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<DIV>Interesting, Gray. Would you also discuss some details about chopped filler
(say 1/16" to 1/2" chop length) and continuous strand filler? Also how material
stiffness and impact resistance are affected?</DIV>
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<DIV>MattK</DIV>
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<DIV> </DIV>
<DIV>In a message dated 1/17/2005 1:18:24 PM Eastern Standard Time,
amirneshati@earthlink.net writes:</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=mailto:gfowler@raytheon.com href="mailto:gfowler@raytheon.com">Gray E
Fowler</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=mailto: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> </FONT></BLOCKQUOTE></DIV>
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