Normally, a fillet of carbon or glass is transfering loads along a secondary surface such as a spar being attached to a rib so there is a lot more to do with geometry and dispersing loads than just mixing various moduii of the materials.  In all cases, you have a stiff area that transitions to a more flexible area and you are trying to avoid making a hinge.  By this I mean look at what the BIDS are doing.  They are attaching a rib to a spar or something like that, which already has vastly stiffer properties due to its geometry than a carbon laminate when the loads are applied edge on.  And that completely ignores the effects of resin which can be huge.  Jeffco and others make toughened epoxies that are a family of resins that are better in impact, peel, flex and aging in general.  That is much higher performance than what we currently use and for the relatively few extra dollars, is surprising we haven't done it....But then again, you don't see a lot of Lancairs falling out of the sky because the epoxied in tabs fatigued and failed from flexing after a collision.  Obviously the existing system is adequate, even though there are options at no extra weight that are stronger.

 

So in general, I am less concerned about that than miss-using mixed modulus fabrics in structures.  For example, a cool looking fabric I did some laminates of was a braided tube of Kevlar and carbon.  It looked great!  The yellow and black looked like a snake skin.  The problem is if it comes under severe stress, the higher modulus fiber will take the strain before the lower on will.  This is always the case.  So I have a good looking tough skin over a carbon laminate that I don't rely on to take major loads. 

 

As an example, take a laminate of 50% carbon and 50% Eglass.  the modulus of "standard" T-700 type carbon is 33 million. and the glass is about 1/3 of that.  In other words, the glass will stretch about 3 times as much before it fails.  So if you pull a rope made of the stuff to where the carbon strarts braeaking, the glass has only contributed 1/3 of its ultimate to the rope.  It may add a lot of toughness thoug so there are potentially synergistic effects but you need to test it.

 

One thing that surprises me is that I haven't seen anywhere in the manual that tells you to taper the laminate.  It's something I always do since structurally, a taper is more efficient at distributing a load.  For example, if a fillet is to have two layers of 2" glass tape, it's better to have one 1.75" and the other 2.25" since you reduce the tendancy to have a hard spot.  Those of us that are anal may also lightly grind the edge to get rid of non structural (and finger biting) nibs.  Cosmetically better too of course.  On some of the boats I've built, the test is to run your wife's nylons over the surface and if it snags, do some sanding.  I HATE fishing in the bilge for something that's gotten loose and in the search come up with hands that look like they tangled with a cheese grater.

 

Happy laminating and grinding!

 

Dan