UD enclosed within laminate

[vinhermes]

Hello,

This may be a silly question for you but looking at laminate drawings for a hi-tech carbon fibre foam core boat (catamaran 30m) I have realised that when using uni directional reinforcements for local loads, the structural engineers always enclose the UD in the nominal laminate.

Basically, there is never a UD ply as outer or inner skin, it is always covered by at least a 300gsm woven roving.

Why would one do that? IS UD more prone to peeling? Is it weaker if not covered? It is just for external transverse stresses? Internal due to poisson's ratio?

Lastly, what if you keep the UD as your last ply and you come to bond something to it (maybe a bulkhead)? Would one be in trouble to do this?

Thanks by advance to lighten this. Vincent

 

[Compositepro]

Uni-directional plies are far more prone to peeling than woven. It is not just that the whole ply can peel but the surface filaments will start to peel at any surface scratch. Also, micro-cracks in UD can be very long, whereas in woven they are very limited in length.

 

[berkshire]

In addition to Composite pro's remarks, you want the unidirectional fibers protected so that they do not get cut by any later refinishing work on the boat.
B.E.

 

[allcomposites]

In addition to the good previous remarks. UD are prone to easily propagate micro-cracks after small impacts. Their fatigue life is then highly reduced.

 

[vinhermes]

Thank you very much.

It is logical. I will adopt this good practice...

Vincent

 

[RPstress]

People also worry about surface UD being more vulnerable to impacts than woven, even using just one ply of woven as a surface ply for a thick UD laminate. I'm not sure how much difference this sort of thing really makes, but it falls into standard practice. You are probably wise to follow suit.

Cloth prepreg also tends to be a bit more resin rich than UD prepreg, and solid prepreg UD can look a bit ratty.