Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml@lancaironline.net
Date: Mon, 10 Nov 2003 14:14:13 -0500
Message-ID: <redirect-2717657@logan.com>
From: Newlan2dl@aol.com
Subject: composite design, composite safety
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One area that many folks think of as a really cool thing is mixing materials.  Just for cosmetics for instance, I took a Kevlar/carbon braid and placed that over a tube part (the rest was carbon unidirectional) and clear finished it.  It looked like a yello and black snake skin and very slick!  But the key is I used it for cosmetics.  Sure it added a LOT of strength but I you aren't carefull, you can actually be designing in a failure.  

This is overly simplified but demonstrates the concept:  

Speaking here of virgin fiber properties only, common grades of carbon has a modulus about 3 times greater than Eglass (33 million v. 10.5 for Eglass v. 12.5 for S2glass).  Common carbon is also typically not much stronger than Eglass fibers at about 500,000 psi. For this demonstration, let's just say carbon is 3X as "stiff" as Eglass (what's in our planes)and the same strength.  

Now, comparing strain to failure, it's about 4.5-5% for Eglass and about 1.5% for carbon.  So if you had a 100" piece of Eglass laminate and the same length of a carbon laminate, the fiberglass will be stretched 4.5" when it breaks and the carbon will have stretched just 1.5" but they both break at 500,000 pounds.  So what happens when the fiberglass has stretched 1.5"?  The load applied is only about 1/3 that of the carbon.

So lets see what happens to someone that says they want a little stiffer part and make it half carbon and half glass.  As load is applied, the carbon is taking most of the load with the glass adding only a fraction of the strength.  The carbon will fail while the glass is only 1/3 of the way to it's ultimate strength.

So while there are very valid reasons for mixing materials, it has to kept in mind what is actually being asked of these materials.  Frequently materials are mixed to give greater impact resistance or toughness but not generally to improve strength since the higher modulus material always takes the load.

One other point that I can illustrate with the above example is that many parts (such as boats) are built using cheap Orthopthalic resin and glass.  This resin may have a strain to failure of only 2% so the resin fractures before the fiberglass takes a load!  

Thus one rule for ALL COMPOSITE CONSTRUCTION:

THE MATRIX RESIN MUST ALWAYS HAVE A GREATER STRAIN TO FAILURE THAN THE FIBERS!  And this is one reason I keep harping on the use of toughened resins with greater strain to failure, greater strength and more flexibility and toughness.

So when you see carbon spars with glass shear webs in between, does that mean it's unsafe?  No, the spar caps are taking the laod and the glass is acting as a shear web.  And when we tab in carbon parts with glass, we are using the glass as a flange to distribute load, not as the primary load memeber.  Done the other way, say with fiberglass caps and carbon shear webs, yeah, I'd be concerned.

Dan Newland