nsrca.org</font><font size=3> </font>
<p><font size=1 face="sans-serif">01/15/2005 09:35 AM</font><font size=3> </font><font size=1 face="sans-serif"><br>
Please respond to discussion</font><font size=3> </font>
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To: "discussion nsrca" <discussion@nsrca.org></font><font size=3> </font><font size=1 face="sans-serif"><br>
cc: </font><font size=3> </font><font size=1 face="sans-serif"><br>
Subject: epoxy joint</font></table>
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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).</font><font size=3> </font><font size=2 face="Arial"><br>
In this case it would be composite horns to balsa. No fillet is needed.</font><font size=3> </font><font size=2 face="Arial"><br>
Anybody wanna jump in?</font><font size=3> <br>
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