Laminate sequence

[Bizcocho]

Hello everybody, I have recently entered in the world of composites so I still have too many questions without answer...
I have designed a part with a balanced and simmetric laminate with 4 cuatriaxial woven fabric plies(so 16 plies of 0-45+45-90º). The part will be made by hand lay-up. The manufacturer tells me that he needs an extra ply of chopped strand mat as the first ply in contact with the mould. If this extra ply is added, the simmetry will no longer exist.
My question is how important it is to include another chopped mat ply opposite to this one to have the simmetry. I have already some problems with the weight of the part (it is like a 2m diameter cilinder) so I would rather not use one more ply. As the mat has clearly worse properties than the fabric, maybe I can avoid it. I suppose both solutions are possible, but I would like to know if you think the effect of the unsymmetry in this case could be important.

thanks %-

 

[Albigger]

Why do they need the chopped mat ply?

Just curious.

 

[Bizcocho]

Hi!

apparently, the chopped mat ply adapts better to the mould walls, it is more flexible, so it reduces the risk of air trapping and imperfections at corners and angles. I would prefer to remove it, but they have said that it is more risky...

 

[TGund]

Chopped mat is also good against the mold for asthetic reasons (less print through of the woven rovings/yarns). If this is one layer of 1oz to 1.5oz/sq ft of chopped mat on a 16 ply stack, I doubt its existance will have much effect on the bend-twisting, etc. coupling. Especially if this is a hand laminated part and the plies are laid up in steps and allowed to cure. Having a low shrink resin system will also mask any coupling issues.

 

[berkshire]

Bizcocho
What you need here is a veil cloth for the first layer.
here is one manufacturer there are others.

http://www.freemansupply.com/Flexiveil130Fiberg.htm

This will give you a skin coat without adding a lot of unnecessary weight to your part.
B.E.

 

[Compositepro]

Chopped mat surface layers and gel coats are used in almost all wet laid parts made in the marine and commercial industries. They also use high shrink polyester resins that show alot of fabric print-through. Mat and gel coat is almost never used in aerospace parts because they add weight and crack under thermocylcing.

It is often best to stay with standard industry practice unless you can clearly show why not. The reason that some things are done a certain way are often forgotten or misunderstood but when you try to change you may relearn the reason.

The mat does create an unbalance layup, but in a cylindrical part it will not create unbalanced stresses. Thin flat panels are always the most difficult because it takes very little stress to warp. But, on the other hand, it takes very little force or stiffening to eliminate the warp.

 

[Bizcocho]

Thank you all, your comments are very useful!

The part, while having a cylindrical shape in half of the body, has also many different surfaces including flat panels of 2x1 meters. Trying to be concise, I described the piece too simplified. We will use isophtalic resin and E type fiberglass for some surfaces and the same resin with carbon fiber in other areas. First, there will be a layer of gelcoat and a thin mat ply. All layers will be applied by hand and allowed to cure (as some of you mentioned).
I agree with Compositepro about the idea of being within standard industry practices. And as TGund says, I hope that such thin layer will not have an important effect on the whole, although the mat is in an external surface where higher bending stresses normally occur.
By the way, could the possible warp be checked by doing a standard uniaxial tension test on the laminate? and, Has a stiffener attached to the laminate wall the effect of unbalancing the laminate?

thanks,
Bizcocho

 

[RPstress]

I suspect that a part this complicated (especially mixing carbon and glass with a polyester matrix) will deform a bit anyway. Having a bit of reasonably light chopped strand mat in the mix will probably not make much difference.

Warpage is typically due to cure (especially if cured at elevated temperature) or temperature excursions in service, and is rarely an issue when due to coupling (because of an unbalanced layup) under applied load. I do not believe that a uniaxial test would tell you anything useful.

Just making a flat panel maybe half a metre or a metre square with your proposed layup might give you a clue as to what deformations will happen during cure and the sort of forces needed to keep it flat. However, the behavior of the actual part, with its complicated geometry and mix of materials in different areas, will be significantly different from any such simple test parts.

Predicting spring of composite laminates is very, very difficult. Experience of similar parts is about the only way to estimate it, though there are beginning to be some numerate FE-based techniques which can help quantify it.

Coupling under load due to asymmetry is more amenable to analysis, but, as said above, is rarely a problem in practice.

The surface strains will indeed be highest, but stresses in a composite depend on layer stiffness. A layer of chopped strand mat is no more of a weak point than the rather weak resin at the surface.