Quote:
But the obvious corollary is that since unidirectional fibres are strong in only one direction, you need twice as many layers if you want the composite to be equally strong/stiff in all directions.
I suppose that should be compared to the twice as many layers to make it as strong in any direction at all using woven material. Considering the varying weights of the materials and also that the unidirectional materials come stacked in multiple layers, I'm having a hard time really following this logic. It's all about straw men with you.
Another problem with the woven materials is that half the thread come to an early end on the sides of the fabric. You have to overlap those edges or provide another layer to cross those seams. If you overlap the layers that puts a flaw in the material where other layers have to cross that bump. When I built samples it was possible to get them to fail in that area. Making choices about this is just another issue in determining an appropriate stackup.
I'll clarify an earlier comment about cars and marketing. It is seldom an issue in a car what to do about 10 Kilograms. You don't want it to be heavy, but typically in competition cars there are reasonable minimum weights and it would matter less in a street car. Typically you can make tradeoffs somewhere.
If you look at airplanes they are much more at the mercy of physics than marketing. It's a more honest endeavor. An example would be Porsche deciding to use their decades of experience with air cooled engines to build an aviation power plant for light airplanes. After an effort in 10s of millions of dollars, on top of all their other existing experience and production abilities - they failed by 10 kilograms and in fact never sold a single unit. This compared to engines designed in the 1940's-1950's.