<DIV>All this talk of Joints has me Buzzing......... <IMG src="http://us.i1.yimg.com/us.yimg.com/i/mesg/tsmileys2/14.gif"></DIV>
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<DIV>Brian</DIV>
<DIV><BR><BR><B><I>John Crozier <sjcrozier@comcast.net></I></B> wrote:</DIV>
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<DIV><FONT face=Arial size=2>Gray,</FONT></DIV>
<DIV><FONT face=Arial size=2>My next project will use composite horns inserted into precision, re-enforced slots in the balsa elevator/aileron.</FONT></DIV>
<DIV><FONT face=Arial size=2>Instructions say "Might want to add some milled fiberglass to the epoxy".</FONT></DIV>
<DIV><FONT face=Arial size=2>Your response confirms my notion not to do that.</FONT></DIV>
<DIV><FONT face=Arial size=2>Thanx..croz (chose your "joints" wisely, even on the mainland))</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></TD></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></BLOCKQUOTE>