The outer layer of a sandwich composite has to withstand tensile loads and the inner layer compression loads in case of an impact.
When we compare the different laminate materials:
Tensile strength [MPa] E-glass 2400, Kevlar 3000, Carbon 4000,
we see that carbon is the strongest material, but in case of an accident Kevlar has it's large advantage over the other materials.
Strain at break point E-Glass 4.5%, Kevlar 2%, Carbon 1.2%.
The 1.2% figure for carbon fibre indicates that though very strong it is also very brittle and will break shortly after the load stress passes yield point.
So the advantage of Kevlar as outer layer is, that it is lighter than glass, stronger in tensile strenght and not brittle.
In case of repair Kevlar will fuzz at the surface if you want to sand it, and it is hard to cut. It must be covered with glass or gel coat.
Here the source for the above mentioned with more technical background:
http://oneoceankayaks.com/Sandcore.htmhttp://www.auf.asn.au/scratchbuilder/composites.html
