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<DIV><FONT face=Arial size=2>"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></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Awesome! and embarrassing!</FONT></DIV>
<DIV><FONT face=Arial size=2>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></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>But you just made future building
easier...</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>John Ferrell <BR><A
href="http://DixieNC.US">http://DixieNC.US</A><BR></FONT></DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=gfowler@raytheon.com href="mailto:gfowler@raytheon.com">Gray E
Fowler</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=discussion@nsrca.org
href="mailto:discussion@nsrca.org">discussion@nsrca.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Monday, January 17, 2005 9:57
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: epoxy joint</DIV>
<DIV><BR></DIV><BR><FONT face=sans-serif size=2>John,</FONT> <BR><BR><FONT
face=sans-serif size=2>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> <BR><FONT face=sans-serif size=2>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>
<BR><FONT face=sans-serif size=2>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> <BR><BR><FONT face=sans-serif size=2>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><BR><BR><FONT
face=sans-serif size=2>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><BR><BR><BR>Gray Fowler<BR>Principal Chemical Engineer<BR>Composites
Engineering</FONT> <BR><BR><BR>
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<TD><FONT face=sans-serif size=1><B>"John Crozier"
<sjcrozier@comcast.net></B></FONT> <BR><FONT face=sans-serif
size=1>Sent by: discussion-request@nsrca.org</FONT>
<P><FONT face=sans-serif size=1>01/15/2005 09:35 AM</FONT> <BR><FONT
face=sans-serif size=1>Please respond to discussion</FONT> <BR></P>
<TD><FONT face=Arial size=1> </FONT><BR><FONT
face=sans-serif size=1> To:
"discussion nsrca" <discussion@nsrca.org></FONT>
<BR><FONT face=sans-serif size=1> cc:
</FONT> <BR><FONT face=sans-serif size=1>
Subject: epoxy
joint</FONT></TR></TBODY></TABLE><BR><BR><BR><FONT face=Arial size=2>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> <BR><FONT face=Arial size=2>In this case it would be composite
horns to balsa. No fillet is needed.</FONT> <BR><FONT face=Arial
size=2>Anybody wanna jump in?</FONT> <BR><BR></BLOCKQUOTE></BODY></HTML>