Higher internal stresses that lead to micro-cracking, more brittle parts, print-through, and warping. Most of what I've seen published on this subject was to promote benefits of heavy fabrics which is lower cost material and fabrication, so the drawbacks were not covered.
You also of course limit the exact thicknesses of laminate that you can make; you may think you need 1.10 mm or something, but with 0.25 mm plies you have to go over to 1.25 mm (weight penalty of 13.6%), whereas with 0.125 mm plies you could use 1.125 mm (weight penalty 2.3%). It can be moot: if you need a 1.15 m thickness then you have to go for 1.25 mm whether you've got 0.125 mm plies or 0.25 mm.
Achieving the desired ply direction proportions in a given thickness is also more problematic as plies get thicker.
Compositepro - what causes the increase in internal stresses and the brittleness?
Swcomposites - is the reduction in the compression strength resulted from increase in the weaviness of ply due to heavier tow (i.e. 12K fibre)??
RPstress & all - what ifI use a combination of different gsm fabric e.g.200gsm and 400gsm fabrics, would some of the above problems improved? and what additional problems it will cause??
by the way, is there lower K but higher gsm fabric?? and if so would it resolve some of the above problems??
You can use combinations of ply thickness if you want, at the price of inventory and complication (if it's prepreg you get also risks like different combinations of out-life, though that sort of thing can happen anyway, and if it's infused in-plane different plies can have different permeabilities).
There are no particular mechanical property penalties in mixing and matching that I'm aware of. Others may know better.
Re k vs. areal weight, this depends on the fabric supplier (and possibly on the fiber supplier). Tows can be spread out before being woven together within limits. The tows in a typical typical fabric aren't round, they're spread out something like 5 or 10:1 in a flattish somewhat oval shape. I'm not sure how this sort of thing affects mechanical properties.
If you want a cured ply thickness of 0.25 mm (10 thou) and the material is from a commercial suppier who deals with 320k tow, then it needs to be spread out something like 20:1.
Not sure how good fabric suppliers are at bunching tows up.
One other practical point about ply thickness, thinner plies drape more easily.
