epoxy joint

[Gray E Fowler]

"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?"
 
MattK


Chopped fibers actually work well. A chopped fiberglass of about 1.0" can 
approach most of the properties of continuos fiberglass(as long as the 
fiber volume is sufficient). Shorter fiber such as 0.5" and 0.25" increase 
most of the properties of neat epoxy resin also. In essence they are long 
enough to transfer some load through the matrix.  An 8% by weight chopped 
fiberglass in epoxy looks like wet cat hair. Pack enough fiber in (fiber 
volumes greater than 30%) and even a FILLET can now transfer load and can become useful.  Of course all of this is 
way too much for our applications.
Chopped fibers suck in my industry because we need absolute predicability. 
Not knowing exactly how the fiber is oriented or what the void volume is 
is impossible to analyze (FEA).  Chopped fiber is never as good or as 
light as continuous fiber. Chopped fiber does not give you "Maximum 
efficiency of materials" which means  it is too heavy to rely on. On 
Aeroslave planes, I mix 3-4% chopped carbon fiber to beef up the 
microballoon-epoxy corner fill material.
First thing I had to learn to do when transitioning from building missiles 
to building pattern planes is not to build it too strong. Remember.....If 
you cannot break it then you made it too heavy!



Gray Fowler
Principal Chemical Engineer
Composites Engineering
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