Axial Compression Layup

General Composites Discussions Engineering Talk
#1

Hi,

I want to make some tubes that will take an axial compression force. I know that carbon fibre is generally more suited to bending or torsional forces so I wanted to ask if anyone has any advice or references on designing tubes for axial compression? So far I’m just overbuilding them with 0, 90, 45deg layers but I would like to put a bit more science into the design.

(Imagine a three legged stool with carbon fibre legs - how do I optimise the layup to make the lightest possible legs)

#2

Im not an expert in design, but if its predominantly an axial load, then the majority of fibres should be in the axial direction (ie 90% perhaps). Then, depending on other conditions such as possible impact, other off-axis loading that may be experienced or other failure criteria,then other orientations should be added as appropriate. If theres no real impact and you’re worried about resisting buckling due to a thin column, then I would add some 90 degree plies in there too. You could also add two 45s.
0 degree plies should likely be placed on the outermost surfaces (ie furthest away from the neutral axis) to increase bending resistance also. So say there were 8 plies in total, I would place 0,90,0,0,0,0,90,0 or the likes. Again, not something im overly proficient in but that’s what I would do.

#3

  1. Try to make the layup balanced and symmetrical.

  2. Have at least 10% of the fibers in each direction (0/45/-45/90). Since the load is well known, I would use mostly uni, with possibly a woven (plain weave or twill) for the 45 degree plies.

  3. Make the outermost and innermost plies those that are in the least load bearing direction. Preferably the surface plies should be woven for damage tolerance.

#4

Coriolis, thanks - point 1 and 2 I understand.
Point 3 - whats the reason? I was under the impression it was better to have the most load bearing plies to the outside?

#5

It completely depends on what you are trying to resist. If your part is subject to impact then you dont want your most load bearing plies on the outside as they may be damaged easily. If you want to increase bending stiffness, axial plies the furthermost point from the neutral axis (ie outermost) will increase this but you run the risk of them being damaged if subjected to impact for example

#6

Thanks Findhan, I think I understand now.